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Subject2.2.18: severe performance problem (high load, low mem, idle CPU)
Hello,

we experienced a severe performance problem on a PentiumPro 200 MHz,
64MB RAM, 128MB swap:

Due to many processes being started in a short time, the system load
went up to 53, and the 9GB SCSI disk was working heavily. At that time
I suspected no severe problem, and I was busy doing something else.
However after almost three hours the system load was still at about 40
with the old processes not yet finished. (The processes typically take
2 to 5 seconds to finish, and need about 4MB memory each).

At that point I became active.

In top I was surprised that the CPU claimed to be more than 90% idle,
while the swap space was exceeded. But the memory wasn't really tight;
cached and buffers were about 12MB together. So basically the situation
should have gone away. Should, but didn't.

The kernel running was that from SuSE Linux 7.1 (Linux version 2.2.18
(root@Pentium.suse.de) (gcc version 2.95.2 19991024 (release)) #1 Fri
Jan 19 22:10:35 GMT 2001). So maybe the defect is an "enhancement" done
by SuSE. Anyway:

In top I noticed that the processes to finish were all mostly swapped
out, and they showed a zero in the "PRI" column. Usually runnable
processes have more "fuel" there. It seems to me swapped out processes
did not get their fules reloaded. The processes all had a "D" status
(blocked on I/O). Also it seemed that processes that share a lot of
data are not favoured enough when paging in. If a page is shared 10
times, paging that one in would help 10 processes. Instead the kernel
seemed to swap in and out a few kB wihout getting any process done.

I decided to kill a few non-essential processes to improve the
situation. No help. I added an extra 32MB swapfile, so the buffers and
shared went up to ver 30MB, but still no process finished. The CPU
still was quite "idle".

So I decided to kill the processes in question. After several seconds,
no process had terminated however. (Maybe due to the code to handle the
signal being paged out). Then I did a kill -9 to the processes which
finally helped.

So to summarize:
1) paged out processes seem not to get enough CPU
2) paged out shared pages seem not to get enough priority to be swapped
in
3) On low memory situations the schedulting algorithm seems to perform
poor

For 3) I sould imagine doing a round-robing scheduling with extended
time-slice (while still being fair, i.e. run them rarely but longer)
for massively swapped out processes, hoping that one of them will
finish soon. That way maybe more of the working set will be paged in,
enabbling some progress.

I don't have the top screen saved, but I have a ps -aux. The 40
processes being paged out were all displayed with a %CPU of "0.0".
The ps command with 7.4% CPU was the highest value. The kernel pager
also seemed to be non-busy:

USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 1 0.0 0.0 400 52 ? S Mar22 0:22 init [3]
root 2 0.0 0.0 0 0 ? SW Mar22 0:03 [kflushd]
root 3 0.0 0.0 0 0 ? SW Mar22 0:01 [kupdate]
root 4 0.0 0.0 0 0 ? SW Mar22 6:58 [kswapd]
root 5 0.0 0.0 0 0 ? SW< Mar22 0:00
[mdrecoveryd]
...
daemon 32528 0.0 2.0 4984 1352 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32529 0.0 2.0 4984 1352 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32531 0.0 2.7 5008 1760 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32533 0.0 2.5 4984 1640 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32539 0.0 3.1 5008 2044 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32540 0.0 1.9 4984 1276 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32542 0.0 1.4 4984 948 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32547 0.0 2.1 4984 1404 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32548 0.0 2.1 4984 1380 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32549 0.0 1.9 4984 1284 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32550 0.0 1.1 4984 768 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32555 0.0 2.3 4984 1504 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32556 0.0 1.8 4984 1224 ? D 14:42 0:04
/etc/mail/dirty-h
daemon 32557 0.0 1.9 4984 1244 ? D 14:42 0:04
/etc/mail/dirty-h
...

These were some of the processes that should have finished.

Regards,
Ulrich

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