Re: What is the PREEMPTION option good for?


On Sun, 26 Nov 2006, Ivan Voras wrote:

Robert Watson wrote:

There's a known performance regression with PREEMPTION and loopback network traffic on UP or UP-like systems due to a poor series of context switches occuring in the network stack. If your benchmark involves the above web load over the loopback, that could be the source of what you're seeing. If it's not loopback traffic, then that's not the source of the problem.

The dynamic stuff is accessing the database (fairly intensively) over the loopback.

This may be significantly affected by preemption then.

You might try fiddling with kern.sched.ipiwakeup.enabled and see what the effect is, btw -- this controls whether or not the scheduler wakes up another idle CPU to run a thread when waking up that thread, rather than queuing it to run which may occur on the other CPU at the next clock tick.

Try this with or without PREEMPTION?

They're independent twiddles, and can be frobbed separately. If you can easily measure performance in the different configurations, seeing a table of permutations and results would be very nice to see what happens :-).

Robert N M Watson
Computer Laboratory
University of Cambridge
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