Messages in this thread | | | Date | Mon, 18 May 2009 17:42:41 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH RFC] v5 expedited "big hammer" RCU grace periods |
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* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > i might be missing something fundamental here, but why not just > > have per CPU helper threads, all on the same waitqueue, and wake > > them up via a single wake_up() call? That would remove the SMP > > cross call (wakeups do immediate cross-calls already). > > My concern with this is that the cache misses accessing all the > processes on this single waitqueue would be serialized, slowing > things down. In contrast, the bitmask that smp_call_function() > traverses delivers on the order of a thousand CPUs' worth of bits > per cache miss. I will give it a try, though.
At least if you go via the migration threads, you can queue up requests to them locally. But there's going to be cachemisses _anyway_, since you have to access them all from a single CPU, and then they have to fetch details about what to do, and then have to notify the originator about completion.
Ingo
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