On Mon, Nov 8, 2010 at 17:18, Adam Kocoloski <[email protected]> wrote: > On Nov 8, 2010, at 3:28 PM, Randall Leeds wrote: > >> On Mon, Nov 8, 2010 at 12:22, Paul Davis <[email protected]> wrote: >>> On Mon, Nov 8, 2010 at 3:04 PM, Randall Leeds <[email protected]> >>> wrote: >>>> Whoops. Hit send too early, but I think I got everything in there that >>>> I wanted to say. >>>> >>>> As for the ref counter bottleneck, I just pushed to >>>> https://github.com/tilgovi/couchdb/tree/ets_ref_count >>>> This branch uses a public ets for the ref_counter. I think I managed >>>> to linear the updates over the {total, RefCtr} keys in the ets table >>>> such that there should be no race conditions but please, please take a >>>> look at this if you have time. >>>> >>>> It seems to pass the ref_counter tests, but I still need to handle >>>> giving away ownership of the ets table. Right now I use couch_server >>>> as the heir so I can use only one ETS table for all couch_ref_counter >>>> processes, but the couch_server just crashes if it actually receives >>>> the 'ETS-TRANSFER' message. If I can't find an easy way to hand the >>>> table to another couch_ref_counter whenever the owner exits I may just >>>> break the encapsulation of the module a bit by leaving couch_server as >>>> the owner and ignoring that message. >>>> >>>> Thanks, guys. My gut says we're going to get some nice numbers when >>>> all this is done. >>>> >>>> -Randall >>>> >>>> On Mon, Nov 8, 2010 at 11:56, Randall Leeds <[email protected]> >>>> wrote: >>>>> Thanks to both of you for getting this conversation going again and >>>>> for the work on the patch and testing, Filipe. >>>>> >>>>> On Sun, Nov 7, 2010 at 12:49, Adam Kocoloski <[email protected]> wrote: >>>>>> On Nov 7, 2010, at 3:29 PM, Filipe David Manana wrote: >>>>>> >>>>>>> On Sun, Nov 7, 2010 at 8:09 PM, Adam Kocoloski <[email protected]> >>>>>>> wrote: >>>>>>>> On Nov 7, 2010, at 2:52 PM, Filipe David Manana wrote: >>>>>>>> >>>>>>>>> On Sun, Nov 7, 2010 at 7:20 PM, Adam Kocoloski <[email protected]> >>>>>>>>> wrote: >>>>>>>>>> On Nov 7, 2010, at 11:35 AM, Filipe David Manana wrote: >>>>>>>>>> >>>>>>>>>>> Also, with this patch I verified (on Solaris, with the 'zpool iostat >>>>>>>>>>> 1' command) that when running a writes only test with relaximation >>>>>>>>>>> (200 write processes), disk write activity is not continuous. >>>>>>>>>>> Without >>>>>>>>>>> this patch, there's continuous (every 1 second) write activity. >>>>>>>>>> >>>>>>>>>> I'm confused by this statement. You must be talking about >>>>>>>>>> relaximation runs with delayed_commits = true, right? Why do you >>>>>>>>>> think you see larger intervals between write activity with the >>>>>>>>>> optimization from COUCHDB-767? Have you measured the time it takes >>>>>>>>>> to open the extra FD? In my tests that was a sub-millisecond >>>>>>>>>> operation, but maybe you've uncovered something else. >>>>>>>>> >>>>>>>>> No, it happens for tests with delayed_commits = false. The only >>>>>>>>> possible explanation I see for the variance might be related to the >>>>>>>>> Erlang VM scheduler decisions about when to start/run that process. >>>>>>>>> Nevertheless, I dont know the exact cause, but the fsync run frequency >>>>>>>>> varies a lot. >>>>>>>> >>>>>>>> I think it's worth investigating. I couldn't reproduce it on my >>>>>>>> plain-old spinning disk MacBook with 200 writers in relaximation; the >>>>>>>> IOPS reported by iostat stayed very uniform. >>>>>>>> >>>>>>>>>>> For the goal of not having readers getting blocked by fsync calls >>>>>>>>>>> (and >>>>>>>>>>> write calls), I would propose using a separate couch_file process >>>>>>>>>>> just >>>>>>>>>>> for read operations. I have a branch in my github for this (with >>>>>>>>>>> COUCHDB-767 reverted). It needs to be polished, but the relaximation >>>>>>>>>>> tests are very positive, both reads and writes get better response >>>>>>>>>>> times and throughput: >>>>>>>>>>> >>>>>>>>>>> https://github.com/fdmanana/couchdb/tree/2_couch_files_no_batch_reads >>>>>>>>>> >>>>>>>>>> I'd like to propose an alternative optimization, which is to keep a >>>>>>>>>> dedicated file descriptor open in the couch_db_updater process and >>>>>>>>>> use that file descriptor for _all_ IO initiated by the db_updater. >>>>>>>>>> The advantage is that the db_updater does not need to do any message >>>>>>>>>> passing for disk IO, and thus does not slow down when the incoming >>>>>>>>>> message queue is large. A message queue much much larger than the >>>>>>>>>> number of concurrent writers can occur if a user writes with >>>>>>>>>> batch=ok, and it can also happen rather easily in a BigCouch cluster. >>>>>>>>> >>>>>>>>> I don't see how that will improve things, since all write operations >>>>>>>>> will still be done in a serialized manner. Since only couch_db_updater >>>>>>>>> writes to the DB file, and since access to the couch_db_updater is >>>>>>>>> serialized, to me it only seems that you're solution avoids one level >>>>>>>>> of indirection (the couch_file process). I don't see how, when using a >>>>>>>>> couch_file only for writes, you get the message queue for that >>>>>>>>> couc_file process full of write messages. >>>>>>>> >>>>>>>> It's the db_updater which gets a large message queue, not the >>>>>>>> couch_file. The db_updater ends up with a big backlog of update_docs >>>>>>>> messages that get in the way when it needs to make gen_server calls to >>>>>>>> the couch_file process for IO. It's a significant problem in R13B, >>>>>>>> probably less so in R14B because of some cool optimizations by the OTP >>>>>>>> team. >>>>>>> >>>>>>> So, let me see if I get it. The couch_db_updater process is slow >>>>>>> picking the results of the calls to the couch_file process because its >>>>>>> mailbox is full of update_docs messages? >>>>>> >>>>>> Correct. Each call to the couch_file requires a selective receive on >>>>>> the part of the db_updater in order to get the response, and prior to >>>>>> R14 that selective receive needed to match against every message in the >>>>>> mailbox. It's really a bigger problem in couch_server, which uses a >>>>>> gen_server call to increment a reference counter before handing the >>>>>> #db{} to the client, since every request to any DB has to talk to >>>>>> couch_server first. Best, >>>>>> >>>>>> Adam >>>>> >>>>> Adam, >>>>> I think the problem is made worse by a backed up db_updater, but the >>>>> db_updater becomes backed up because it makes more synchronous calls >>>>> to the couch_file than a reader does, handling only one update >>>>> operation at a time while readers queue up on the couch_file in >>>>> parallel. >>>>> >>>>> Filipe, >>>>> Using a separate fd for writes at the couch_file level is not the >>>>> answer. The db_updater has to read the btree before it can write, >>>>> incurring multiple trips through the couch_file message queue between >>>>> queuing append_term requests and processing its message queue for new >>>>> updates. Using two file descriptors keeps the readers out of the way >>>>> of the writers only if you select which fd to use at the db-operation >>>>> level and not the file-operation level. Perhaps two couch_file >>>>> processes is better. Fairness should be left to the operating system >>>>> I/O scheduler once reads don'. This seems seems like the best way >>>>> forward to me right now. Let's try to crunch some numbers on it soon. >>>>> >>>>> I couldn't find a solution I liked that was fair to readers and >>>>> writers at any workload with only one file descriptor. The btree cache >>>>> alleviates this problem a bit because the read path becomes much >>>>> faster and therefore improves database reads and writes. >>>>> >>>>> As to the patch, I'd think we need the readers and writers separated >>>>> into two separate couch_files. That way the updater can perform its >>>>> reads on the "writer" fd, otherwise writers suffer starvation because >>>>> readers go directly into the couch_file queue in parallel instead of >>>>> serializing through something like db_updater. >>>>> >>>> >>> >>> Wasn't there a branch or patch somehwere that just removed the >>> ref_counter code entirely and used monitors/links to make sure >>> everything behaved correctly? I'm not sure I ever saw it to see how >>> dramatic and/or scary it was, but it might be another approach to >>> consider. >>> >> >> Adam should chime in. I think BigCouch got rid of the ref counter in >> favor of something else, but last I asked him about it he said there >> might be a small edge case race condition. How critical that is I >> can't say. It may be that that edge case is tolerable and recoverable. > > BigCouch uses a protected couch_dbs ets table to store all open #dbs and a > public couch_lru ets table for LRU updates. Reference counting is > accomplished by having clients monitor and demonitor the couch_file. The > least recently used database is determined by folding over the couch_lru > table. The race condition Randall is referring to is the following: > > 1) client process grabs a #db{} record from the couch_dbs ets table > 2) couch_server calculates that the LRU DB is the one the client just looked > up > 3) couch_server checks the monitored_by field for the #db.fd process and > finds no clients are monitoring it > 4) couch_server kills the DB > 5) client updates the LRU time (too late) > 6) client monitors the #db.fd, but it's already dead > > Practically speaking, it's been a non-issue, but it does exist. For what > it's worth, a similar race condition existed in older (0.10.x and below) > versions of CouchDB, where the client would increment the ref counter after > receiving the DB from couch_server. The current implementation in CouchDB > avoids the race condition by having couch_server increment the ref counter on > behalf of the client, but that approach has serious performance and stability > implications for BigCouch. > > I'll try to take a look at Randall's github branch tonight. Using monitors > for reference counting feels really right to me, but as it's not possible to > monitor a process on behalf of someone else I'm not sure it's possible to > avoid the race that I described with monitors. Regards, > > Adam
That seems like a pretty easy thing to detect (monitor fails) and handle gracefully, but check over my code and let me know what you all want to do. I just pushed some changes to squash out the last little edge problems with it. I added one extra test case to see that it cleans itself up properly and now make check and futon both pass. I should be around on IRC most of the night (GMT -8) so ping me if something doesn't make sense. Cheers, Randall
