From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f41.google.com (mail-pb0-f41.google.com [209.85.160.41]) by kanga.kvack.org (Postfix) with ESMTP id 6E20E6B0035 for ; Sun, 22 Dec 2013 21:11:23 -0500 (EST) Received: by mail-pb0-f41.google.com with SMTP id jt11so4742567pbb.14 for ; Sun, 22 Dec 2013 18:11:23 -0800 (PST) Received: from LGEMRELSE1Q.lge.com (LGEMRELSE1Q.lge.com. [156.147.1.111]) by mx.google.com with ESMTP id yd9si11216407pab.31.2013.12.22.18.11.20 for ; Sun, 22 Dec 2013 18:11:22 -0800 (PST) Date: Mon, 23 Dec 2013 11:11:19 +0900 From: Joonsoo Kim Subject: Re: [PATCH v3 13/14] mm, hugetlb: retry if failed to allocate and there is concurrent user Message-ID: <20131223021118.GA2487@lge.com> References: <1387349640-8071-1-git-send-email-iamjoonsoo.kim@lge.com> <1387349640-8071-14-git-send-email-iamjoonsoo.kim@lge.com> <20131219170202.0df2d82a2adefa3ab616bdaa@linux-foundation.org> <20131220140153.GC11295@suse.de> <1387608497.3119.17.camel@buesod1.americas.hpqcorp.net> <20131223004438.GA19388@lge.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131223004438.GA19388@lge.com> Sender: owner-linux-mm@kvack.org List-ID: To: Davidlohr Bueso Cc: Mel Gorman , Andrew Morton , Rik van Riel , Michal Hocko , "Aneesh Kumar K.V" , KAMEZAWA Hiroyuki , Hugh Dickins , Davidlohr Bueso , David Gibson , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Wanpeng Li , Naoya Horiguchi , Hillf Danton , aswin@hp.com On Mon, Dec 23, 2013 at 09:44:38AM +0900, Joonsoo Kim wrote: > On Fri, Dec 20, 2013 at 10:48:17PM -0800, Davidlohr Bueso wrote: > > On Fri, 2013-12-20 at 14:01 +0000, Mel Gorman wrote: > > > On Thu, Dec 19, 2013 at 05:02:02PM -0800, Andrew Morton wrote: > > > > On Wed, 18 Dec 2013 15:53:59 +0900 Joonsoo Kim wrote: > > > > > > > > > If parallel fault occur, we can fail to allocate a hugepage, > > > > > because many threads dequeue a hugepage to handle a fault of same address. > > > > > This makes reserved pool shortage just for a little while and this cause > > > > > faulting thread who can get hugepages to get a SIGBUS signal. > > > > > > > > > > To solve this problem, we already have a nice solution, that is, > > > > > a hugetlb_instantiation_mutex. This blocks other threads to dive into > > > > > a fault handler. This solve the problem clearly, but it introduce > > > > > performance degradation, because it serialize all fault handling. > > > > > > > > > > Now, I try to remove a hugetlb_instantiation_mutex to get rid of > > > > > performance degradation. > > > > > > > > So the whole point of the patch is to improve performance, but the > > > > changelog doesn't include any performance measurements! > > > > > > > > > > I don't really deal with hugetlbfs any more and I have not examined this > > > series but I remember why I never really cared about this mutex. It wrecks > > > fault scalability but AFAIK fault scalability almost never mattered for > > > workloads using hugetlbfs. The most common user of hugetlbfs by far is > > > sysv shared memory. The memory is faulted early in the lifetime of the > > > workload and after that it does not matter. At worst, it hurts application > > > startup time but that is still poor motivation for putting a lot of work > > > into removing the mutex. > > > > Yep, important hugepage workloads initially pound heavily on this lock, > > then it naturally decreases. > > > > > Microbenchmarks will be able to trigger problems in this area but it'd > > > be important to check if any workload that matters is actually hitting > > > that problem. > > > > I was thinking of writing one to actually get some numbers for this > > patchset -- I don't know of any benchmark that might stress this lock. > > > > However I first measured the amount of cycles it costs to start an > > Oracle DB and things went south with these changes. A simple 'startup > > immediate' calls hugetlb_fault() ~5000 times. For a vanilla kernel, this > > costs ~7.5 billion cycles and with this patchset it goes up to ~27.1 > > billion. While there is naturally a fair amount of variation, these > > changes do seem to do more harm than good, at least in real world > > scenarios. > > Hello, > > I think that number of cycles is not proper to measure this patchset, > because cycles would be wasted by fault handling failure. Instead, it > targeted improved elapsed time. Could you tell me how long it > takes to fault all of it's hugepages? > > Anyway, this order of magnitude still seems a problem. :/ > > I guess that cycles are wasted by zeroing hugepage in fault-path like as > Andrew pointed out. > > I will send another patches to fix this problem. Hello, Davidlohr. Here goes the fix on top of this series. Thanks. -------------->8---------------------------