Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753299AbcCVU1e (ORCPT ); Tue, 22 Mar 2016 16:27:34 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44766 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752326AbcCVU1b (ORCPT ); Tue, 22 Mar 2016 16:27:31 -0400 From: Jeff Moyer To: Jens Axboe Cc: , , Subject: Re: [PATCH 6/6] writeback: throttle buffered writeback References: <1458669320-6819-1-git-send-email-axboe@fb.com> <1458669320-6819-7-git-send-email-axboe@fb.com> <56F1A8D0.4060403@fb.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, 22 Mar 2016 16:27:29 -0400 In-Reply-To: <56F1A8D0.4060403@fb.com> (Jens Axboe's message of "Tue, 22 Mar 2016 14:19:28 -0600") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 717 Lines: 22 Jens Axboe writes: > 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. Ah, makes perfect sense. Thanks! -Jeff