Re: [SCHED] Totally WRONG prority calculation with specific test-case(since 2.6.10-bk12)
From: Peter Williams
Date: Tue Dec 27 2005 - 19:04:39 EST
Paolo Ornati wrote:
On Tue, 27 Dec 2005 19:09:18 +0100
Paolo Ornati <ornati@xxxxxxxxxxxxx> wrote:
Hello,
I've found an easy-to-reproduce-for-me test case that shows a totally
wrong priority calculation: basically a CPU-intensitive process gets
better priority than a disk-intensitive one (dd if=bigfile
of=/dev/null ...).
Seems impossible, isn't it?
---- THE NUMBERS with 2.6.15-rc7 -----
The test-case is the Xvid encoding of dvd-ripped track with transcode
(using "dvd::rip" interface). The copied-and-pasted command line is
this:
mkdir -m 0775 -p '/home/paolo/tmp/test/tmp' &&
cd /home/paolo/tmp/test/tmp && dr_exec transcode -H 10 -a 2 -x vob,null
-i /home/paolo/tmp/test/vob/003 -w 1198,50 -b 128,0,0 -s 1.972
--a52_drc_off -f 25 -Y 52,8,52,8 -B 27,10,8 -R 1 -y xvid4,null
-o /dev/null --print_status 20 && echo DVDRIP_SUCCESS mkdir -m 0775 -p
'/home/paolo/tmp/test/tmp' && cd /home/paolo/tmp/test/tmp && dr_exec
transcode -H 10 -a 2 -x vob -i /home/paolo/tmp/test/vob/003 -w 1198,50
-b 128,0,0 -s 1.972 --a52_drc_off -f 25 -Y 52,8,52,8 -B 27,10,8 -R 2 -y
xvid4 -o /home/paolo/tmp/test/avi/003/test-003.avi --print_status 20 &&
echo DVDRIP_SUCCESS
Here there is a TOP snapshot while running it:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
5721 paolo 16 0 115m 18m 2428 R 84.4 3.7 0:15.11 transcode
5736 paolo 25 0 50352 4516 1912 R 8.4 0.9 0:01.53 tcdecode
5725 paolo 15 0 115m 18m 2428 S 4.6 3.7 0:00.84 transcode
5738 paolo 18 0 115m 18m 2428 S 0.8 3.7 0:00.15 transcode
5734 paolo 25 0 20356 1140 920 S 0.6 0.2 0:00.12 tcdemux
5731 paolo 25 0 47312 2540 1996 R 0.4 0.5 0:00.08 tcdecode
5319 root 15 0 166m 16m 2584 S 0.2 3.2 0:25.06 X
5444 paolo 16 0 87116 22m 15m R 0.2 4.6 0:04.05 konsole
5716 paolo 16 0 10424 1160 876 R 0.2 0.2 0:00.06 top
5735 paolo 25 0 22364 1436 932 S 0.2 0.3 0:00.01 tcextract
DD running alone:
paolo@tux /mnt $ mount space/; time dd if=space/bigfile of=/dev/null bs=1M count=128; umount space/
128+0 records in
128+0 records out
real 0m4.052s
user 0m0.000s
sys 0m0.209s
DD while transcoding:
paolo@tux /mnt $ mount space/; time dd if=space/bigfile of=/dev/null bs=1M count=128; umount space/
128+0 records in
128+0 records out
real 0m26.121s
user 0m0.001s
sys 0m0.255s
---------------------------------------
I've tried older kernels finding that 2.6.11 is the first affected.
Going on with testing...
2.6.11-rc[1-5]:
2.6.11-rc3 bad
2.6.11-rc1 bad
2.6.10-bk[1-14]
2.6.10-bk7 good
2.6.10-bk11 good
2.6.10-bk13 bad
2.6.10-bk12 bad
So the problem was introduced with:
>> 2.6.10-bk12 09-Jan-2005 <<
The exact behaviour is different with 2.6.11/12/13/14... for example:
with 2.6.11 the priority of "transcode" is initially set to ~25 and go
down to 17/18 when running DD.
The problem doesn't seem 100% reproducible with every kernel, sometimes
a "BAD" kernel looks "GOOD"... or maybe it was me confused by too
much compile/install/reboot/test work ;)
Other INFO:
- I'm on x86_64
- preemption ON/OFF doesn't make any differences
Can anyone reproduce this?
IOW: is this affecting only my machine?
Hello Con and Ingo... I've found that the above problem goes away
by reverting this:
http://linux.bkbits.net:8080/linux-2.6/cset@41e054c6pwNQXzErMxvfh4IpLPXA5A?nav=index.html|src/|src/include|src/include/linux|related/include/linux/sched.h
--------------------------------------------------
[PATCH] sched: remove_interactive_credit
Special casing tasks by interactive credit was helpful for preventing fully
cpu bound tasks from easily rising to interactive status.
However it did not select out tasks that had periods of being fully cpu
bound and then sleeping while waiting on pipes, signals etc. This led to a
more disproportionate share of cpu time.
Backing this out will no longer special case only fully cpu bound tasks,
and prevents the variable behaviour that occurs at startup before tasks
declare themseleves interactive or not, and speeds up application startup
slightly under certain circumstances. It does cost in interactivity
slightly as load rises but it is worth it for the fairness gains.
Signed-off-by: Con Kolivas <kernel@xxxxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx>
--------------------------------------------------
Maybe this change has revealed a scheduler weakness ?
I'm glad to test any patch or give more data :)
Bye,
Any chance of you applying the PlugSched patches and seeing how the
other schedulers that it contains handle this situation?
The patch at:
<http://prdownloads.sourceforge.net/cpuse/plugsched-6.1.6-for-2.6.15-rc5.patch?download>
should apply without problems to the 2.6.15-rc7 kernel.
Very Brief Documentation:
You can select a default scheduler at kernel build time. If you wish to
boot with a scheduler other than the default it can be selected at boot
time by adding:
cpusched=<scheduler>
to the boot command line where <scheduler> is one of: ingosched,
nicksched, staircase, spa_no_frills, spa_ws, spa_svr or zaphod. If you
don't change the default when you build the kernel the default scheduler
will be ingosched (which is the normal scheduler).
The scheduler in force on a running system can be determined by the
contents of:
/proc/scheduler
Control parameters for the scheduler can be read/set via files in:
/sys/cpusched/<scheduler>/
Peter
--
Peter Williams pwil3058@xxxxxxxxxxxxxx
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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