From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 668C86B004D for ; Wed, 17 Jun 2009 18:43:52 -0400 (EDT) Date: Thu, 18 Jun 2009 00:41:49 +0200 From: Johannes Weiner Subject: Re: [patch v3] swap: virtual swap readahead Message-ID: <20090617224149.GA16104@cmpxchg.org> References: <20090609190128.GA1785@cmpxchg.org> <20090611143122.108468f1.kamezawa.hiroyu@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090611143122.108468f1.kamezawa.hiroyu@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org To: KAMEZAWA Hiroyuki Cc: Andrew Morton , Rik van Riel , Hugh Dickins , Andi Kleen , Wu Fengguang , Minchan Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Thu, Jun 11, 2009 at 02:31:22PM +0900, KAMEZAWA Hiroyuki wrote: > On Tue, 9 Jun 2009 21:01:28 +0200 > Johannes Weiner wrote: > > [resend with lists cc'd, sorry] > > > > +static int swap_readahead_ptes(struct mm_struct *mm, > > + unsigned long addr, pmd_t *pmd, > > + swp_entry_t *entries, > > + unsigned long cluster) > > +{ > > + unsigned long window, min, max, limit; > > + spinlock_t *ptl; > > + pte_t *ptep; > > + int i, nr; > > + > > + window = cluster << PAGE_SHIFT; > > + min = addr & ~(window - 1); > > + max = min + cluster; > > Johannes, I wonder there is no reason to use "alignment". I am wondering too. I digged into the archives but the alignment comes from a change older than what history.git documents, so I wasn't able to find written down justification for this. > I think we just need to read "nearby" pages. Then, this function's > scan range should be > > [addr - window/2, addr + window/2) > or some. > > And here, too > > + if (!entries) /* XXX: shmem case */ > > + return swapin_readahead_phys(entry, gfp_mask, vma, addr); > > + pmin = swp_offset(entry) & ~(cluster - 1); > > + pmax = pmin + cluster; > > pmin = swp_offset(entry) - cluster/2. > pmax = swp_offset(entry) + cluster/2. > > I'm sorry if I miss a reason for using "alignment". Perhas someone else knows a good reason for it, but I think it could even be harmful. Chances are that several processes fault around the same slots simultaneously. By letting them all start at the same aligned offset we have a maximum race between them and they all allocate pages for the same slots concurrently. By placing the window unaligned we decrease this overlapping, so it sounds like a good idea. It would increase the amount of readahead done even more, though, and Fengguang already measured degradation in IO latency with my patch, so this probably needs more changes to work well. -- 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