From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 30 Apr 2008 12:33:53 +0100 (BST) From: Hugh Dickins Subject: Re: Page Faults slower in 2.6.25-rc9 than 2.6.23 In-Reply-To: <20080430135035.b0b02533.kamezawa.hiroyu@jp.fujitsu.com> Message-ID: References: <20080430135035.b0b02533.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: KAMEZAWA Hiroyuki Cc: Ross Biro , linux-mm@kvack.org, lkml List-ID: On Wed, 30 Apr 2008, KAMEZAWA Hiroyuki wrote: > On Tue, 29 Apr 2008 09:10:36 -0400 > "Ross Biro" wrote: > > I don't know if this has been noticed before. I was benchmarking my > > page table relocation code and I noticed that on 2.6.25-rc9 page > > faults take 10% more time than on 2.6.22. This is using lmbench > > running on an intel x86_64 system. The good news is that the page > > table relocation code now only adds a 1.6% slow down to page faults. > > It seems lmbench's pagefault program uses 'page fault by READ'. > Then, this patch affects. (this patch was added at 2.6.24-rc?.) > == > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=557ed1fa2620dc119adb86b34c614e152a629a80 > == > By it, ZERO_PAGE is not used for page fault in anonymous mapping. I'd wondered about that one too, but no: lmbench lat_pagefault uses a shared mmap of an ordinary file (not /dev/zero), so the ZERO_PAGE changes should have no effect on it whatsoever. I notice that test is expecting msync(,,MS_INVALIDATE) to do something it's never done on Linux (a kind of drop caches for the range). We've never done anything with MS_INVALIDATE, beyond permitting the flag: I think you find problems however you try to go about implementing it (and it might even originate from a UNIX which couldn't do shared mmap coherently). So I wonder if that test is erratic because of it. Hugh -- 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