softclock swis not bound to specific cpu

Robert Watson rwatson at
Mon Oct 19 15:05:42 UTC 2009

On Sun, 18 Oct 2009, Taku YAMAMOTO wrote:

> I noticed that the softclock threads didn't seem to be bound to any cpu.
> I'm not sure whether it's the Right Thing (TM) to bind them to the 
> corresponding cpus though: it might be good to give the scheduler a chance 
> to rebalance callouts.
> I'm about to test the modification like the attached diff. Comments are 
> welcome.

Yes, I think the intent is that they have a "soft" affinity to the CPU where 
the lapic timer is firing, but not a hard binding, allowing them to migrate if 
required.  It would be interesting to measure how effective that soft affinity 
is in practice under various loads -- presumably the goal would be for the 
softclock thread to migrate if a higher (lower) priority thread is hogging the 

Robert N M Watson
Computer Laboratory
University of Cambridge

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