SCHED_ULE & niceness / rtprio
jroberson at chesapeake.net
Tue Jan 1 20:56:17 PST 2008
On Tue, 20 Nov 2007, Kris Kennaway wrote:
> Yar Tikhiy wrote:
>> Hi all,
>> SCHED_ULE seems to do an unfair job to processes with low niceness
>> or with real-time priority. Here are my observations:
>> A few days ago I noticed that music (played by Totem, Gnome's default
>> player) would pause for a fraction of second each time I did something
>> in X/Gnome, such as switched between windows, clicked on a link in
>> the web browser, etc. Then I found that music was jerky only if
>> the player ran with a negative niceness or a real-time priority:
>> As soon as I returned it to niceness 0 and normal priority, sound
>> became totally seamless notwithstanding my activity in X.
>> The approximate value required for the effect to appear was niceness
>> as low as -5 or RT priority as high as 10; niceness -1 or rtprio 1
>> wasn't enough.
>> Curious, I substituted SCHED_4BSD for SCHED_ULE in my otherwise
>> GENERIC kernel, and the jerkiness of sound was gone irrespective
>> of the niceness or RT priority of the player.
>> To rule out other possible causes, I also tried kernels with SCHED_ULE
>> but without SMP or without debug stuff (INVARIANTS+WITNESS), but
>> the issue was there in both cases, unlike in the case of SCHED_4BSD.
>> Of course, X+Gnome+stuff isn't the clearest environment for debugging
>> schedulers, but multimedia apps are rather sensitive to scheduling
>> quality. This case should be rather obvious: When I click in an
>> inactive window, some processes are woken that have been idle.
>> After that the high-priority player isn't scheduled long enough for
>> the hardware audo buffer to drain, although it would be scheduled
>> soon if it had normal priority.
>> Did I hit a known issue?
> Others have reported it, but I don't know if Jeff has had time to investigate
I tried to reproduce this recently by running mplayer with various nice
values while scrolling around a lot in firefox. This is on a 1.4ghz
machine. I didn't encounter any problems.
If anyone is having nice related problems with ULE I'd love a simple test
case that shows it. I tried installing boinc which was mentioned in other
threads but had trouble contacting the servers and getting it setup.
Something simpler would be very beneficial.
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