From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx155.postini.com [74.125.245.155]) by kanga.kvack.org (Postfix) with SMTP id 825C16B0031 for ; Wed, 11 Sep 2013 19:21:51 -0400 (EDT) Received: from /spool/local by e35.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 11 Sep 2013 17:21:50 -0600 Received: from d03relay02.boulder.ibm.com (d03relay02.boulder.ibm.com [9.17.195.227]) by d03dlp01.boulder.ibm.com (Postfix) with ESMTP id 2DC351FF001A for ; Wed, 11 Sep 2013 17:21:46 -0600 (MDT) Received: from d03av04.boulder.ibm.com (d03av04.boulder.ibm.com [9.17.195.170]) by d03relay02.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r8BNLml0326290 for ; Wed, 11 Sep 2013 17:21:48 -0600 Received: from d03av04.boulder.ibm.com (loopback [127.0.0.1]) by d03av04.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r8BNLmOM013757 for ; Wed, 11 Sep 2013 17:21:48 -0600 Message-ID: <5230FB0A.70901@linux.vnet.ibm.com> Date: Wed, 11 Sep 2013 16:21:46 -0700 From: Cody P Schafer MIME-Version: 1.0 Subject: Re: [RFC][PATCH] mm: percpu pages: up batch size to fix arithmetic?? errror References: <20130911220859.EB8204BB@viggo.jf.intel.com> <5230F7DD.90905@linux.vnet.ibm.com> In-Reply-To: <5230F7DD.90905@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Dave Hansen Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, cl@linux.com 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@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org