Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.43-00

Mike Galbraith
2005-04-05T15:18:30Z

At 01:57 AM 4/5/2005 -0600, Zwane Mwaikambo wrote:
>On Tue, 5 Apr 2005, Esben Nielsen wrote:
>
> > > I'm sure a lot of the yield() users could be converted to
> > > schedule_timeout(), some of the users i saw were for low memory
> conditions
> > > where we want other tasks to make progress and complete so that we a bit
> > > more free memory.
> > >
> >
> > Easy, but damn ugly. Completions are the right answer. The memory system
> > needs a queue system where tasks can sleep (with a timeout) until the
> > right amount of memory is available instead of half busy-looping.
>
>I agree entirely, that would definitely be a better way to go eventually.

I wouldn't bet on it. There used to be a queue - minus the
timeout. Throughput improved markedly with it's removal.

That said, yield()s in the kernel can be quite evil. I once instrumented
semaphores, and under hefty load, frequently found tasks waiting for a
semaphore held by someone in the expired array. When you've got a busy
cpu, that wait can be _extremely_ painful. The yield()s in mm/*.c are long
gone (thank god), but after a quick grep/peek, I can imagine the one in
free_more_memory() causing some throughput grief in a cpu intense
environment, and the one in __wait_on_freeing_inode() would punt you into
the dungeon while you're holding the inode lock... that looks like it could
be pretty unpleasant.

-Mike