From dev-return-13133-apmail-couchdb-dev-archive=couchdb.apache.org@couchdb.apache.org Mon Nov 08 20:28:40 2010 Return-Path: Delivered-To: apmail-couchdb-dev-archive@www.apache.org Received: (qmail 58307 invoked from network); 8 Nov 2010 20:28:40 -0000 Received: from unknown (HELO mail.apache.org) (140.211.11.3) by 140.211.11.9 with SMTP; 8 Nov 2010 20:28:40 -0000 Received: (qmail 37385 invoked by uid 500); 8 Nov 2010 20:29:11 -0000 Delivered-To: apmail-couchdb-dev-archive@couchdb.apache.org Received: (qmail 37346 invoked by uid 500); 8 Nov 2010 20:29:11 -0000 Mailing-List: contact dev-help@couchdb.apache.org; run by ezmlm Precedence: bulk List-Help: List-Unsubscribe: List-Post: List-Id: Reply-To: dev@couchdb.apache.org Delivered-To: mailing list dev@couchdb.apache.org Received: (qmail 37338 invoked by uid 99); 8 Nov 2010 20:29:11 -0000 Received: from nike.apache.org (HELO nike.apache.org) (192.87.106.230) by apache.org (qpsmtpd/0.29) with ESMTP; Mon, 08 Nov 2010 20:29:11 +0000 X-ASF-Spam-Status: No, hits=0.0 required=10.0 tests=FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,SPF_PASS,T_TO_NO_BRKTS_FREEMAIL X-Spam-Check-By: apache.org Received-SPF: pass (nike.apache.org: domain of randall.leeds@gmail.com designates 209.85.161.52 as permitted sender) Received: from [209.85.161.52] (HELO mail-fx0-f52.google.com) (209.85.161.52) by apache.org (qpsmtpd/0.29) with ESMTP; Mon, 08 Nov 2010 20:29:04 +0000 Received: by fxm8 with SMTP id 8so356981fxm.11 for ; Mon, 08 Nov 2010 12:28:44 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:mime-version:received:received:in-reply-to :references:date:message-id:subject:from:to:content-type :content-transfer-encoding; bh=NPC4U8d+WyQ/8Zd8DrhQW7G+BZQKqMiqt8JHKWH9sBs=; b=JuueqijCFDyo97f/1heDfWjA5iR0ilyb/Cwv0+C859PRqMEBlGaqzOdz/F++0yQKBS NwjpLVWCzN5kXhNg6zwHO99Yp3tAd94k0lK+myczsZz5npLUuvHRM013JI3VnmkQEvv2 d/Ls+IxRHn4nCHs33e6r9CW7pLRnvoQRu2jp8= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type:content-transfer-encoding; b=rIPif1MIh+g0uyC8XfuQcaahC+RHJ2PuaVPW65KuXkDYIZ/mf21WCwUpEUHrU7O/v8 Oc6bsGnrhBfRjL3yy1T9jf5IpfF2cFWoM7CGtm/VfgritQY2QNsyUzBu1BpuRwG8H7gr fCwLru97fv8RKqTp/RA7LHDB7c9wbfGxkja+c= MIME-Version: 1.0 Received: by 10.223.122.133 with SMTP id l5mr4231517far.52.1289248124462; Mon, 08 Nov 2010 12:28:44 -0800 (PST) Received: by 10.223.79.13 with HTTP; Mon, 8 Nov 2010 12:28:44 -0800 (PST) In-Reply-To: References: <48F6147B-1B71-4712-A3D6-C2781D762D60@apache.org> Date: Mon, 8 Nov 2010 12:28:44 -0800 Message-ID: Subject: Re: About possibly reverting COUCHDB-767 From: Randall Leeds To: dev@couchdb.apache.org Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Virus-Checked: Checked by ClamAV on apache.org On Mon, Nov 8, 2010 at 12:22, Paul Davis wrot= e: > On Mon, Nov 8, 2010 at 3:04 PM, Randall Leeds w= rote: >> 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 wr= ote: >>> 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 wrot= e: >>>> On Nov 7, 2010, at 3:29 PM, Filipe David Manana wrote: >>>> >>>>> On Sun, Nov 7, 2010 at 8:09 PM, Adam Kocoloski = wrote: >>>>>> On Nov 7, 2010, at 2:52 PM, Filipe David Manana wrote: >>>>>> >>>>>>> On Sun, Nov 7, 2010 at 7:20 PM, Adam Kocoloski wrote: >>>>>>>> On Nov 7, 2010, at 11:35 AM, Filipe David Manana wrote: >>>>>>>> >>>>>>>>> Also, with this patch I verified (on Solaris, with the 'zpool ios= tat >>>>>>>>> 1' command) that when running a writes only test with relaximatio= n >>>>>>>>> (200 write processes), disk write activity is not continuous. Wit= hout >>>>>>>>> this patch, there's continuous (every 1 second) write activity. >>>>>>>> >>>>>>>> I'm confused by this statement. You must be talking about relaxima= tion runs with delayed_commits =3D true, right? =C2=A0Why do you think you = see larger intervals between write activity with the optimization from COUC= HDB-767? =C2=A0Have you measured the time it takes to open the extra FD? = =C2=A0In my tests that was a sub-millisecond operation, but maybe you've un= covered something else. >>>>>>> >>>>>>> No, it happens for tests with delayed_commits =3D 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 freque= ncy >>>>>>> varies a lot. >>>>>> >>>>>> I think it's worth investigating. =C2=A0I couldn't reproduce it on m= y plain-old spinning disk MacBook with 200 writers in relaximation; the IOP= S 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 relaximat= ion >>>>>>>>> tests are very positive, both reads and writes get better respons= e >>>>>>>>> times and throughput: >>>>>>>>> >>>>>>>>> https://github.com/fdmanana/couchdb/tree/2_couch_files_no_batch_r= eads >>>>>>>> >>>>>>>> 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 th= at file descriptor for _all_ IO initiated by the db_updater. =C2=A0The adva= ntage is that the db_updater does not need to do any message passing for di= sk IO, and thus does not slow down when the incoming message queue is large= . =C2=A0A message queue much much larger than the number of concurrent writ= ers can occur if a user writes with batch=3Dok, and it can also happen rath= er easily in a BigCouch cluster. >>>>>>> >>>>>>> I don't see how that will improve things, since all write operation= s >>>>>>> will still be done in a serialized manner. Since only couch_db_upda= ter >>>>>>> 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 lev= el >>>>>>> of indirection (the couch_file process). I don't see how, when usin= g 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. =C2=A0The db_updater ends up with a big backlog of update_docs messag= es that get in the way when it needs to make gen_server calls to the couch_= file process for IO. =C2=A0It's a significant problem in R13B, probably les= s 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 it= s >>>>> mailbox is full of update_docs messages? >>>> >>>> Correct. =C2=A0Each call to the couch_file requires a selective receiv= e 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 mai= lbox. =C2=A0It's really a bigger problem in couch_server, which uses a gen_= server call to increment a reference counter before handing the #db{} to th= e client, since every request to any DB has to talk to couch_server first. = =C2=A0Best, >>>> >>>> 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.