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SubjectRe: Preemption patch, some more feedback
On Mon, 2001-09-10 at 11:41, Francis Galiegue wrote:
> Machine is Athlon 650, AMD Viper chipset, 256 MB RAM. Kernel is
> 2.4.9-ac10 + preempt patch + irc_conntrack patch from iptables.
> The preempt patch largely improves multimedia latency (no surprise on
> that), I can watch a DivX smoothly (with mplayer, gfx being Matrox G400)
> and compile various stuff behind.
> However, a very simple command destroys this completely:
> cat /dev/zero >/dev/null
> DivX playback then becomes sluggish, no visible difference in this case
> between stock kernel and "preempt" kernel.

A long-term lock must be held for the duration of `cat /dev/zero >
/dev/null' -- i dont know if it is in the access to /dev/null or
/dev/zero or in the basic file operation itself.

as long as a lock is held, preemption can not occur.

what do we do? for the short term, and the benefit of everyone (UP, SMP,
and preemption users) we need to eliminate long-held locks with a better

in the long term, we can look at having the preemption patch use various
different types of locks (priority locks, spin then sleep locks, etc.)

Robert M. Love
rml at
rml at

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