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Date: Thu, 9 Apr 2009 23:59:46 +0200 From: Ingo Molnar <mingo@...e.hu> To: Jan Kara <jack@...e.cz>, Linus Torvalds <torvalds@...ux-foundation.org>, Jens Axboe <jens.axboe@...cle.com>, Theodore Ts'o <tytso@....edu> Cc: Andrew Morton <akpm@...ux-foundation.org>, Alan Cox <alan@...rguk.ukuu.org.uk>, Arjan van de Ven <arjan@...radead.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Oleg Nesterov <oleg@...hat.com>, Roland McGrath <roland@...hat.com> Subject: updated: ext3 IO latency measurements on v2.6.30-rc1 This is an update to the ext3+CFQ latency measurements i did early in the merge window - originally with a v2.6.29 based kernel. Today i've repeated my measurements under v2.6.30-rc1, using the exact same system and the exact same workload. The quick executive summary: _what a difference a week of upstream development makes_! :-) Here are the specific details, as a reply to my earlier mail: * Ingo Molnar <mingo@...e.hu> wrote: > * Jan Kara <jack@...e.cz> wrote: > > > > So tell me again how the VM can rely on the filesystem not > > > blocking at random points. > > > > I can write a patch to make writepage() in the non-"mmapped > > creation" case non-blocking on journal. But I'll also have to find > > out whether it really helps something. But it's probably worth > > trying... > > _all_ the problems i ever had with ext3 were 'collateral damage' > type of things: simple writes (sometimes even reads) getting > serialized on some large [but reasonable] dirtying activity > elsewhere - even if the system was still well within its > hard-dirty-limit threshold. > > So it sure sounds like an area worth improving, and it's not that > hard to reproduce either. Take a system with enough RAM but only a > single disk, and do this in a kernel tree: > > sync > echo 3 > /proc/sys/vm/drop_caches > > while :; do > date > make mrproper 2>/dev/null >/dev/null > make defconfig 2>/dev/null >/dev/null > make -j32 bzImage 2>/dev/null >/dev/null > done & > > Plain old kernel build, no distcc and no icecream. Wait a few > minutes for the system to reach equilibrium. There's no tweaking > anywhere, kernel, distro and filesystem defaults used everywhere: > > aldebaran:/home/mingo/linux/linux> ./compile-test > Thu Mar 26 10:33:03 CET 2009 > Thu Mar 26 10:35:24 CET 2009 > Thu Mar 26 10:36:48 CET 2009 > Thu Mar 26 10:38:54 CET 2009 > Thu Mar 26 10:41:22 CET 2009 > Thu Mar 26 10:43:41 CET 2009 > Thu Mar 26 10:46:02 CET 2009 > Thu Mar 26 10:48:28 CET 2009 > > And try to use the system while this workload is going on. Use Vim > to edit files in this kernel tree. Use plain _cat_ - and i hit > delays all the time - and it's not the CPU scheduler but all IO > related. Under .30-rc1 i couldnt hit a single (!) annoying delay during half an hour of trying. The "Vim experience" is _totally_ smooth with a load average of 40+. And this is with default, untweaked ext3 - not even ext4. I'm impressed. > I have such an ext3 based system where i can do such tests and > where i dont mind crashes and data corruption either, so if you > send me experimental patches against latet -git i can try them > immediately. The system has 16 CPUs, 12GB of RAM and a single > disk. > > Btw., i had this test going on that box while i wrote some simple > scripts in Vim - and it was a horrible experience. The worst wait > was well above one minute - Vim just hung there indefinitely. Not > even Ctrl-Z was possible. I captured one such wait, it was hanging > right here: > > aldebaran:~/linux/linux> cat /proc/3742/stack > [<ffffffff8034790a>] log_wait_commit+0xbd/0x110 > [<ffffffff803430b2>] journal_stop+0x1df/0x20d > [<ffffffff8034421f>] journal_force_commit+0x28/0x2d > [<ffffffff80331c69>] ext3_force_commit+0x2b/0x2d > [<ffffffff80328b56>] ext3_write_inode+0x3e/0x44 > [<ffffffff802ebb9d>] __sync_single_inode+0xc1/0x2ad > [<ffffffff802ebed6>] __writeback_single_inode+0x14d/0x15a > [<ffffffff802ebf0c>] sync_inode+0x29/0x34 > [<ffffffff80327453>] ext3_sync_file+0xa7/0xb4 > [<ffffffff802ef17d>] vfs_fsync+0x78/0xaf > [<ffffffff802ef1eb>] do_fsync+0x37/0x4d > [<ffffffff802ef228>] sys_fsync+0x10/0x14 > [<ffffffff8020bd1b>] system_call_fastpath+0x16/0x1b > [<ffffffffffffffff>] 0xffffffffffffffff > > It took about 120 seconds for it to recover. These delays are definitely below 300 msecs now. (100 msecs is roughly the lag i can still notice in typing) > And it's not just sys_fsync(). The script i wrote tests file read > latencies. I have created 1000 files with the same size (all copies > of kernel/sched.c ;-), and tested their cache-cold plain-cat > performance via: > > for ((i=0;i<1000;i++)); do > printf "file #%4d, plain reading it took: " $i > /usr/bin/time -f "%e seconds." cat $i >/dev/null > done > > I.e. plain, supposedly high-prio reads. The result is very common > hickups in read latencies: > > file # 579 (253560 bytes), reading it took: 0.08 seconds. > file # 580 (253560 bytes), reading it took: 0.05 seconds. > file # 581 (253560 bytes), reading it took: 0.01 seconds. > file # 582 (253560 bytes), reading it took: 0.01 seconds. > file # 583 (253560 bytes), reading it took: 4.61 seconds. > file # 584 (253560 bytes), reading it took: 1.29 seconds. > file # 585 (253560 bytes), reading it took: 3.01 seconds. > file # 586 (253560 bytes), reading it took: 7.74 seconds. > file # 587 (253560 bytes), reading it took: 3.22 seconds. > file # 588 (253560 bytes), reading it took: 0.05 seconds. > file # 589 (253560 bytes), reading it took: 0.36 seconds. > file # 590 (253560 bytes), reading it took: 7.39 seconds. > file # 591 (253560 bytes), reading it took: 7.58 seconds. > file # 592 (253560 bytes), reading it took: 7.90 seconds. > file # 593 (253560 bytes), reading it took: 8.78 seconds. > file # 594 (253560 bytes), reading it took: 8.01 seconds. > file # 595 (253560 bytes), reading it took: 7.47 seconds. > file # 596 (253560 bytes), reading it took: 11.52 seconds. > file # 597 (253560 bytes), reading it took: 10.33 seconds. > file # 598 (253560 bytes), reading it took: 8.56 seconds. > file # 599 (253560 bytes), reading it took: 7.58 seconds. This test is totally smooth now: file # 6 (253560 bytes), reading it took: 0.05 seconds. file # 7 (253560 bytes), reading it took: 0.11 seconds. file # 8 (253560 bytes), reading it took: 0.12 seconds. file # 9 (253560 bytes), reading it took: 0.06 seconds. file # 10 (253560 bytes), reading it took: 0.05 seconds. file # 11 (253560 bytes), reading it took: 0.11 seconds. file # 12 (253560 bytes), reading it took: 0.09 seconds. file # 13 (253560 bytes), reading it took: 0.09 seconds. file # 14 (253560 bytes), reading it took: 0.03 seconds. file # 15 (253560 bytes), reading it took: 0.08 seconds. file # 16 (253560 bytes), reading it took: 0.15 seconds. file # 17 (253560 bytes), reading it took: 0.06 seconds. file # 18 (253560 bytes), reading it took: 0.13 seconds. file # 19 (253560 bytes), reading it took: 0.16 seconds. file # 20 (253560 bytes), reading it took: 0.29 seconds. file # 21 (253560 bytes), reading it took: 0.18 seconds. file # 22 (253560 bytes), reading it took: 0.28 seconds. file # 23 (253560 bytes), reading it took: 0.04 seconds. 290 msecs was the worst in thes series above. The vim read+write test takes longer: aldebaran:~/linux/linux/test-files/src> ./vim-test file # 0 (253560 bytes), Vim-opening it took: 2.35 seconds. file # 1 (253560 bytes), Vim-opening it took: 2.09 seconds. file # 2 (253560 bytes), Vim-opening it took: 2.20 seconds. file # 3 (253560 bytes), Vim-opening it took: 2.14 seconds. file # 4 (253560 bytes), Vim-opening it took: 2.15 seconds. file # 5 (253560 bytes), Vim-opening it took: 2.13 seconds. file # 6 (253560 bytes), Vim-opening it took: 2.11 seconds. file # 7 (253560 bytes), Vim-opening it took: 2.09 seconds. file # 8 (253560 bytes), Vim-opening it took: 2.03 seconds. file # 9 (253560 bytes), Vim-opening it took: 2.03 seconds. file # 10 (253560 bytes), Vim-opening it took: 2.06 seconds. file # 11 (253560 bytes), Vim-opening it took: 2.19 seconds. file # 12 (253560 bytes), Vim-opening it took: 2.07 seconds. file # 13 (253560 bytes), Vim-opening it took: 2.02 seconds. I suspect that is to be expected? The test does: vim -c "q" $i 2>/dev/null >/dev/null 2 seconds is OK-ish - when close+writing a file i mentally expect some short delay anyway. I think vim fsyncs the swap file in that case as well. I havent actually experienced any such delays while editing files. > The system's RAM is ridiculously under-utilized, 96.1% is free, only > 3.9% is utilized: > > total used free shared buffers cached > Mem: 12318192 476732 11841460 0 48324 142936 > -/+ buffers/cache: 285472 12032720 > Swap: 4096564 0 4096564 > > Dirty data in /proc/meminfo fluctuates between 0.4% and 1.6% of > total RAM. (the script removes the freshly build kernel object > files, so the workload is pretty steady.) > > The peak of 1.6% looks like this: > > Dirty: 118376 kB > Dirty: 143784 kB > Dirty: 161756 kB > Dirty: 185084 kB > Dirty: 210524 kB > Dirty: 213348 kB > Dirty: 200124 kB > Dirty: 122152 kB > Dirty: 121508 kB > Dirty: 121512 kB > > (1 second snapshots) > > So the problems are all around the place and they are absolutely, > trivially reproducible. And this is how a default ext3 based distro > and the default upstream kernel will present itself to new Linux > users and developers. It's not a pretty experience. > > Oh, and while at it - also a job control complaint. I tried to > Ctrl-C the above script: > > file # 858 (253560 bytes), reading it took: 0.06 seconds. > file # 859 (253560 bytes), reading it took: 0.02 seconds. > file # 860 (253560 bytes), reading it took: 5.53 seconds. > file # 861 (253560 bytes), reading it took: 3.70 seconds. > file # 862 (253560 bytes), reading it took: 0.88 seconds. > file # 863 (253560 bytes), reading it took: 0.04 seconds. > file # 864 (253560 bytes), reading it took: ^C0.69 seconds. > file # 865 (253560 bytes), reading it took: ^C0.49 seconds. > file # 866 (253560 bytes), reading it took: ^C0.01 seconds. > file # 867 (253560 bytes), reading it took: ^C0.02 seconds. > file # 868 (253560 bytes), reading it took: ^C^C0.01 seconds. > file # 869 (253560 bytes), reading it took: ^C^C0.04 seconds. > file # 870 (253560 bytes), reading it took: ^C^C^C0.03 seconds. > file # 871 (253560 bytes), reading it took: ^C0.02 seconds. > file # 872 (253560 bytes), reading it took: ^C^C0.02 seconds. > file # 873 (253560 bytes), reading it took: > ^C^C^C^Caldebaran:~/linux/linux/test-files/src> > > I had to hit Ctrl-C numerous times before Bash would honor it. > This to is a very common thing on large SMP systems. I'm willing > to test patches until all these problems are fixed. Any takers? This Bash bug still occurs and is highly annoying when using scripts on SMP Linux boxes and trying to Ctrl-C out of them. But all in one, the ext3 and IO latency problems seem to be thoroughly cured! To me the past 1 week has made more difference in filesystems and IO interactivity than all filesystems development done in the past 5+ years, combined. Kudos! Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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