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. >
