Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752214AbcCVUTl (ORCPT ); Tue, 22 Mar 2016 16:19:41 -0400 Received: from mx0b-00082601.pphosted.com ([67.231.153.30]:45072 "EHLO mx0b-00082601.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750901AbcCVUTj (ORCPT ); Tue, 22 Mar 2016 16:19:39 -0400 Subject: Re: [PATCH 6/6] writeback: throttle buffered writeback To: Jeff Moyer References: <1458669320-6819-1-git-send-email-axboe@fb.com> <1458669320-6819-7-git-send-email-axboe@fb.com> CC: , , From: Jens Axboe Message-ID: <56F1A8D0.4060403@fb.com> Date: Tue, 22 Mar 2016 14:19:28 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [192.168.54.13] X-Proofpoint-Spam-Reason: safe X-FB-Internal: Safe X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-03-22_09:,, signatures=0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 637 Lines: 18 On 03/22/2016 02:12 PM, Jeff Moyer wrote: > Hi, Jens, > > Jens Axboe writes: > >> If the device has write back caching, 'wb_cache_delay' delays by >> this amount of usecs when a write completes before allowing more. > > What's the reason behind that? For classic write back caching, the cache can absorb a bunch of writes shortly, which means that the completion cost only shows a small part of the overall cost. This means that if we just throttle on completion, then when the device starts committing to media, then we'll end up starving other IO anyway. This knob is a way to attempt to tame that. -- Jens Axboe