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

[Andrew Morton <akpm@osdl.org>]

On Mon, 14 Aug 2006 18:36:44 +0200
Andrzej Szymanski <szymans@agh.edu.pl> wrote:

> 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)

Which filesystem?

If ext3 in ordered-data mode: any fsync() will sync the whole filesystem
(it has to).  Mounting with `-o writeback' should help.