Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754569Ab0AEO7M (ORCPT ); Tue, 5 Jan 2010 09:59:12 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754472Ab0AEO7L (ORCPT ); Tue, 5 Jan 2010 09:59:11 -0500 Received: from mx1.redhat.com ([209.132.183.28]:31681 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754496Ab0AEO7J convert rfc822-to-8bit (ORCPT ); Tue, 5 Jan 2010 09:59:09 -0500 From: Jeff Moyer To: Corrado Zoccolo Cc: Vivek Goyal , Jens Axboe , Linux-Kernel , Shaohua Li , Gui Jianfeng Subject: Re: [PATCH] cfq-iosched: non-rot devices do not need read queue merging References: <20091230213439.GQ4489@kernel.dk> <1262211768-10858-1-git-send-email-czoccolo@gmail.com> <20100104144711.GA7968@redhat.com> <4e5e476b1001040836p2c8d7486x807a1a89b61c2458@mail.gmail.com> <4e5e476b1001041037x6aa63be6ncfa523a7df78bb0d@mail.gmail.com> <20100104185100.GF7968@redhat.com> <4e5e476b1001041237v71952c8ewaaef3778353f7521@mail.gmail.com> X-PGP-KeyID: 1F78E1B4 X-PGP-CertKey: F6FE 280D 8293 F72C 65FD 5A58 1FF8 A7CA 1F78 E1B4 X-PCLoadLetter: What the f**k does that mean? Date: Tue, 05 Jan 2010 09:58:52 -0500 In-Reply-To: <4e5e476b1001041237v71952c8ewaaef3778353f7521@mail.gmail.com> (Corrado Zoccolo's message of "Mon, 4 Jan 2010 21:37:05 +0100") Message-ID: User-Agent: Gnus/5.110011 (No Gnus v0.11) Emacs/23.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2365 Lines: 58 Corrado Zoccolo writes: > On Mon, Jan 4, 2010 at 8:04 PM, Jeff Moyer wrote: >> Vivek Goyal writes: >>>> >>> Hi Corrado, >>>> >>> >>>> >>> What's the reason that reads don't benefit from merging queues and hence >>>> >>> merging requests and only writes do on SSD? >>>> >> >>>> >> On SSDs, reads are just limited by the maximum transfer rate, and >>>> >> larger (i.e. merged) reads will just take proportionally longer. >>>> > >>>> > This is simply not true.  You can get more bandwidth from an SSD (I just >>>> > checked numbers for 2 vendors' devices) by issuing larger read requests, >>>> > no matter whether the access pattern is sequential or random. >>>> I know, but the performance increase given the size is sublinear, and >>>> the situation here is slightly different. >>>> In order for the requests to be merged, they have to be submitted concurrently. >>>> So you have to compare 2 concurrent requests of size x with one >>>> request of size 2*x (with some CPU overhead). >>>> Moreover, you always pay the CPU overhead, even if you can't do the >>>> merging, and you must be very lucky to keep merging, because it means >>>> the two processes are working in lockstep; it is not sufficient that >>>> the requests are just nearby, as for rotational disks. >>>> >>> >>> For jeff, at least "dump" utility threads were kind of working in lockstep >>> for writes and he gained significantly by merging these queues together. >> >> Actually, it was for reads. >> >>> So the argument is that CPU overhead saving in this case is more substantial >>> as compared to gains made by lockstep read threads. I think we shall have to >>> have some numbers to justify that. >> >> Agreed.  Corrado, I know you don't have the hardware, so I'll give this >> a run through the read-test2 program and see if it regresses at all. > Great. I ran the test program 50 times, and here are the results: ==> vanilla <== Mean: 163.22728 Population Std. Dev.: 0.55401 ==> patched <== Mean: 162.91558 Population Std. Dev.: 1.08612 This looks acceptable to me. Cheers, Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/