From: Bruce Momjian Date: Tue, 18 Mar 2003 01:36:01 +0000 (+0000) Subject: Add to mmap discussion. X-Git-Tag: REL7_4_BETA1~921 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=6fdd71c133361a3caae7530b2a14ddd245dd3aa8;p=postgresql Add to mmap discussion. --- diff --git a/doc/TODO.detail/mmap b/doc/TODO.detail/mmap index b2eac95ddd..58a549ef58 100644 --- a/doc/TODO.detail/mmap +++ b/doc/TODO.detail/mmap @@ -2014,3 +2014,395 @@ KwvG7YLsJ+xpsTUS67KD+4M= --HjNkcEWJ4DMx36DP-- +From pgsql-performance-owner+M1354=pgman=candle.pha.pa.us@postgresql.org Fri Mar 7 01:09:07 2003 +Return-path: +Received: from relay2.pgsql.com (relay2.pgsql.com [64.49.215.143]) + by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h27693604295 + for ; Fri, 7 Mar 2003 01:09:05 -0500 (EST) +Received: from postgresql.org (postgresql.org [64.49.215.8]) + by relay2.pgsql.com (Postfix) with ESMTP id 95CD2EDFD3B + for ; Fri, 7 Mar 2003 01:09:03 -0500 (EST) +X-Original-To: pgsql-performance@postgresql.org +Received: from perrin.int.nxad.com (internal.ext.nxad.com [69.1.70.251]) + by postgresql.org (Postfix) with ESMTP id F16034768E2 + for ; Fri, 7 Mar 2003 01:04:33 -0500 (EST) +Received: by perrin.int.nxad.com (Postfix, from userid 1001) + id 7969A21065; Thu, 6 Mar 2003 22:04:12 -0800 (PST) +Date: Thu, 6 Mar 2003 22:04:12 -0800 +From: Sean Chittenden +To: Neil Conway +cc: Tom Lane , + Christopher Kings-Lynne , + PostgreSQL Performance +Subject: Re: [PERFORM] [COMMITTERS] pgsql-server/ /configure /configure.in rc/incl ... +Message-ID: <20030307060412.GA19138@perrin.int.nxad.com> +References: <20030306031656.1876F4762E0@postgresql.org> <032f01c2e390$b1842b20$6500a8c0@fhp.internal> <11077.1046921667@sss.pgh.pa.us> <033f01c2e392$71476570$6500a8c0@fhp.internal> <12228.1046922471@sss.pgh.pa.us> <20030306094117.GA79234@perrin.int.nxad.com> <15071.1046964336@sss.pgh.pa.us> <20030307003640.GF79234@perrin.int.nxad.com> <1046998072.10527.67.camel@tokyo> +MIME-Version: 1.0 +Content-Type: multipart/signed; micalg=pgp-sha1; + protocol="application/pgp-signature"; boundary="KsGdsel6WgEHnImy" +Content-Disposition: inline +In-Reply-To: <1046998072.10527.67.camel@tokyo> +User-Agent: Mutt/1.4i +X-PGP-Key: finger seanc@FreeBSD.org +X-PGP-Fingerprint: 3849 3760 1AFE 7B17 11A0 83A6 DD99 E31F BC84 B341 +X-Web-Homepage: http://sean.chittenden.org/ +Precedence: bulk +Sender: pgsql-performance-owner@postgresql.org +Status: OR + +--KsGdsel6WgEHnImy +Content-Type: text/plain; charset=us-ascii +Content-Disposition: inline +Content-Transfer-Encoding: quoted-printable + +> > I don't have my copy of Steven's handy (it's some 700mi away atm +> > otherwise I'd cite it), but if Tom or someone else has it handy, look +> > up the example re: the performance gain from read()'ing an mmap()'ed +> > file versus a non-mmap()'ed file. The difference is non-trivial and +> > _WELL_ worth the time given the speed increase. +>=20 +> Can anyone confirm this? If so, one easy step we could take in this +> direction would be adapting COPY FROM to use mmap(). + +Weeee! Alright, so I got to have some fun writing out some simple +tests with mmap() and friends tonight. Are the results interesting? +Absolutely! Is this a simple benchmark? Yup. Do I think it +simulates PostgreSQL? Eh, not particularly. Does it demonstrate that +mmap() is a win and something worth implementing? I sure hope so. Is +this a test program to demonstrate the ideal use of mmap() in +PostgreSQL? No. Is it a place to start a factual discussion? I hope +so. + +I have here four tests that are conditionalized by cpp. + +# The first one uses read() and write() but with the buffer size set +# to the same size as the file. +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -o test-= +mmap test-mmap.c +/usr/bin/time ./test-mmap > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is the same as the file size +Number of iterations: 100000 +Start time: 1047013002.412516 +Time: 82.88178 + +Completed tests + 82.09 real 2.13 user 68.98 sys + +# The second one uses read() and write() with the default buffer size: +# 65536 +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL= +T_READSIZE=3D1 -o test-mmap test-mmap.c +/usr/bin/time ./test-mmap > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is default read size: 65536 +Number of iterations: 100000 +Start time: 1047013085.16204 +Time: 18.155511 + +Completed tests + 18.16 real 0.90 user 14.79 sys +# Please note this is significantly faster, but that's expected + +# The third test uses mmap() + madvise() + write() +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL= +T_READSIZE=3D1 -DDO_MMAP=3D1 -o test-mmap test-mmap.c +/usr/bin/time ./test-mmap > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is the same as the file size +Number of iterations: 100000 +Start time: 1047013103.859818 +Time: 8.4294203644 + +Completed tests + 7.24 real 0.41 user 5.92 sys +# Faster still, and twice as fast as the normal read() case + +# The last test only calls mmap()'s once when the file is opened and +# only msync()'s, munmap()'s, close()'s the file once at exit. +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL= +T_READSIZE=3D1 -DDO_MMAP=3D1 -DDO_MMAP_ONCE=3D1 -o test-mmap test-mmap.c +/usr/bin/time ./test-mmap > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is the same as the file size +Number of iterations: 100000 +Start time: 1047013111.623712 +Time: 1.174076 + +Completed tests + 1.18 real 0.09 user 0.92 sys +# Substantially faster + + +Obviously this isn't perfect, but reading and writing data is faster +(specifically moving pages through the VM/OS). Doing partial writes +from mmap()'ed data should be faster along with scanning through +mmap()'ed portions of - or completely mmap()'ed - files because the +pages are already loaded in the VM. PostgreSQL's LRU file descriptor +cache could easily be adjusted to add mmap()'ing of frequently +accessed files (specifically, system catalogs come to mind). It's not +hard to figure out how often particular files are accessed and to +either _avoid_ mmap()'ing a file that isn't accessed often, or to +mmap() files that _are_ accessed often. mmap() does have a cost, but +I'd wager that mmap()'ing the same file a second or third time from a +different process would be more efficient. The speedup of searching +through an mmap()'ed file may be worth it, however, to mmap() all +files if the system is under a tunable resource limit +(max_mmaped_bytes?). + +If someone is so inclined or there's enough interest, I can reverse +this test case so that data is written to an mmap()'ed file, but the +same performance difference should hold true (assuming this isn't a +write to a tape drive ::grin::). + +The URL for the program used to generate the above tests is at: + +http://people.freebsd.org/~seanc/mmap_test/ + + +Please ask if you have questions. -sc + +--=20 +Sean Chittenden + +--KsGdsel6WgEHnImy +Content-Type: application/pgp-signature +Content-Disposition: inline + +-----BEGIN PGP SIGNATURE----- +Comment: Sean Chittenden + +iD8DBQE+aDZc3ZnjH7yEs0ERAid6AJ9/TAYMUx2+ZcD2680OlKJBj5FzrACgquIG +PBNCzM0OegBXrPROJ/uIKDM= +=y7O6 +-----END PGP SIGNATURE----- + +--KsGdsel6WgEHnImy-- + +From pgsql-performance-owner+M1358=pgman=candle.pha.pa.us@postgresql.org Fri Mar 7 16:47:38 2003 +Return-path: +Received: from relay2.pgsql.com (relay2.pgsql.com [64.49.215.143]) + by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id h27LlX429809 + for ; Fri, 7 Mar 2003 16:47:35 -0500 (EST) +Received: from postgresql.org (postgresql.org [64.49.215.8]) + by relay2.pgsql.com (Postfix) with ESMTP id D40CBEDFE05 + for ; Fri, 7 Mar 2003 16:47:32 -0500 (EST) +X-Original-To: pgsql-performance@postgresql.org +Received: from perrin.int.nxad.com (internal.ext.nxad.com [69.1.70.251]) + by postgresql.org (Postfix) with ESMTP id 913B5474E44 + for ; Fri, 7 Mar 2003 16:46:50 -0500 (EST) +Received: by perrin.int.nxad.com (Postfix, from userid 1001) + id A55392105B; Fri, 7 Mar 2003 13:46:30 -0800 (PST) +Date: Fri, 7 Mar 2003 13:46:30 -0800 +From: Sean Chittenden +To: Tom Lane +cc: Neil Conway , + Christopher Kings-Lynne , + PostgreSQL Performance +Subject: Re: [PERFORM] [COMMITTERS] pgsql-server/ /configure /configure.in rc/incl ... +Message-ID: <20030307214630.GI79234@perrin.int.nxad.com> +References: <032f01c2e390$b1842b20$6500a8c0@fhp.internal> <11077.1046921667@sss.pgh.pa.us> <033f01c2e392$71476570$6500a8c0@fhp.internal> <12228.1046922471@sss.pgh.pa.us> <20030306094117.GA79234@perrin.int.nxad.com> <15071.1046964336@sss.pgh.pa.us> <20030307003640.GF79234@perrin.int.nxad.com> <1046998072.10527.67.camel@tokyo> <20030307060412.GA19138@perrin.int.nxad.com> <29933.1047047386@sss.pgh.pa.us> +MIME-Version: 1.0 +Content-Type: multipart/signed; micalg=pgp-sha1; + protocol="application/pgp-signature"; boundary="TALVG7vV++YnpwZG" +Content-Disposition: inline +In-Reply-To: <29933.1047047386@sss.pgh.pa.us> +User-Agent: Mutt/1.4i +X-PGP-Key: finger seanc@FreeBSD.org +X-PGP-Fingerprint: 3849 3760 1AFE 7B17 11A0 83A6 DD99 E31F BC84 B341 +X-Web-Homepage: http://sean.chittenden.org/ +Precedence: bulk +Sender: pgsql-performance-owner@postgresql.org +Status: OR + +--TALVG7vV++YnpwZG +Content-Type: text/plain; charset=us-ascii +Content-Disposition: inline +Content-Transfer-Encoding: quoted-printable + +> > Absolutely! Is this a simple benchmark? Yup. Do I think it +> > simulates PostgreSQL? Eh, not particularly. + +I think quite a few of these Q's would have been answered by reading +the code/Makefile.... + +> This would be on what OS? + +FreeBSD, but it shouldn't matter. Any reasonably written VM should +have similar numbers (though BSD is generally regarded as having the +best VM, which, I think Linux poached not that long ago, iirc +::grimace::). + +> What hardware? + +My ultra-pathetic laptop with some fine - overly-noisy and can hardly +buildworld - IDE drives. + +> What size test file? + +In this case, only 72K. I've just updated the test program to use an +array of files though. + +> Do the "iterations" mean so many reads of the entire file, or so +> many buffer-sized read requests? + +In some cases, yes. With the file mmap()'ed, sorta. One of the test +cases (the one that did it in ~8s), mmap()'ed and munmap()'ed the file +every iteration and was twice as fast as the vanilla read() call. + +> Did the mmap case actually *read* anything, or just map and unmap +> the file? + +Nope, read it and wrote it out to stdout (which was redirected to +/dev/null). + +> Also, what did you do to normalize for the effects of the test file +> being already in kernel disk cache after the first test? + +That honestly doesn't matter too much since I wasn't testing the rate +of reading in files from my hard drive, only the OS's ability to +read/write pages of data around. In any case, I've updated my test +case to iterate through an array of files instead of just reading in a +copy of /etc/services. My laptop is generally a poor benchmark for +disk read performance given it takes 8hrs to buildworld, over 12hrs to +build mozilla, 18 for KDE, and about 48hrs for Open Office. :) +Someone with faster disks may want to try this and report back, but it +doesn't matter much in terms of relevancy for considering the benefits +of mmap(). The point is that there are calls that can be used that +substantially speed up read()'s and write()'s by allowing the VM to +align pages of data and give hints about its usage. For the sake of +argument re: the previously done tests, I'll reverse the order in +which I ran them and I bet dime to dollar that the times will be +identical. + +% make = + ~/open_source/mmap_test +cp -f /etc/services ./services +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL= +T_READSIZE=3D1 -DDO_MMAP=3D1 -DDO_MMAP_ONCE=3D1 -o mmap-test mmap-test.c +/usr/bin/time ./mmap-test > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is the same as the file size +Number of iterations: 100000 +Start time: 1047064672.276544 +Time: 1.281477 + +Completed tests + 1.29 real 0.10 user 0.92 sys +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL= +T_READSIZE=3D1 -DDO_MMAP=3D1 -o mmap-test mmap-test.c +/usr/bin/time ./mmap-test > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is the same as the file size +Number of iterations: 100000 +Start time: 1047064674.266191 +Time: 7.486622 + +Completed tests + 7.49 real 0.41 user 6.01 sys +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL= +T_READSIZE=3D1 -o mmap-test mmap-test.c +/usr/bin/time ./mmap-test > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is default read size: 65536 +Number of iterations: 100000 +Start time: 1047064682.288637 +Time: 19.35214 + +Completed tests + 19.04 real 0.88 user 15.43 sys +gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -o mmap-= +test mmap-test.c +/usr/bin/time ./mmap-test > /dev/null +Beginning tests with file: services + +Page size: 4096 +File read size is the same as the file size +Number of iterations: 100000 +Start time: 1047064701.867031 +Time: 82.4294540875 + +Completed tests + 81.57 real 2.10 user 69.55 sys + + +Here's the updated test that iterates through. Ooh! One better, the +files I've used are actual data files from ~pgsql. The new benchmark +iterates through the list of files and and calls bench() once for each +file and restarts at the first file after reaching the end of its +list (ARGV). + +Whoa, if these tests are even close to real world, then we at the very +least should be mmap()'ing the file every time we read it (assuming +we're reading more than just a handful of bytes): + +find /usr/local/pgsql/data -type f | /usr/bin/xargs /usr/bin/time ./mmap-te= +st > /dev/null +Page size: 4096 +File read size is the same as the file size +Number of iterations: 100000 +Start time: 1047071143.463360 +Time: 12.109530 + +Completed tests + 12.11 real 0.36 user 6.80 sys + +find /usr/local/pgsql/data -type f | /usr/bin/xargs /usr/bin/time ./mmap-te= +st > /dev/null +Page size: 4096 +File read size is default read size: 65536 +Number of iterations: 100000 +.... [been waiting here for >40min now....] + + +Ah well, if these tests finish this century, I'll post the results in +a bit, but it's pretty clearly a win. In terms of the data that I'm +copying, I'm copying ~700MB of data from my test DB on my laptop. I +only have 256MB of RAM so I can pretty much promise you that the data +isn't in my system buffers. If anyone else would like to run the +tests or look at the results, please check it out: + +o1 and o2 should be the only targets used if FILES is bigger than the +RAM on the system. o3's by far and away the fastest, but only in rare +cases will a DBA have more RAM than data. But, as mentioned earlier, +the LRU cache could easily be modified to munmap() infrequently +accessed files to keep the size of mmap()'ed data down to a reasonable +level. + +The updated test programs are at: + +http://people.FreeBSD.org/~seanc/mmap_test/ + +-sc + +--=20 +Sean Chittenden + +--TALVG7vV++YnpwZG +Content-Type: application/pgp-signature +Content-Disposition: inline + +-----BEGIN PGP SIGNATURE----- +Comment: Sean Chittenden + +iD8DBQE+aRM23ZnjH7yEs0ERAoqhAKCFgmhpvNMqe9tucoFvK1H6J50z2QCeIZEI +mgBHwu/H1pe1sXIX9UG2V+I= +=cFRQ +-----END PGP SIGNATURE----- + +--TALVG7vV++YnpwZG-- +