From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx106.postini.com [74.125.245.106]) by kanga.kvack.org (Postfix) with SMTP id 7BE546B0072 for ; Thu, 25 Oct 2012 17:21:33 -0400 (EDT) Date: Fri, 26 Oct 2012 00:22:51 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page Message-ID: <20121025212251.GA31749@shutemov.name> References: <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> <20121025140524.17083937.akpm@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121025140524.17083937.akpm@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: "Kirill A. Shutemov" , Andrea Arcangeli , linux-mm@kvack.org, Andi Kleen , "H. Peter Anvin" , linux-kernel@vger.kernel.org On Thu, Oct 25, 2012 at 02:05:24PM -0700, Andrew Morton wrote: > 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?? nr_to_scan == 0 asks for the fast path. shrinker callback can shink, if it thinks it's good idea. > > > 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. What about ratelimit on alloc path to force fallback if we allocate to often? Is it good idea? > 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? Okay, I'll prepare patch. -- Kirill A. Shutemov -- 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