Messages in this thread | | | Subject | Re: hackbench regression due to commit 9dfc6e68bfe6e | From | "Zhang, Yanmin" <> | Date | Tue, 06 Apr 2010 16:28:17 +0800 |
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On Tue, 2010-04-06 at 10:27 +0900, Tejun Heo wrote: > Hello, > > On 04/06/2010 02:30 AM, Pekka Enberg wrote: > >> Hmnmmm... The dynamic percpu areas use page tables and that data is used > >> in the fast path. Maybe the high thread count causes tlb trashing? > > > > Hmm indeed. I don't see anything particularly funny in the SLUB percpu > > conversion so maybe this is a more issue with the new percpu > > allocator? > > By default, percpu allocator embeds the first chunk in the kernel > linear mapping and accesses there shouldn't involve any TLB overhead. > >From the second chunk on, they're mapped page-by-page into vmalloc > area. This can be updated to use larger page mapping but 2M page > per-cpu is pretty large and the trade off hasn't been right yet. > > The amount reserved for dynamic allocation in the first chunk is > determined by PERCPU_DYNAMIC_RESERVE constant in > include/linux/percpu.h. It's currently 20k on 64bit machines and 12k > on 32bit. The intention was to size this such that most common stuff > is allocated from this area. The 20k and 12k are numbers that I > pulled out of my ass :-) with the custom config I used. Now that more > stuff has been converted to dynamic percpu, it's quite possible that > the area is too small. Can you please try to increase the size of the > area (say 2 or 4 times) and see whether the performance regression > goes away? Thanks. I tried 2 and 4 times and didn't see much improvement. I checked /proc/vamallocinfo and it doesn't have item of pcpu_get_vm_areas when I use 4 times of PERCPU_DYNAMIC_RESERVE.
I used perf to collect dtlb misses and LLC misses. dtlb miss data is not stable. Sometimes, we have a bigger dtlb miss, but get a better result.
LLC misses data are more stable. Only LLC-load-misses is the clear sign now. LLC-store-misses has no big difference.
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