From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with SMTP id DC1656B005D for ; Thu, 6 Aug 2009 06:13:15 -0400 (EDT) Message-ID: <4A7AAE07.1010202@redhat.com> Date: Thu, 06 Aug 2009 13:18:47 +0300 From: Avi Kivity MIME-Version: 1.0 Subject: Re: [RFC] respect the referenced bit of KVM guest pages? References: <20090805024058.GA8886@localhost> <20090805155805.GC23385@random.random> <20090806100824.GO23385@random.random> In-Reply-To: <20090806100824.GO23385@random.random> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Andrea Arcangeli Cc: Wu Fengguang , Rik van Riel , "Dike, Jeffrey G" , "Yu, Wilfred" , "Kleen, Andi" , Hugh Dickins , Andrew Morton , Christoph Lameter , KOSAKI Motohiro , Mel Gorman , LKML , linux-mm List-ID: On 08/06/2009 01:08 PM, Andrea Arcangeli wrote: > After some conversation it seems reactivating on large systems > generates troubles to the VM as young bit have excessive time to be > reactivated, giving troubles to shrink active list. I see that, so > then the check should be still nuked, but the unconditional > deactivation should happen instead. Otherwise it's trivial to put the > VM to its knees and DoS it with a simple mmap of a file with MAP_EXEC > as parameter of mmap. My whole point is that deciding if activating or > deactivating pages can't be in function of VM_EXEC, and clearly it > helps on desktops but then it probably is a signal that the VM isn't > good enough by itself to identify the important working set using > young bits and stuff on desktop systems, and if there's a good reason > to not activate, we shouldn't activate the VM_EXEC either as anything > and anybody can generate a file mapping with VM_EXEC set... > Reasonable; if you depend on a hint from userspace, that hint can be used against you. > Likely we need a cut-off point, if we detect it takes more than X > seconds to scan the whole active list, we start ignoring young bits, > as young bits don't provide any meaningful information then and they > just hang the VM in preventing it to shrink active list and looping > over it endlessy with million pages inside that list. But on small > systems if inactive list is short it may be too quick to just clear > the young bit and only giving it time to be re-enabled in inactive > list. That may be the source of the problem. Actually I'm speculating > here, because I barely understood that this is swapin... not sure > exactly what this regression is about but testing the patch posted is > good idea and it will tell us if we just need to dynamically > differentiating the algorithm between large and small systems and start > ignoring young bits only at some point. > How about, for every N pages that you scan, evict at least 1 page, regardless of young bit status? That limits overscanning to a N:1 ratio. With N=250 we'll spend at most 25 usec in order to locate one page to evict. -- error compiling committee.c: too many arguments to function -- 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