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.
