Re: [PERFORM] Intermittent hangs with 9.2
We haven't seen any issues since we decreased shared_buffers. We also tuned some of the longer running / more frequently executed queries, so that may have had an effect as well, but my money would be on the shared_buffers change. If the issue re-appears I'll try to get a perf again and post back, but if you don't hear from me again you can assume the problem is solved. Thank you all again for the help. -Dave On Fri, Sep 13, 2013 at 11:05 AM, David Whittaker d...@iradix.com wrote: On Fri, Sep 13, 2013 at 10:52 AM, Merlin Moncure mmonc...@gmail.comwrote: On Thu, Sep 12, 2013 at 3:06 PM, David Whittaker d...@iradix.com wrote: Hi All, We lowered shared_buffers to 8G and increased effective_cache_size accordingly. So far, we haven't seen any issues since the adjustment. The issues have come and gone in the past, so I'm not convinced it won't crop up again, but I think the best course is to wait a week or so and see how things work out before we make any other changes. Thank you all for your help, and if the problem does reoccur, we'll look into the other options suggested, like using a patched postmaster and compiling for perf -g. Thanks again, I really appreciate the feedback from everyone. Interesting -- please respond with a follow up if/when you feel satisfied the problem has gone away. Andres was right; I initially mis-diagnosed the problem (there is another issue I'm chasing that has a similar performance presentation but originates from a different area of the code). That said, if reducing shared_buffers made *your* problem go away as well, then this more evidence that we have an underlying contention mechanic that is somehow influenced by the setting. Speaking frankly, under certain workloads we seem to have contention issues in the general area of the buffer system. I'm thinking (guessing) that the problems is usage_count is getting incremented faster than the buffers are getting cleared out which is then causing the sweeper to spend more and more time examining hotly contended buffers. This may make no sense in the context of your issue; I haven't looked at the code yet. Also, I've been unable to cause this to happen in simulated testing. But I'm suspicious (and dollars to doughnuts '0x347ba9' is spinlock related). Anyways, thanks for the report and (hopefully) the follow up. merlin You guys have taken the time to help me through this, following up is the least I can do. So far we're still looking good.
Re: [PERFORM] Intermittent hangs with 9.2
On Thu, Sep 12, 2013 at 3:06 PM, David Whittaker d...@iradix.com wrote: Hi All, We lowered shared_buffers to 8G and increased effective_cache_size accordingly. So far, we haven't seen any issues since the adjustment. The issues have come and gone in the past, so I'm not convinced it won't crop up again, but I think the best course is to wait a week or so and see how things work out before we make any other changes. Thank you all for your help, and if the problem does reoccur, we'll look into the other options suggested, like using a patched postmaster and compiling for perf -g. Thanks again, I really appreciate the feedback from everyone. Interesting -- please respond with a follow up if/when you feel satisfied the problem has gone away. Andres was right; I initially mis-diagnosed the problem (there is another issue I'm chasing that has a similar performance presentation but originates from a different area of the code). That said, if reducing shared_buffers made *your* problem go away as well, then this more evidence that we have an underlying contention mechanic that is somehow influenced by the setting. Speaking frankly, under certain workloads we seem to have contention issues in the general area of the buffer system. I'm thinking (guessing) that the problems is usage_count is getting incremented faster than the buffers are getting cleared out which is then causing the sweeper to spend more and more time examining hotly contended buffers. This may make no sense in the context of your issue; I haven't looked at the code yet. Also, I've been unable to cause this to happen in simulated testing. But I'm suspicious (and dollars to doughnuts '0x347ba9' is spinlock related). Anyways, thanks for the report and (hopefully) the follow up. merlin -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Intermittent hangs with 9.2
On Fri, Sep 13, 2013 at 10:52 AM, Merlin Moncure mmonc...@gmail.com wrote: On Thu, Sep 12, 2013 at 3:06 PM, David Whittaker d...@iradix.com wrote: Hi All, We lowered shared_buffers to 8G and increased effective_cache_size accordingly. So far, we haven't seen any issues since the adjustment. The issues have come and gone in the past, so I'm not convinced it won't crop up again, but I think the best course is to wait a week or so and see how things work out before we make any other changes. Thank you all for your help, and if the problem does reoccur, we'll look into the other options suggested, like using a patched postmaster and compiling for perf -g. Thanks again, I really appreciate the feedback from everyone. Interesting -- please respond with a follow up if/when you feel satisfied the problem has gone away. Andres was right; I initially mis-diagnosed the problem (there is another issue I'm chasing that has a similar performance presentation but originates from a different area of the code). That said, if reducing shared_buffers made *your* problem go away as well, then this more evidence that we have an underlying contention mechanic that is somehow influenced by the setting. Speaking frankly, under certain workloads we seem to have contention issues in the general area of the buffer system. I'm thinking (guessing) that the problems is usage_count is getting incremented faster than the buffers are getting cleared out which is then causing the sweeper to spend more and more time examining hotly contended buffers. This may make no sense in the context of your issue; I haven't looked at the code yet. Also, I've been unable to cause this to happen in simulated testing. But I'm suspicious (and dollars to doughnuts '0x347ba9' is spinlock related). Anyways, thanks for the report and (hopefully) the follow up. merlin You guys have taken the time to help me through this, following up is the least I can do. So far we're still looking good.
Re: [PERFORM] Intermittent hangs with 9.2
Hi All, We lowered shared_buffers to 8G and increased effective_cache_size accordingly. So far, we haven't seen any issues since the adjustment. The issues have come and gone in the past, so I'm not convinced it won't crop up again, but I think the best course is to wait a week or so and see how things work out before we make any other changes. Thank you all for your help, and if the problem does reoccur, we'll look into the other options suggested, like using a patched postmaster and compiling for perf -g. Thanks again, I really appreciate the feedback from everyone. -Dave On Wed, Sep 11, 2013 at 1:17 PM, Andres Freund and...@2ndquadrant.comwrote: On 2013-09-11 07:43:35 -0500, Merlin Moncure wrote: I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. Try converting it to something more meaningful with addr2line, that often has more sucess. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. Reducing shared buffers to around 2gb will probably make the problem go away That profile doesn't really look like one of the problem you are referring to would look like. Based on the profile I'd guess it's possible that you're seing problems with GIN's fastupdate mechanism. Try ALTER INDEX whatever SET (FASTUPDATE = OFF); VACUUM whatever's_table for all gin indexes. It's curious that set_config_option is so high in the profile... Any chance you could recompile postgres with -fno-omit-frame-pointers in CFLAGS? That would allow you to use perf -g. The performance price of that usually is below 1% for postgres. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training Services
Re: [PERFORM] Intermittent hangs with 9.2
On 10/09/13 20:04, David Whittaker wrote: On Tue, Sep 10, 2013 at 11:26 AM, Andrew Dunstan and...@dunslane.net mailto:and...@dunslane.net wrote: On 09/10/2013 11:04 AM, David Whittaker wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: | + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics| I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. I have seen cases like this with very high shared_buffers settings. 24Gb for shared_buffers is quite high, especially on a 48Gb box. What happens if you dial that back to, say, 12Gb? I'd be willing to give it a try. I'd really like to understand what's going on here though. Can you elaborate on that? Why would 24G of shared buffers be too high in this case? The machine is devoted entirely to PG, so having PG use half of the available RAM to cache data doesn't feel unreasonable. Here is what I have recently learned. The root cause is crash safety and checkpoints. This is certainly something you want. When you write to the database these operations first occur in the buffer cache and the particular buffer you write to is marked dirty. The cache is organized in chunks of 8kb. Additionally write operations are also committed to the WAL. A checkpoint iterates over all dirty buffers writing them to the database. After that all buffers are clean again. Now, if you write to a clean buffer it gets entirely written to the WAL. That means after
Re: [PERFORM] Intermittent hangs with 9.2
On Tue, Sep 10, 2013 at 02:04:57PM -0400, David Whittaker wrote: Hi Andrew, On Tue, Sep 10, 2013 at 11:26 AM, Andrew Dunstan and...@dunslane.netwrote: On 09/10/2013 11:04 AM, David Whittaker wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: | + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics| I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. I have seen cases like this with very high shared_buffers settings. 24Gb for shared_buffers is quite high, especially on a 48Gb box. What happens if you dial that back to, say, 12Gb? I'd be willing to give it a try. I'd really like to understand what's going on here though. Can you elaborate on that? Why would 24G of shared buffers be too high in this case? The machine is devoted entirely to PG, so having PG use half of the available RAM to cache data doesn't feel unreasonable. Some of the overhead of bgwriter and checkpoints is more or less linear in the size of shared_buffers. If your shared_buffers is large a lot of data could be dirty when a checkpoint starts, resulting in an I/O spike ... (although we've spread checkpoints in recent pg versions, so this should be less a problem nowadays). Another reason is that the OS cache is also being used for reads and writes and with a large shared_buffers there is a risk of doubly cached data (in the OS cache + in shared_buffers). In an ideal world most frequently used blocks should be in shared_buffers and less frequently used block in the OS cache .. cheers andrew -- No trees were killed in the creation of this message. However, many electrons were terribly inconvenienced. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Intermittent hangs with 9.2
On Tue, Sep 10, 2013 at 10:04 AM, David Whittaker d...@iradix.com wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. Reducing shared buffers to around 2gb will probably make the problem go away *) What's your ratio reads to writes (approximately)? *) How many connections when it happens. Do connections pile on after that? *) Are you willing to run custom patched postmaster to help troubleshoot the problem? merlin -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Intermittent hangs with 9.2
On 2013-09-11 07:43:35 -0500, Merlin Moncure wrote: I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. Try converting it to something more meaningful with addr2line, that often has more sucess. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. Reducing shared buffers to around 2gb will probably make the problem go away That profile doesn't really look like one of the problem you are referring to would look like. Based on the profile I'd guess it's possible that you're seing problems with GIN's fastupdate mechanism. Try ALTER INDEX whatever SET (FASTUPDATE = OFF); VACUUM whatever's_table for all gin indexes. It's curious that set_config_option is so high in the profile... Any chance you could recompile postgres with -fno-omit-frame-pointers in CFLAGS? That would allow you to use perf -g. The performance price of that usually is below 1% for postgres. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training Services -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Intermittent hangs with 9.2
On Wed, Sep 11, 2013 at 12:17 PM, Andres Freund and...@2ndquadrant.com wrote: On 2013-09-11 07:43:35 -0500, Merlin Moncure wrote: I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. Try converting it to something more meaningful with addr2line, that often has more sucess. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. Reducing shared buffers to around 2gb will probably make the problem go away That profile doesn't really look like one of the problem you are referring to would look like. yup -- I think you're right. merlin -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Intermittent hangs with 9.2
Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. Thanks, Dave
Re: [PERFORM] Intermittent hangs with 9.2
On 09/10/2013 11:04 AM, David Whittaker wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: | + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics| I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. I have seen cases like this with very high shared_buffers settings. 24Gb for shared_buffers is quite high, especially on a 48Gb box. What happens if you dial that back to, say, 12Gb? cheers andrew -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Intermittent hangs with 9.2
On Tue, Sep 10, 2013 at 11:04:21AM -0400, David Whittaker wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. Thanks, Dave Hi Dave, A load average of 100+ means that you have that many processes waiting to run yet you only have 16 cpus. You really need to consider using a connection pooler like pgbouncer to keep your connection count in the 16-32 range. Regards, Ken -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Intermittent hangs with 9.2
Hi Andrew, On Tue, Sep 10, 2013 at 11:26 AM, Andrew Dunstan and...@dunslane.netwrote: On 09/10/2013 11:04 AM, David Whittaker wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: | + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics| I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. I have seen cases like this with very high shared_buffers settings. 24Gb for shared_buffers is quite high, especially on a 48Gb box. What happens if you dial that back to, say, 12Gb? I'd be willing to give it a try. I'd really like to understand what's going on here though. Can you elaborate on that? Why would 24G of shared buffers be too high in this case? The machine is devoted entirely to PG, so having PG use half of the available RAM to cache data doesn't feel unreasonable. cheers andrew
Re: [PERFORM] Intermittent hangs with 9.2
On Tue, Sep 10, 2013 at 8:04 AM, David Whittaker d...@iradix.com wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. I'm curious about the spike it outbound network usage. If the database is hung and no longer responding to queries, what is it getting sent over the network? Can you snoop on that traffic? CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. There have been reports that using very large shared_buffers can cause a lot of contention issues in the kernel, for some kernels. The usual advice is not to set shared_buffers above 8GB. The operating system can use the rest of the memory to cache for you. Also, using a connection pooler and lowering the number of connections to the real database has solved problems like this before. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. How are your tracking those? Is it log_min_duration_statement or something else? Cheers, Jeff
[PERFORM] Intermittent hangs with 9.2
Hi Ken, On Tue, Sep 10, 2013 at 11:33 AM, k...@rice.edu k...@rice.edu wrote: On Tue, Sep 10, 2013 at 11:04:21AM -0400, David Whittaker wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. Some other changes we've made to postgresql.conf: synchronous_commit = off maintenance_work_mem = 1GB wal_level = hot_standby wal_buffers = 16MB max_wal_senders = 10 wal_keep_segments = 5000 checkpoint_segments = 128 checkpoint_timeout = 30min checkpoint_completion_target = 0.9 max_connections = 500 The server is a Dell Poweredge R900 with 4 Xeon E7430 processors, 48GB of RAM, running Cent OS 6.3. So far we‘ve tried disabling Transparent Huge Pages after I found a number of resources online that indicated similar interrupt/context switch issues, but it hasn’t resolve the problem. I managed to catch it happening once and run a perf which showed: + 41.40% 48154 postmaster 0x347ba9 f 0x347ba9 + 9.55% 10956 postmaster 0x2dc820 f set_config_option + 8.64%9946 postmaster 0x5a3d4 f writeListPage + 5.75%6609 postmaster 0x5a2b0 f ginHeapTupleFastCollect + 2.68%3084 postmaster 0x192483 f build_implied_join_equality + 2.61%2990 postmaster 0x187a55 f build_paths_for_OR + 1.86%2131 postmaster 0x794aa f get_collation_oid + 1.56%1822 postmaster 0x5a67e f ginHeapTupleFastInsert + 1.53%1766 postmaster 0x1929bc f distribute_qual_to_rels + 1.33%1558 postmaster 0x249671 f cmp_numerics I‘m not sure what 0x347ba9 represents, or why it’s an address rather than a method name. That's about the sum of it. Any help would be greatly appreciated and if you want any more information about our setup, please feel free to ask. Thanks, Dave Hi Dave, A load average of 100+ means that you have that many processes waiting to run yet you only have 16 cpus. You really need to consider using a connection pooler like pgbouncer to keep your connection count in the 16-32 range. That would make sense if the issues corresponded to increased load, but they don't. I understand that the load spike is caused by waiting processed, but it doesn't seem to correspond to a transaction spike. The number of transactions per second appear to stay in-line with normal usage when these issues occur. I do see an increase in postmaster processes when it happens, but they don't seem to have entered a transaction yet. Coupled with the fact that cpu usage is all system time, and the context switch / interrupt spikes, I feel like something must be going on behind the scenes leading to these problems. I'm just not sure what that something is. Regards, Ken
[PERFORM] Intermittent hangs with 9.2
Hi Jeff On Tue, Sep 10, 2013 at 1:44 PM, Jeff Janes jeff.ja...@gmail.com wrote: On Tue, Sep 10, 2013 at 8:04 AM, David Whittaker d...@iradix.com wrote: Hi All, I've been seeing a strange issue with our Postgres install for about a year now, and I was hoping someone might be able to help point me at the cause. At what seem like fairly random intervals Postgres will become unresponsive to the 3 application nodes it services. These periods tend to last for 10 - 15 minutes before everything rights itself and the system goes back to normal. During these periods the server will report a spike in the outbound bandwidth (from about 1mbs to about 5mbs most recently), a huge spike in context switches / interrupts (normal peaks are around 2k/8k respectively, and during these periods they‘ve gone to 15k/22k), and a load average of 100+. I'm curious about the spike it outbound network usage. If the database is hung and no longer responding to queries, what is it getting sent over the network? Can you snoop on that traffic? It seems curious to me as well. I don't know if one, or a few of the pg connections are streaming out data and somehow blocking the others in the process, or the data could be unrelated to pg. If you can suggest a tool I could use to monitor the data transfer continuously and get some type of summary of what happened after the issue reoccurs, I'd appreciate it. Otherwise, I'll try to get in and catch some specifics the next time it happens. CPU usage stays relatively low, but it’s all system time reported, user time goes to zero. It doesn‘t seem to be disk related since we’re running with a shared_buffers setting of 24G, which will fit just about our entire database into memory, and the IO transactions reported by the server, as well as the disk reads reported by Postgres stay consistently low. There have been reports that using very large shared_buffers can cause a lot of contention issues in the kernel, for some kernels. The usual advice is not to set shared_buffers above 8GB. The operating system can use the rest of the memory to cache for you. Also, using a connection pooler and lowering the number of connections to the real database has solved problems like this before. We're going to implement both of these changes tonight. I was going to go with 12G for shared_buffers based on Andrew's suggestion, but maybe I'll go down to 8 if that seems to be the magic number. We're also going to decrease the max connections from 500 to 100 and decrease the pooled connections per server. We‘ve recently started tracking how long statements take to execute, and we’re seeing some really odd numbers. A simple delete by primary key, for example, from a table that contains about 280,000 rows, reportedly took 18h59m46.900s. An update by primary key in that same table was reported as 7d 17h 58m 30.415s. That table is frequently accessed, but obviously those numbers don't seem reasonable at all. How are your tracking those? Is it log_min_duration_statement or something else? We're using log_min_duration_statement = 1000, sending the log messages to syslog, then analyzing with pg_badger. Cheers, Jeff
