Linux scheduling threads for disk vs. sound card io

I haven't been using zinf, on Linux, for a while... because it would
skip fairly regularly. (Also, MyMusic got to be dog slow adding new
music to > 20GB of music) If that's fixed, you can ignore the remainder
of this, if not, here's something I read, while reading about the
'anticipatory scheduler' that will, soon, be a part of the Linux
kernel:

> From: Andrew Morton
> Subject: Re: iosched: impact of streaming read on read-many-files
> Date: Fri, 21 Feb 2003 13:11:58 -0800
> Andrea Arcangeli wrote:
> >
> > CFQ is made for multimedia desktop usage only, you want to be sure
> > mplayer or xmms will never skip frames, not for parallel cp reading
> > floods of data at max speed like a database with zillon of threads.
For
> > multimedia not to skip frames 1M/sec is  more than enough bandwidth,
> > doesn't matter if the huge database in background runs much slower
as
> > far as you never skip a frame.
> 
> These applications are broken.  The kernel shouldn't be bending over
> backwards trying to fix them up.  Because this will never ever work as
well
> as fixing the applications.
> 
> The correct way to design such an application is to use an RT thread
to
> perform the display/audio device I/O and a non-RT thread to perform
the disk
> I/O.  The disk IO thread keeps the shared 8 megabyte buffer full.  The
RT
> thread mlocks that buffer.
> 
> The deadline scheduler will handle that OK.  The anticipatory
scheduler
> (which is also deadline) will handle it better.
> 
> If an RT thread performs disk I/O it is bust, and we should not try to
fix
> it.  The only place where VFS/VM/block needs to care for RT tasks is
in the
> page allocator.  Because even well-designed RT tasks need to allocate
pages.
> 
> The 2.4 page allocator has a tendency to cause 5-10 second stalls for
a
> single page allocation when the system is under writeout load.  That
is fixed
> in 2.5, but special-casing RT tasks in the allocator would make sense.


Tom Kimpton.

----
Unix IS user friendly. It's just picky who it's friends with.


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