excellent. if automove=2 is too aggressive you'll see that come in in a hit ratio reduction.
the new branch works with automove=2 as well, but it will attempt to rescue valid items in the old slab if possible. I'll still be working on it for another few hours today though. I'll mail again when I'm done. On Tue, 29 Sep 2015, Scott Mansfield wrote: > I have the first commit (slab_automove=2) running in prod right now. Later > today will be a full load production test of the latest code. I'll just let > it run for a few days unless I spot any problems. We have good metrics for > latency et. al. from the client side, though network normally dwarfs > memcached time. > > On Tuesday, September 29, 2015 at 3:10:03 AM UTC-7, Dormando wrote: > That's unfortunate. > > I've done some more work on the branch: > https://github.com/memcached/memcached/pull/112 > > It's not completely likely you would see enough of an improvement from > the > new default mode. However if your item sizes change gradually, items are > reclaimed during expiration, or get overwritten (and thus freed in the > old > class), it should work just fine. I have another patch coming which > should > help though. > > Open to feedback from any interested party. > > On Fri, 25 Sep 2015, Scott Mansfield wrote: > > > I have it running internally, and it runs fine under normal load. > It's difficult to put it into the line of fire for a production workload > because of social reasons... As well it's a degenerate case that we normally > don't run in to (and actively try to avoid). I'm going to run some heavier > load tests on it today. > > > > On Wednesday, September 9, 2015 at 10:23:32 AM UTC-7, Scott Mansfield > wrote: > > I'm working on getting a test going internally. I'll let you > know how it goes. > > > > > > Scott Mansfield > > On Mon, Sep 7, 2015 at 2:33 PM, dormando wrote: > > Yo, > > > > https://github.com/dormando/memcached/commits/slab_rebal_next - > would you > > mind playing around with the branch here? You can see the start > options in > > the test. > > > > This is a dead simple modification (a restoration of a feature > that was > > arleady there...). The test very aggressively writes and is > able to shunt > > memory around appropriately. > > > > The work I'm exploring right now will allow savings of items > being > > rebalanced from, and increasing the aggression of page moving > without > > being so brain damaged about it. > > > > But while I'm poking around with that, I'd be interested in > knowing if > > this simple branch is an improvement, and if so how much. > > > > I'll push more code to the branch, but the changes should be > gated behind > > a feature flag. > > > > On Tue, 18 Aug 2015, 'Scott Mansfield' via memcached wrote: > > > > > > > > No worries man, you're doing us a favor. Let me know if > there's anything you need from us, and I promise I'll be quicker this time :) > > > > > > On Aug 18, 2015 12:01 AM, "dormando" <dorm...@rydia.net> > wrote: > > > Hey, > > > > > > I'm still really interested in working on this. I'll be > taking a careful > > > look soon I hope. > > > > > > On Mon, 3 Aug 2015, Scott Mansfield wrote: > > > > > > > I've tweaked the program slightly, so I'm adding a > new version. It prints more stats as it goes and runs a bit faster. > > > > > > > > On Monday, August 3, 2015 at 1:20:37 AM UTC-7, Scott > Mansfield wrote: > > > > Total brain fart on my part. Apparently I had > memcached 1.4.13 on my path (who knows how...) Using the actual one that I've > built works. Sorry for the confusion... can't believe I didn't realize that > before. I'm testing against the compiled one now to see how it behaves. > > > > On Monday, August 3, 2015 at 1:15:06 AM UTC-7, > Dormando wrote: > > > > You sure that's 1.4.24? None of those > fail for me :( > > > > > > > > On Mon, 3 Aug 2015, Scott Mansfield wrote: > > > > > > > > > The command line I've used that will > start is: > > > > > > > > > > memcached -m 64 -o > slab_reassign,slab_automove > > > > > > > > > > > > > > > the ones that fail are: > > > > > > > > > > > > > > > memcached -m 64 -o > slab_reassign,slab_automove,lru_crawler,lru_maintainer > > > > > > > > > > memcached -o lru_crawler > > > > > > > > > > > > > > > I'm sure I've missed something during > compile, though I just used ./configure and make. > > > > > > > > > > > > > > > On Monday, August 3, 2015 at 12:22:33 > AM UTC-7, Scott Mansfield wrote: > > > > > I've attached a pretty simple > program to connect, fill a slab with data, and then fill another slab slowly > with data of a different size. I've been trying to get memcached to run with > the lru_crawler and lru_maintainer flags, but I get ' > > > > > > > > > > Illegal suboption "(null)"' every > time I try to start with either in any configuration. > > > > > > > > > > > > > > > I haven't seen it start to move > slabs automatically with a freshly installed 1.2.24. > > > > > > > > > > > > > > > On Tuesday, July 21, 2015 at > 4:55:17 PM UTC-7, Scott Mansfield wrote: > > > > > I realize I've not given > you the tests to reproduce the behavior. I should be able to soon. Sorry > about the delay here. > > > > > In the mean time, I wanted to bring up > a possible secondary use of the same logic to move items on slab rebalancing. > I think the system might benefit from using the same logic to crawl the pages > in a slab and compact the data in the background. In the case where we have > memory that is assigned to the slab but not > > being used > > > because > > > > of replaced > > > > > or TTL'd out data, returning the memory > to a pool of free memory will allow a slab to grow with that memory first > instead of waiting for an event where memory is needed at that instant. > > > > > > > > > > It's a change in approach, from > reactive to proactive. What do you think? > > > > > > > > > > On Monday, July 13, 2015 at 5:54:11 PM > UTC-7, Dormando wrote: > > > > > > First, more detail for you: > > > > > > > > > > > > We are running 1.4.24 in > production and haven't noticed any bugs as of yet. The new LRUs seem to be > working well, though we nearly always run memcached scaled to hold all data > without evictions. Those with evictions are behaving well. Those without > evictions haven't seen crashing or any other noticeable > bad > > behavior. > > > > > > > > > > Neat. > > > > > > > > > > > > > > > > > OK, I think I see an area where > I was speculating on functionality. If you have a key in slab 21 and then the > same key is written again at a larger size in slab 23 I assumed that the > space in 21 was not freed on the second write. With that assumption, the LRU > crawler would not free up that space. Also just > > by observation > > > in > > > > the > > > > > macro, the space is not freed > > > > > > fast enough to be effective, in > our use case, to accept the writes that are happening. Think in the hundreds > of millions of "overwrites" in a 6 - 10 hour period across a cluster. > > > > > > > > > > Internally, "items" (a key/value > pair) are generally immutable. The only > > > > > time when it's not is for > INCR/DECR, and it still becomes immutable if two > > > > > INCR/DECR's collide. > > > > > > > > > > What this means, is that the new > item is staged in a piece of free memory > > > > > while the "upload" stage of the > SET happens. When memcached has all of the > > > > > data in memory to replace the > item, it does an internal swap under a lock. > > > > > The old item is removed from the > hash table and LRU, and the new item gets > > > > > put in its place (at the head of > the LRU). > > > > > > > > > > Since items are refcounted, this > means that if other users are downloading > > > > > an item which just got replaced, > their memory doesn't get corrupted by the > > > > > item changing out from underneath > them. They can continue to read the old > > > > > item until they're done. When the > refcount reaches zero the old memory is > > > > > reclaimed. > > > > > > > > > > Most of the time, the item > replacement happens then the old memory is > > > > > immediately removed. > > > > > > > > > > However, this does mean that you > need *one* piece of free memory to > > > > > replace the old one. Then the old > memory gets freed after that set. > > > > > > > > > > So if you take a memcached > instance with 0 free chunks, and do a rolling > > > > > replacement of all items (within > the same slab class as before), the first > > > > > one would cause an eviction from > the tail of the LRU to get a free chunk. > > > > > Every SET after that would use > the chunk freed from the replacement of the > > > > > previous memory. > > > > > > > > > > > After that last sentence I > realized I also may not have explained well enough the access pattern. The > keys are all overwritten every day, but it takes some time to write them all > (obviously). We see a huge increase in the bytes metric as if the new data > for the old keys was being written for the first > time. > > Since the > > > "old" > > > > slab for > > > > > the same key doesn't > > > > > > proactively release memory, it > starts to fill up the cache and then start evicting data in the new slab. > Once that happens, we see evictions in the old slab because of the algorithm > you mentioned (random picking / freeing of memory). Typically we don't see > any use for "upgrading" an item as the new data > > would be entirely > > > > new and > > > > > should wholesale replace the > > > > > > old data for that key. More > specifically, the operation is always set, with different data each day. > > > > > > > > > > Right. Most of your problems will > come from two areas. One being that > > > > > writing data aggressively into > the new slab class (unless you set the > > > > > rebalancer to always-replace > mode), the mover will make memory available > > > > > more slowly than you can insert. > So you'll cause extra evictions in the > > > > > new slab class. > > > > > > > > > > The secondary problem is from the > random evictions in the previous slab > > > > > class as stuff is chucked on the > floor to make memory moveable. > > > > > > > > > > > As for testing, we'll be able > to put it under real production workload. I don't know what kind of data you > mean you need for testing. The data stored in the caches are highly > confidential. I can give you all kinds of metrics, since we collect most of > the ones that are in the stats and some from the stats > > slabs output. If > > > > you have > > > > > some specific ones that > > > > > > need collecting, I'll double > check and make sure we can get those. Alternatively, it might be most > beneficial to see the metrics in person :) > > > > > > > > > > I just need stats snapshots here > and there, and actually putting the thing > > > > > under load. When I did the LRU > work I had to beg for several months > > > > > before anyone tested it with a > production load. This slows things down and > > > > > demotivates me from working on > the project. > > > > > > > > > > Unfortunately my dayjob keeps me > pretty busy so ~internet~ would probably > > > > > be best. > > > > > > > > > > > I can create a driver program > to reproduce the behavior on a smaller scale. It would write e.g. 10k keys of > 10k size, then rewrite the same keys with different size data. I'll work on > that and post it to this thread when I can reproduce the behavior locally. > > > > > > > > > > Ok. There're slab rebalance unit > tests in the t/ directory which do things > > > > > like this, and I've used > mc-crusher to slam the rebalancer. It's pretty > > > > > easy to run one config to load up > 10k objects, then flip to the other > > > > > using the same key namespace. > > > > > > > > > > > Thanks, > > > > > > Scott > > > > > > > > > > > > On Saturday, July 11, 2015 at > 12:05:54 PM UTC-7, Dormando wrote: > > > > > > Hey, > > > > > > > > > > > > On Fri, 10 Jul 2015, > Scott Mansfield wrote: > > > > > > > > > > > > > We've seen issues > recently where we run a cluster that typically has the majority of items > overwritten in the same slab every day and a sudden change in data size > evicts a ton of data, affecting downstream systems. To be clear that is our > problem, but I think there's a tweak in memcached that might > > be useful and > > > > another > > > > > possible feature that > > > > > > would be even > > > > > > > better. > > > > > > > The data that is > written to this cache is overwritten every day, though the TTL is 7 days. One > slab takes up the majority of the space in the cache. The application wrote > e.g. 10KB (slab 21) every day for each key consistently. One day, a change > occurred where it started writing 15KB (slab 23), > > causing a migration > > > > of data > > > > > from one slab to > > > > > > another. We had -o > > > > > > > > slab_reassign,slab_automove=1 set on the server, causing large numbers of > evictions on the initial slab. Let's say the cache could hold the data at > 15KB per key, but the old data was not technically TTL'd out in it's old > slab. This means that memory was not being freed by the lru crawler thread (I > > think) because > > > its > > > > expiry > > > > > had not come > > > > > > around. > > > > > > > > > > > > > > lines 1199 and 1200 in > items.c: > > > > > > > if ((search->exptime != > 0 && search->exptime < current_time) || is_flushed(search)) { > > > > > > > > > > > > > > If there was a check to > see if this data was "orphaned," i.e. that the key, if accessed, would map to > a different slab than the current one, then these orphans could be reclaimed > as free memory. I am working on a patch to do this, though I have > reservations about performing a hash on the key on the > > lru crawler > > > > thread (if > > > > > the hash is not > > > > > > already available). > > > > > > > I have very little > experience in the memcached codebase so I don't know the most efficient way > to do this. Any help would be appreciated. > > > > > > > > > > > > There seems to be a > misconception about how the slab classes work. A key, > > > > > > if already existing in a > slab, will always map to the slab class it > > > > > > currently fits into. The > slab classes always exist, but the amount of > > > > > > memory reserved for each > of them will shift with the slab_reassign. ie: 10 > > > > > > pages in slab class 21, > then memory pressure on 23 causes it to move over. > > > > > > > > > > > > So if you examine a key > that still exists in slab class 21, it has no > > > > > > reason to move up or down > the slab classes. > > > > > > > > > > > > > Alternatively, and > possibly more beneficial is compaction of data in a slab using the same set > of criteria as lru crawling. Understandably, compaction is a very difficult > problem to solve since moving the data would be a pain in the ass. I saw a > couple of discussions about this in the mailing list, > > though I didn't > > > > see any > > > > > firm thoughts about > > > > > > it. I think it > > > > > > > can probably be done in > O(1) like the lru crawler by limiting the number of items it touches each > time. Writing and reading are doable in O(1) so moving should be as well. Has > anyone given more thought on compaction? > > > > > > > > > > > > I'd be interested in > hacking this up for you folks if you can provide me > > > > > > testing and some data to > work with. With all of the LRU work I did in > > > > > > 1.4.24, the next things I > wanted to do is a big improvement on the slab > > > > > > reassignment code. > > > > > > > > > > > > Currently it picks > essentially a random slab page, empties it, and moves > > > > > > the slab page into the > class under pressure. > > > > > > > > > > > > One thing we can do is > first examine for free memory in the existing slab, > > > > > > IE: > > > > > > > > > > > > - Take a page from slab 21 > > > > > > - Scan the page for valid > items which need to be moved > > > > > > - Pull free memory from > slab 21, migrate the item (moderately complicated) > > > > > > - When the page is empty, > move it (or give up if you run out of free > > > > > > chunks). > > > > > > > > > > > > The next step is to pull > from the LRU on slab 21: > > > > > > > > > > > > - Take page from slab 21 > > > > > > - Scan page for valid > items > > > > > > - Pull free memory from > slab 21, migrate the item > > > > > > - If no memory free, > evict tail of slab 21. use that chunk. > > > > > > - When the page is empty, > move it. > > > > > > > > > > > > Then, when you hit this > condition your least-recently-used data gets > > > > > > culled as new data > migrates your page class. This should match a natural > > > > > > occurrance if you would > already be evicting valid (but old) items to make > > > > > > room for new items. > > > > > > > > > > > > A bonus to using the free > memory trick, is that I can use the amount of > > > > > > free space in a slab > class as a heuristic to more quickly move slab pages > > > > > > around. > > > > > > > > > > > > If it's still necessary > from there, we can explore "upgrading" items to a > > > > > > new slab class, but that > is much much more complicated since the item has > > > > > > to shift LRU's. Do you > put it at the head, the tail, the middle, etc? It > > > > > > might be impossible to > make a good generic decision there. > > > > > > > > > > > > What version are you > currently on? If 1.4.24, have you seen any > > > > > > instability? I'm > currently torn between fighting a few bugs and start on > > > > > > improving the slab > rebalancer. > > > > > > > > > > > > -Dormando > > > > > > > > > > > > > > > > > > On Saturday, July 11, 2015 at > 12:05:54 PM UTC-7, Dormando wrote: > > > > > > Hey, > > > > > > > > > > > > On Fri, 10 Jul 2015, > Scott Mansfield wrote: > > > > > > > > > > > > > We've seen issues > recently where we run a cluster that typically has the majority of items > overwritten in the same slab every day and a sudden change in data size > evicts a ton of data, affecting downstream systems. To be clear that is our > problem, but I think there's a tweak in memcached that might > > be useful and > > > > another > > > > > possible feature that > > > > > > would be even > > > > > > > better. > > > > > > > The data that is > written to this cache is overwritten every day, though the TTL is 7 days. One > slab takes up the majority of the space in the cache. The application wrote > e.g. 10KB (slab 21) every day for each key consistently. One day, a change > occurred where it started writing 15KB (slab 23), > > causing a migration > > > > of data > > > > > from one slab to > > > > > > another. We had -o > > > > > > > > slab_reassign,slab_automove=1 set on the server, causing large numbers of > evictions on the initial slab. Let's say the cache could hold the data at > 15KB per key, but the old data was not technically TTL'd out in it's old > slab. This means that memory was not being freed by the lru crawler thread (I > > think) because > > > its > > > > expiry > > > > > had not come > > > > > > around. > > > > > > > > > > > > > > lines 1199 and 1200 in > items.c: > > > > > > > if ((search->exptime != > 0 && search->exptime < current_time) || is_flushed(search)) { > > > > > > > > > > > > > > If there was a check to > see if this data was "orphaned," i.e. that the key, if accessed, would map to > a different slab than the current one, then these orphans could be reclaimed > as free memory. I am working on a patch to do this, though I have > reservations about performing a hash on the key on the > > lru crawler > > > > thread (if > > > > > the hash is not > > > > > > already available). > > > > > > > I have very little > experience in the memcached codebase so I don't know the most efficient way > to do this. Any help would be appreciated. > > > > > > > > > > > > There seems to be a > misconception about how the slab classes work. A key, > > > > > > if already existing in a > slab, will always map to the slab class it > > > > > > currently fits into. The > slab classes always exist, but the amount of > > > > > > memory reserved for each > of them will shift with the slab_reassign. ie: 10 > > > > > > pages in slab class 21, > then memory pressure on 23 causes it to move over. > > > > > > > > > > > > So if you examine a key > that still exists in slab class 21, it has no > > > > > > reason to move up or down > the slab classes. > > > > > > > > > > > > > Alternatively, and > possibly more beneficial is compaction of data in a slab using the same set > of criteria as lru crawling. Understandably, compaction is a very difficult > problem to solve since moving the data would be a pain in the ass. I saw a > couple of discussions about this in the mailing list, > > though I didn't > > > > see any > > > > > firm thoughts about > > > > > > it. I think it > > > > > > > can probably be done in > O(1) like the lru crawler by limiting the number of items it touches each > time. Writing and reading are doable in O(1) so moving should be as well. Has > anyone given more thought on compaction? > > > > > > > > > > > > I'd be interested in > hacking this up for you folks if you can provide me > > > > > > testing and some data to > work with. With all of the LRU work I did in > > > > > > 1.4.24, the next things I > wanted to do is a big improvement on the slab > > > > > > reassignment code. > > > > > > > > > > > > Currently it picks > essentially a random slab page, empties it, and moves > > > > > > the slab page into the > class under pressure. > > > > > > > > > > > > One thing we can do is > first examine for free memory in the existing slab, > > > > > > IE: > > > > > > > > > > > > - Take a page from slab 21 > > > > > > - Scan the page for valid > items which need to be moved > > > > > > - Pull free memory from > slab 21, migrate the item (moderately complicated) > > > > > > - When the page is empty, > move it (or give up if you run out of free > > > > > > chunks). > > > > > > > > > > > > The next step is to pull > from the LRU on slab 21: > > > > > > > > > > > > - Take page from slab 21 > > > > > > - Scan page for valid > items > > > > > > - Pull free memory from > slab 21, migrate the item > > > > > > - If no memory free, > evict tail of slab 21. use that chunk. > > > > > > - When the page is empty, > move it. > > > > > > > > > > > > Then, when you hit this > condition your least-recently-used data gets > > > > > > culled as new data > migrates your page class. This should match a natural > > > > > > occurrance if you would > already be evicting valid (but old) items to make > > > > > > room for new items. > > > > > > > > > > > > A bonus to using the free > memory trick, is that I can use the amount of > > > > > > free space in a slab > class as a heuristic to more quickly move slab pages > > > > > > around. > > > > > > > > > > > > If it's still necessary > from there, we can explore "upgrading" items to a > > > > > > new slab class, but that > is much much more complicated since the item has > > > > > > to shift LRU's. Do you > put it at the head, the tail, the middle, etc? It > > > > > > might be impossible to > make a good generic decision there. > > > > > > > > > > > > What version are you > currently on? If 1.4.24, have you seen any > > > > > > instability? I'm > currently torn between fighting a few bugs and start on > > > > > > improving the slab > rebalancer. > > > > > > > > > > > > -Dormando > > > > > > > > > > > > -- > > > > > > > > > > > > --- > > > > > > You received this message > because you are subscribed to the Google Groups "memcached" group. > > > > > > To unsubscribe from this group > and stop receiving emails from it, send an email to > memcached+...@googlegroups.com. > > > > > > For more options, visit > https://groups.google.com/d/optout. > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > --- > > > > > You received this message because you > are subscribed to the Google Groups "memcached" group. > > > > > To unsubscribe from this group and stop > receiving emails from it, send an email to memcached+...@googlegroups.com. > > > > > For more options, visit > https://groups.google.com/d/optout. > > > > > > > > > > > > > > > > > > -- > > > > > > > > --- > > > > You received this message because you are subscribed > to the Google Groups "memcached" group. > > > > To unsubscribe from this group and stop receiving > emails from it, send an email to memcached+...@googlegroups.com. > > > > For more options, visit > https://groups.google.com/d/optout. > > > > > > > > > > > > > > -- > > > > > > --- > > > You received this message because you are subscribed to the > Google Groups "memcached" group. > > > To unsubscribe from this group and stop receiving emails from > it, send an email to memcached+...@googlegroups.com. > > > For more options, visit https://groups.google.com/d/optout. > > > > > > > > > > > > -- > > > > --- > > You received this message because you are subscribed to the Google > Groups "memcached" group. > > To unsubscribe from this group and stop receiving emails from it, > send an email to memcached+...@googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. > > > > > > -- > > --- > You received this message because you are subscribed to the Google Groups > "memcached" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to memcached+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > >
