Re: Strange write starvation on 2.6.17 (and other) kernels

[Grant Coady]

On Mon, 14 Aug 2006 18:36:44 +0200, Andrzej Szymanski <szymans@xxxxxxxxxx> wrote:

>Hi,
>
>I've encountered a strange problem - if an application is sequentially 
>writing a large file on a busy machine, a single write() of 64KB may 
>take even 30 seconds. But if I do fsync() after each write() the maximum 
>time of write()+fsync() is about 0.5 second (the overall performance is, 
>of course, degraded).
>
>The point is, that some applications (samba+smbclient) time out after 
>20s waiting for write().
>
>Does anybody have an idea how to tune the kernel to avoid this strange 
>delay in write()?
>
>I've tried to experiment with cfq and deadline IO scheduler - without 
>success. Decreasing /proc/vm/dirty_ratio to 5% helps a little.
>
>If somebody want to test it, the tool I've written for measuring maximum 
>write() time is here: http://galaxy.agh.edu.pl/~szymans/writetimer
>
>1. Compile writetimer.c
>2. Put a large background read from the disk
>3. Simultaneously write 10 files 200MB each (write() without fsync())
>for i in 1 2 3 4 5 6 7 8 9 0 ; do ./writetimer 200 > testfile$i & done
>4. and with fsync() after each write()
>for i in 1 2 3 4 5 6 7 8 9 0 ; do ./writetimer -200 > testfile$i & done
>(negative file size turns on fsync())
>
>Tested on
>- 2.6.15-23 (512MB RAM, Pentium-M 1.7, Ubuntu 6.06, ATA disk)
>- 2.6.17-1.2145_FC5 (512MB RAM, Pentium-M 1.7, Fedora Core 5, ATA disk)
>- 2.6.12-2.3.legacy_FC3smp (2GB RAM, Fedora Core 3, software RAID 5 on 4 
>ATA disks)

Does top show what's holding up the file I/O?  Rings a vague bell but 
I'm not able to run tests here for a little while until some other stuff 
completed.

Grant.
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