Subject: Re: [PATCH] 2.5.72 O(1) interactivity bugfix
From: Andreas Boman <aboman@midgaard.us>
To: Con Kolivas <kernel@kolivas.org>
Cc: linux kernel mailing list <linux-kernel@vger.kernel.org>
In-Reply-To: <200306191112.49621.kernel@kolivas.org>
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Date: 18 Jun 2003 22:00:42 -0400
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On Wed, 2003-06-18 at 21:12, Con Kolivas wrote:
> On Thu, 19 Jun 2003 10:47, Andreas Boman wrote:
> > On Wed, 2003-06-18 at 19:38, Con Kolivas wrote:
> > > On Thu, 19 Jun 2003 08:59, Andreas Boman wrote:
> > > > On Wed, 2003-06-18 at 18:43, Con Kolivas wrote:
> > > > > On Thu, 19 Jun 2003 03:59, Andreas Boman wrote:
> > > > > > On Wed, 2003-06-18 at 10:43, Con Kolivas wrote:
> > > > > > > --BEGIN PGP SIGNED MESSAGE--
> > > > > > > Hash: SHA1
> > > > > > >
> > > > > > > Hi Ingo, all
> > > > > > >
> > > > > > > While messing with the interactivity code I found what appears to
> > > > > > > be an uninitialised variable (p->sleep_avg), which is responsible
> > > > > > > for all the boost/penalty in the scheduler. Initialising this
> > > > > > > variable to 0 seems to have made absolutely massive improvements
> > > > > > > to system responsiveness under load and completely removed audio
> > > > > > > skips up to doing a make -j64 on my uniprocessor P4 (beyond which
> > > > > > > swap starts being used), without changing the scheduler
> > > > > > > timeslices. This seems to help all 2.4 O(1) based kernels as
> > > > > > > well. Attached is a patch against 2.5.72 but I'm not sure about
> > > > > > > the best place to initialise it.
> > > > > >
> > > > > > Applying this ontop of 2.5.72-mm1 causes more xmms/mpg321/ogg123
> > > > > > skipping than with plain -mm1 here. make -j20 on my up athlon 1900+
> > > > > > with 512M ram causes extreme skipping until the make is killed.
> > > > > > With plain -mm1 I may get _one_ skip at the very begining of a song
> > > > > > during make -j20 (about 50% of the time). Plain -mm1 stops skipping
> > > > > > after 10-15 sec of playback of a song, and even switching desktops
> > > > > > after that doesnt cause skips, with or without make -j20 running
> > > > > > (switching to/from desktops with apps like mozilla, evolution etc.
> > > > > > will cause skips during the first 10-15 sec of a song regardless
> > > > > > what I do it seems).
> > > > > >
> > > > > > Renicing xmms to -15 doesnt change anything with either kernel.
> > > > >
> > > > > Hmm. I got too excited with the fact it improved so much on the 2.4
> > > > > O(1)
> > > >
> > > > Well, I got very exited when I saw your post ;) I guess this is a
> > > > problem all us UP desktop users would like too see solved.
> > > >
> > > > > kernels that I didn't try it hard enough on the 2.5 kernels. I have
> > > > > had people quietly telling me that it isn't uninitialised, but that I
> > > > > am simply resetting it with this patch on new forked processes. It
> > > > > seems the extra changes to the 2.5 scheduler make this patch make
> > > > > things worse?
> > > >
> > > > Yeah, I poked around a bit after I sent my earlier mail to see what may
> > > > be going on and noticed that too. (In activate_task() and sched_exit()
> > > > and some other place iirc)
> > > >
> > > > > I need more testing of the 2.4 one as well to see if it was just my
> > > > > combination of hardware and kernel that was better with this...
> > > >
> > > > I suspect that is the case, yes, or I got unlucky with mine since it
> > > > was extremely bad during the make -j. I'll see if I can get a 2.4.21-ck
> > > > patched up with some other things I need here, and try to reproduce my
> > > > results. That should tell us if it is infact scheduler differences or
> > > > our different setups.
> > >
> > > I had another look at 2.5 and noticed the max sleep avg is set to 10
> > > seconds instead of 2 seconds in 2.4. This could make a _big_ difference
> > > to new forked tasks if they all start out penalised as most
> > > non-interactive. It can take 5 times longer before they get the balance
> > > right. Can you try with this set to 2 or even 1 second on 2.5?
> >
> > Ahh, thanks Con, setting MAX_SLEEP_AVG to 2 *almost* removes all xmms
> > skipping here, a song *may* skip during desktop switches sometime during
> > the first 5 sec or so of playback IFF make -j20 is running. On a mostly
> > idle box (well LoadAvg 3 or so is mostly idle isnt it? ;) desktop
> > switching doesnt cause skips anymore 8)
> 
> That's nice; a MAX_SLEEP_AVG of 1 second will shorten that 5 seconds to half 
> that as well. What you describe makes perfect sense given that achieving a 
> balance is an exponential function where the MSA is the time constant.

Right.

> > Doing make -j20 and staying on the same desktop doesnt cause any
> > skipping at all (but it didnt cause much skipping at all on plain
> > 2.5.72-mm1 either).
> 
> So it is better than the default mm1? (doesnt cause any vs didnt cause much)

Yes.

> > I also applied your p->sleep_avg = 0; stuff (keeping MAX_SLEEP_AVG 2 and
> > HZ 1000) and it behaved just like I described earlier (songs started
> > after the make never stop skipping).
> 
> Well anything started will be penalised initially as being completely 
> non-interactive with the p->sleep_avg = 0. This seems to work fine for normal 
> usage patterns I've found on -ck1, as after a short while it gets a bonus up 
> to interactive. But you say that doesn't happen on 2.5?

It doesnt seem like it, xmms gets completely starved. A song started
before the make -j plays through fine while songs started after the make
-j just skip perpetually until I kill the player or the make.

> > I am fairly sure the winner for me here was the MAX_SLEEP_AVG since I
> > have fiddled with HZ before without it making big noticable differences.
> 
> Yes you're confirming pretty much what I'm finding now that I've played with 
> it a lot more.
> 
> > I havent gotten a 2.4 kernel patched up yet (lazy), but I'll get that
> > done and see how that sleep_avg patch behaves here then.
> 
> Shouldn't be any different than what you've described on 2.5 now, if you make 
> CHILD_PENALTY match that on 2.5 (is 50 in 2.5, was 95 in -ck1)

Right, I'll take a look at all thoose magic numbers so I'll have the
same settings.

	Andreas


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