If you look at the new branch there's a commit explaining the new stats. You can watch slab_reassing_evictions vs slab_reassign_saves. you can also test automove=1 vs automove=2 (please also turn on the lru_maintainer and lru_crawler).
The initial branch you were running didn't add any new stats. It just restored an old feature. On Tue, 29 Sep 2015, Scott Mansfield wrote: > An unrelated prod problem meant I had to stop after about an hour. I'm > turning it on again tomorrow morning. > Are there any new metrics I should be looking at? Anything new in the stats > output? I'm about to take a look at the diffs as well. > > On Tuesday, September 29, 2015 at 12:37:45 PM UTC-7, Dormando wrote: > 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+...@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. > >
