Messages in this thread | | | Date | Sun, 24 Aug 2008 11:01:33 +0200 | From | Andi Kleen <> | Subject | Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu |
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> > Ah so it was already 25% slower even without kmalloc? I thought > > that was with already. That doesn't sound good. Any idea where that slowdown > > comes from? > > Just longer code path, I think. It calls the generic
I did IPI measurements quite some time ago and what I remember from them is that IPI latencies were in the low multiple thousands cycle ballpark.
> smp_call_function_mask(), which then does a popcount on the cpu mask > (which it needs to do anyway), sees only one bit set, and then punts to > the smp_call_function_single() path.
But that is more in the a few tens of cycles (or maybe 1-2 hundreds if you have a NR_CPU==4096 kernel with really large cpumask)
Doesn't really explain a 25% slowdown I would say.
Are you sure there isn't a new cache miss in there or something? Actually it must be even multiple ones to account for such a slow down.
-Andi
-- ak@linux.intel.com
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