Andrzej Szymanski 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)
Thanks,
Andrzej
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Sounds like a problem with the elevator code. As often as the memory
management gets rewritten and busted, no telling where the condition is
coming from. I have not seen this problem on released kernels with the
distros, but I have seen on on post 2.6.14 kernels, which is why I am
not using them. Seems related to user space memory usage in some way,
and not the actual elevator code. Try an older kernel and see if it
goes away.
Jeff
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