Re: Busy-wait delay in qmail 1.03 after upgrading to Linux 2.6

[Mike Fedyk <mfedyk@matchmail.com>]

On Tue, Jan 20, 2004 at 08:22:16PM +0100, Haakon Riiser wrote:
> [Manfred Spraul]
> > If a thread switch happens in the indicated line, then the reader will 
> > loop, until it's timeslice expires - one full timeslice delay between 
> > the two gettimeofday() calls.
> 
> Exactly.  But on 2.6, the delay between the two gettimeofday()
> calls are sometimes up to 300 ms, which is 300 timeslices in
> 2.6, right?  I have never observed more than _one_ timeslice
> delay in 2.4.
> 
> > Running the reader with nice -20 resulted in delays of 200-1000 ms for 
> > each write call, nice 20 resulted in no slow calls. In both cases 100% 
> > cpu load.
> 
> But when the listener and the writer have the same nice value,
> how is it possible to have a delay of 300 ms?  Both the writer
> and the listener are ready to run, so wouldn't a 300 ms delay
> mean that the listener was given the CPU 300 times in a row?

The scheduler can do this for you with its priority modification heuristics.

Try running a test with Nick's scheduler, and see how much your timings
change.

Also, there is a scheduling patch in -mm that's not in 2.6.1 that might
affect you also.