I'm a bit confused by the two different cases you described, so cannot comment specially on your case.
In general if Cassandra is slowing down take a look at the thread pool stats, using nodetool tpstats to see where it is backing up and take at look at the logs to check for excessive GC. If node stats shows the read or mutation stage backing up, check the iostats. Hope that helps. Aaron On 28/04/2011, at 12:32 AM, Nikolay Kоvshov <nkovs...@yandex.ru> wrote: > I have set quite low memory consumption (see my configuration in first > message) and give Cassandra 2.7 Gb of memory. > I cache 1M of 64-bytes keys + 64 Mb memtables. I believe overhead can't be > 500% or so ? > > memtable operations in millions = default 0.3 > > I see now very strange behaviour > > If i fill Cassandra with, say, 100 millions of 64B key + 64B data and after > that I start inserting 64B key + 64 KB data, compaction queue immediately > grows to hundreds and cassandra extremely slows down (it makes aroung 30-50 > operations/sec), then starts to give timeout errors. > > But if I insert 64B key + 64 KB data from the very beginning, cassandra works > fine and makes around 300 operations/sec even when the database exceeds 2-3 TB > > The behaviour is quite complex and i cannot predict the effect of my actions. > What consequences I will have if I turn off compaction ? Can i read somewhere > about what is compaction and why it so heavily depends on what and in which > order i write into cassandra ? > > 26.04.2011, 00:08, "Shu Zhang" <szh...@mediosystems.com>: >> the way I measure actual memtable row sizes is this >> >> write X rows into a cassandra node >> trigger GC >> record heap usage >> trigger compaction and GC >> record heap savings and divide by X for actual cassandra memtable row size >> in memory >> >> Similar process to measure per-key/per-row cache sizes for your data. To >> understand your memtable row overhead size, you can do the above exercise >> with very different data sizes. >> >> Also, you probably know this, but when setting your memory usage ceiling or >> heap size, make sure to leave a few hundred MBs for GC. >> ________________________________________ >> From: Shu Zhang [szh...@mediosystems.com] >> Sent: Monday, April 25, 2011 12:55 PM >> To: user@cassandra.apache.org >> Subject: RE: OOM on heavy write load >> >> How large are your rows? binary_memtable_throughput_in_ >> mb only tracks size of data, but there is an overhead associated with each >> row on the order of magnitude of a few KBs. If your row data sizes are >> really small then the overhead dominates the memory usage and >> binary_memtable_throughput_in_ >> mb end up not limiting your memory usage the way you'd expect. It's a good >> idea to specify memtable_operations_in_millions in that case. If you're not >> sure how big your data is compared to memtable overhead, it's a good idea to >> specify both parameters to effectively put in a memory ceiling no matter >> which dominates your actual memory usage. >> >> It could also be that your key cache is too big, you should measure your key >> sizes and make sure you have enough memory to cache 1m keys (along with your >> memtables). Finally if you have multiple keyspaces (for multiple >> applications) on your cluster, they all share the total available heap, so >> you have to account for that. >> >> There's no measure in cassandra to guard against OOM, you must configure >> nodes such that the max memory usage on each node, that is max size all your >> caches and memtables can potentially grow to, is less than your heap size. >> ________________________________________ >> From: Nikolay Kоvshov [nkovs...@yandex.ru] >> Sent: Monday, April 25, 2011 5:21 AM >> To: user@cassandra.apache.org >> Subject: Re: OOM on heavy write load >> >> I assume if I turn off swap it will just die earlier, no ? What is the >> mechanism of dying ? >> >> From the link you provided >> >> # Row cache is too large, or is caching large rows >> my row_cache is 0 >> >> # The memtable sizes are too large for the amount of heap allocated to the >> JVM >> Is my memtable size too large ? I have made it less to surely fit the >> "magical formula" >> >> Trying to analyze heap dumps gives me the following: >> >> In one case diagram has 3 Memtables about 64 Mb each + 72 Mb "Thread" + 700 >> Mb "Unreachable objects" >> >> suspected threats: >> 7 instances of "org.apache.cassandra.db.Memtable", loaded by >> "sun.misc.Launcher$AppClassLoader @ 0x7f29f4992d68" occupy 456,292,912 >> (48.36%) bytes. >> 25,211 instances of "org.apache.cassandra.io.sstable.SSTableReader", loaded >> by "sun.misc.Launcher$AppClassLoader @ 0x7f29f4992d68" occupy 294,908,984 >> (31.26%) byte >> 72 instances of "java.lang.Thread", loaded by "<system class loader>" occupy >> 143,632,624 (15.22%) bytes. >> >> In other cases memory analyzer hangs trying to parse 2Gb dump >> >> 22.04.2011, 17:26, "Jonathan Ellis" <jbel...@gmail.com>;;: >> >>> (0) turn off swap >>> (1) >>> http://www.datastax.com/docs/0.7/troubleshooting/index#nodes-are-dying-with-oom-errors >>> >>> On Fri, Apr 22, 2011 at 8:00 AM, Nikolay Kоvshov <nkovs...@yandex.ru>;;; >>> wrote: >>>> I am using Cassandra 0.7.0 with following settings >>>> >>>> binary_memtable_throughput_in_mb: 64 >>>> in_memory_compaction_limit_in_mb: 64 >>>> keys_cached 1 million >>>> rows_cached 0 >>>> >>>> RAM for Cassandra 2 GB >>>> >>>> I run very simple test >>>> >>>> 1 Node with 4 HDDs (1 HDD - commitlog and caches, 3 HDDs - data) >>>> 1 KS => 1 CF => 1 Column >>>> >>>> I insert data (random key 64 bytes + value 64 bytes) at a maximum >>>> possible speed, trying to hit disk i/o, calculate speed and make sure >>>> Cassandra stays alive. It doesn't, unfortunately. >>>> After several hundreds millions of inserts Cassandra always does down >>>> by OOM. Getting it up again doesn't help - after inserting some new data >>>> it goes down again. By this time Cassandra goes to swap and has a lot of >>>> tasks pending. I am not inserting anything now and tasks sloooowly >>>> disappear, but it will take her weeks to do all of them. >>>> >>>> compaction type: Minor >>>> column family: Standard1 >>>> bytes compacted: 3661003227 >>>> bytes total in progress: 4176296448 >>>> pending tasks: 630 >>>> >>>> So, what am I (or Cassandra) doing wrong ? I dont want to get Cassandra >>>> crashed without means of repair on heavy load circumstances. >>> -- >>> Jonathan Ellis >>> Project Chair, Apache Cassandra >>> co-founder of DataStax, the source for professional Cassandra support >>> http://www.datastax.com
