From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965736Ab2JYVF2 (ORCPT ); Thu, 25 Oct 2012 17:05:28 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:54354 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760257Ab2JYVF0 (ORCPT ); Thu, 25 Oct 2012 17:05:26 -0400 Date: Thu, 25 Oct 2012 14:05:24 -0700 From: Andrew Morton To: "Kirill A. Shutemov" Cc: "Kirill A. Shutemov" , Andrea Arcangeli , linux-mm@kvack.org, Andi Kleen , "H. Peter Anvin" , linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page Message-Id: <20121025140524.17083937.akpm@linux-foundation.org> In-Reply-To: <20121025204959.GA27251@otc-wbsnb-06> References: <20121018164502.b32791e7.akpm@linux-foundation.org> <20121018235941.GA32397@shutemov.name> <20121023063532.GA15870@shutemov.name> <20121022234349.27f33f62.akpm@linux-foundation.org> <20121023070018.GA18381@otc-wbsnb-06> <20121023155915.7d5ef9d1.akpm@linux-foundation.org> <20121023233801.GA21591@shutemov.name> <20121024122253.5ecea992.akpm@linux-foundation.org> <20121024194552.GA24460@otc-wbsnb-06> <20121024132552.5f9a5f5b.akpm@linux-foundation.org> <20121025204959.GA27251@otc-wbsnb-06> X-Mailer: Sylpheed 3.0.2 (GTK+ 2.20.1; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 25 Oct 2012 23:49:59 +0300 "Kirill A. Shutemov" wrote: > On Wed, Oct 24, 2012 at 01:25:52PM -0700, Andrew Morton wrote: > > On Wed, 24 Oct 2012 22:45:52 +0300 > > "Kirill A. Shutemov" wrote: > > > > > On Wed, Oct 24, 2012 at 12:22:53PM -0700, Andrew Morton wrote: > > > > > > > > I'm thinking that such a workload would be the above dd in parallel > > > > with a small app which touches the huge page and then exits, then gets > > > > executed again. That "small app" sounds realistic to me. Obviously > > > > one could exercise the zero page's refcount at higher frequency with a > > > > tight map/touch/unmap loop, but that sounds less realistic. It's worth > > > > trying that exercise as well though. > > > > > > > > Or do something else. But we should try to probe this code's > > > > worst-case behaviour, get an understanding of its effects and then > > > > decide whether any such workload is realisic enough to worry about. > > > > > > Okay, I'll try few memory pressure scenarios. > > A test program: > > while (1) { > posix_memalign((void **)&p, 2 * MB, 2 * MB); > assert(*p == 0); > free(p); > } > > With this code in background we have pretty good chance to have huge zero > page freeable (refcount == 1) when shrinker callback called - roughly one > of two. > > Pagecache hog (dd if=hugefile of=/dev/null bs=1M) creates enough pressure > to get shrinker callback called, but it was only asked about cache size > (nr_to_scan == 0). > I was not able to get it called with nr_to_scan > 0 on this scenario, so > hzp never freed. hm. It's odd that the kernel didn't try to shrink slabs in this case. Why didn't it?? > I also tried another scenario: usemem -n16 100M -r 1000. It creates real > memory pressure - no easy reclaimable memory. This time callback called > with nr_to_scan > 0 and we freed hzp. Under pressure we fails to allocate > hzp and code goes to fallback path as it supposed to. > > Do I need to check any other scenario? I'm thinking that if we do hit problems in this area, we could avoid freeing the hugepage unless the scan_control.priority is high enough. That would involve adding a magic number or a tunable to set the threshold. Also, it would be beneficial if we can monitor this easily. Perhaps add a counter to /proc/vmstat which tells us how many times that page has been reallocated? And perhaps how many times we tried to allocate it but failed?