Messages in this thread | | | Date | Wed, 11 Sep 2013 16:21:46 -0700 | From | Cody P Schafer <> | Subject | Re: [RFC][PATCH] mm: percpu pages: up batch size to fix arithmetic?? errror |
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On 09/11/2013 04:08 PM, Cody P Schafer wrote: > On 09/11/2013 03:08 PM, Dave Hansen wrote: >> I really don't know where the: >> >> batch /= 4; /* We effectively *= 4 below */ >> ... >> batch = rounddown_pow_of_two(batch + batch/2) - 1; >> >> came from. The round down code at *MOST* does a *= 1.5, but >> *averages* out to be just under 1. >> >> On a system with 128GB in a zone, this means that we've got >> (you can see in /proc/zoneinfo for yourself): >> >> high: 186 (744kB) >> batch: 31 (124kB) >> >> That 124kB is almost precisely 1/4 of the "1/2 of a meg" that we >> were shooting for. We're under-sizing the batches by about 4x. >> This patch kills the /=4. >> >> --- >> diff -puN mm/page_alloc.c~debug-pcp-sizes-1 mm/page_alloc.c >> --- linux.git/mm/page_alloc.c~debug-pcp-sizes-1 2013-09-11 >> 14:41:08.532445664 -0700 >> +++ linux.git-davehans/mm/page_alloc.c 2013-09-11 >> 15:03:47.403912683 -0700 >> @@ -4103,7 +4103,6 @@ static int __meminit zone_batchsize(stru >> batch = zone->managed_pages / 1024; >> if (batch * PAGE_SIZE > 512 * 1024) >> batch = (512 * 1024) / PAGE_SIZE; >> - batch /= 4; /* We effectively *= 4 below */ >> if (batch < 1) >> batch = 1; >> >> _ >> > > Looking back at the first git commit (way before my time), it appears > that the percpu pagesets initially had a ->high and ->low (now removed), > set to batch*6 and batch*2 respectively. I assume the idea was to keep > the number of pages in the percpu pagesets around batch*4, hence the > comment. > > So we have this variable called "batch", and the code is trying to store > the _average_ number of pcp pages we want into it (not the batchsize), > and then we divide our "average" goal by 4 to get a batchsize. All the > comments refer to the size of the pcp pagesets, not to the pcp pageset > batchsize. > > Looking further, in current code we don't refill the pcp pagesets unless > they are completely empty (->low was removed a while ago), and then we > only add ->batch pages. > > Has anyone looked at what type of average pcp sizing the current code > results in?
Also, we may want to consider shrinking pcp->high down from 6*pcp->batch given that the original "6*" choice was based upon ->batch actually being 1/4th of the average pageset size, where now it appears closer to being the average.
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