From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx204.postini.com [74.125.245.204]) by kanga.kvack.org (Postfix) with SMTP id 0AD6B6B0068 for ; Mon, 3 Sep 2012 15:05:52 -0400 (EDT) Message-ID: <5044FF89.5090400@redhat.com> Date: Mon, 03 Sep 2012 15:05:45 -0400 From: Rik van Riel MIME-Version: 1.0 Subject: Re: [patch v4]swap: add a simple random read swapin detection References: <20120827040037.GA8062@kernel.org> <503B8997.4040604@openvz.org> <20120830103612.GA12292@kernel.org> <20120830174223.GB2141@barrios> <20120903072137.GA26821@kernel.org> <20120903083245.GA7674@bbox> <20120903114631.GA5410@kernel.org> <5044FEE3.4050009@openvz.org> In-Reply-To: <5044FEE3.4050009@openvz.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Konstantin Khlebnikov Cc: Shaohua Li , Minchan Kim , "akpm@linux-foundation.org" , "linux-mm@kvack.org" , "fengguang.wu@intel.com" On 09/03/2012 03:02 PM, Konstantin Khlebnikov wrote: > Shaohua Li wrote: >> On Mon, Sep 03, 2012 at 05:32:45PM +0900, Minchan Kim wrote: >> Subject: swap: add a simple random read swapin detection >> >> The swapin readahead does a blind readahead regardless if the swapin is >> sequential. This is ok for harddisk and random read, because read big >> size has >> no penality in harddisk, and if the readahead pages are garbage, they >> can be >> reclaimed fastly. But for SSD, big size read is more expensive than >> small size >> read. If readahead pages are garbage, such readahead only has overhead. >> >> This patch addes a simple random read detection like what file mmap >> readahead >> does. If random read is detected, swapin readahead will be skipped. This >> improves a lot for a swap workload with random IO in a fast SSD. >> >> I run anonymous mmap write micro benchmark, which will triger >> swapin/swapout. >> runtime changes with path >> randwrite harddisk -38.7% >> seqwrite harddisk -1.1% >> randwrite SSD -46.9% >> seqwrite SSD +0.3% >> >> For both harddisk and SSD, the randwrite swap workload run time is >> reduced >> significant. sequential write swap workload hasn't chanage. >> >> Interesting is the randwrite harddisk test is improved too. This might be >> because swapin readahead need allocate extra memory, which further tights >> memory pressure, so more swapout/swapin. > > Generally speaking swapin readahread isn't usable while system is under > memory > pressure. Cache hit isn't very probable, because reclaimer allocates swap > entries in page-LRU order. > > But swapin readahead is very useful if system recovers from memory > pressure, > it helps to read whole swap back to memory (a sort of desktop scenario). > > So, I think we can simply disable swapin readahead while system is under > memory > pressure. For example in time-based manner -- enable it only after grace > period > after last swap_writepage(). Determining "under memory pressure" is pretty hard to do. However, Shaohua's patch provides an easy way to see whether swap readahead is helping (we are getting pages from the swap cache), or whether it is not (pages got evicted before someone faulted on them). In short, Shaohua's patch not only does roughly what you want, it does it in a simple way. -- All rights reversed -- 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