Date: Tue, 20 Nov 2007 20:23:22 +0100 From: Kris Kennaway <kris@FreeBSD.org> To: Yar Tikhiy <yar@comp.chem.msu.su> Cc: jroberson@chesapeake.net, freebsd-current@freebsd.org Subject: Re: SCHED_ULE & niceness / rtprio Message-ID: <4743342A.10507@FreeBSD.org> In-Reply-To: <20071120141403.GE81260@comp.chem.msu.su> References: <20071120141403.GE81260@comp.chem.msu.su>
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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 yet. Kris
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