ext3 IO latency measurements (was: Linux 2.6.29)

[Ingo Molnar <mingo@elte.hu>]

* Jan Kara <jack@suse.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.

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.

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.

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?

	Ingo