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List:       freebsd-arch
Subject:    Re: What is the PREEMPTION option good for?
From:       Robert Watson <rwatson () freebsd ! org>
Date:       2006-11-28 14:23:50
Message-ID: 20061128142218.P44465 () fledge ! watson ! org
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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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