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.
-- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>