On Wednesday 27 August 2008 17:05, David Miller wrote: > From: Nick Piggin <nickpiggin@xxxxxxxxxxxx> > Date: Wed, 27 Aug 2008 16:54:32 +1000 > > > 5% is a pretty nasty performance hit... what sort of benchmarks are we > > talking about here? > > > > I just made some pretty crazy changes to the VM to get "only" around 5 > > or so % performance improvement in some workloads. > > > > What places are making heavy use of cpumasks that causes such a slowdown? > > Hopefully callers can mostly be improved so they don't need to use > > cpumasks for common cases. > > It's almost certainly from the cross-call dispatch call chain. > > As just one example, just to do a TLB flush mm->cpu_vm_mask probably > gets passed around as an aggregate two or three times on the way down > to the APIC programming code on x86. That's two or three 512 byte > copies on the stack :) Yeah, I see. That's stupid isn't it? (Well, I guess it was completely sane when cpumasks were word sized ;)) Hopefully that accounts for a significant chunk... -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
