From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 1DD946B004D for ; Sun, 16 Aug 2009 01:18:28 -0400 (EDT) Date: Sun, 16 Aug 2009 13:15:02 +0800 From: Wu Fengguang Subject: Re: [RFC] respect the referenced bit of KVM guest pages? Message-ID: <20090816051502.GB13740@localhost> References: <20090805155805.GC23385@random.random> <20090806100824.GO23385@random.random> <4A7AAE07.1010202@redhat.com> <20090806102057.GQ23385@random.random> <20090806105932.GA1569@localhost> <4A7AC201.4010202@redhat.com> <20090806130631.GB6162@localhost> <20090806210955.GA14201@c2.user-mode-linux.org> <20090816031827.GA6888@localhost> <4A87829C.4090908@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4A87829C.4090908@redhat.com> Sender: owner-linux-mm@kvack.org To: Rik van Riel Cc: Jeff Dike , Avi Kivity , Andrea Arcangeli , "Yu, Wilfred" , "Kleen, Andi" , Hugh Dickins , Andrew Morton , Christoph Lameter , KOSAKI Motohiro , Mel Gorman , LKML , linux-mm List-ID: On Sun, Aug 16, 2009 at 11:53:00AM +0800, Rik van Riel wrote: > Wu Fengguang wrote: > > On Fri, Aug 07, 2009 at 05:09:55AM +0800, Jeff Dike wrote: > >> Side question - > >> Is there a good reason for this to be in shrink_active_list() > >> as opposed to __isolate_lru_page? > >> > >> if (unlikely(!page_evictable(page, NULL))) { > >> putback_lru_page(page); > >> continue; > >> } > >> > >> Maybe we want to minimize the amount of code under the lru lock or > >> avoid duplicate logic in the isolate_page functions. > > > > I guess the quick test means to avoid the expensive page_referenced() > > call that follows it. But that should be mostly one shot cost - the > > unevictable pages are unlikely to cycle in active/inactive list again > > and again. > > Please read what putback_lru_page does. > > It moves the page onto the unevictable list, so that > it will not end up in this scan again. Yes it does. I said 'mostly' because there is a small hole that an unevictable page may be scanned but still not moved to unevictable list: when a page is mapped in two places, the first pte has the referenced bit set, the _second_ VMA has VM_LOCKED bit set, then page_referenced() will return 1 and shrink_page_list() will move it into active list instead of unevictable list. Shall we fix this rare case? > >> But if there are important mlock-heavy workloads, this could make the > >> scan come up empty, or at least emptier than we might like. > > > > Yes, if the above 'if' block is removed, the inactive lists might get > > more expensive to reclaim. > > Why? Without the 'if' block, an unevictable page may well be deactivated into inactive list (and some time later be moved to unevictable list from there), increasing the inactive list's scanned:reclaimed ratio. Thanks, Fengguang -- 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