lkml.org 
[lkml]   [2006]   [Aug]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectStrange write starvation on 2.6.17 (and other) kernels
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)

Thanks,
Andrzej


-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-08-14 18:39    [W:1.181 / U:0.012 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site