Re: [SCHED] Totally WRONG prority calculation with specific test-case (since 2.6.10-bk12)

[Con Kolivas <kernel@kolivas.org>]

On Wednesday 28 December 2005 08:48, Paolo Ornati wrote:
> On Tue, 27 Dec 2005 19:09:18 +0100
>
> Paolo Ornati <ornati@fastwebnet.it> 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

Looking at your top output I see that transcode command generates 7 processes 
all likely to be using cpu, and your DD slowdown is almost 7 times... I 
assume it all goes away if you nice the transcode up by 3 or more.

> 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/sche
>d.h
>
> --------------------------------------------------
>
> [PATCH] sched: remove_interactive_credit

The issue is that the scheduler interactivity estimator is a state machine and 
can be fooled to some degree, and a cpu intensive task that just happens to 
sleep a little bit gets significantly better priority than one that is fully 
cpu bound all the time. Reverting that change is not a solution because it 
can still be fooled by the same process sleeping lots for a few seconds or so 
at startup and then changing to the cpu mostly-sleeping slightly behaviour. 
This "fluctuating" behaviour is in my opinion worse which is why I removed 
it.

Cheers,
Con

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