From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755350AbdKIXnD (ORCPT ); Thu, 9 Nov 2017 18:43:03 -0500 Received: from mail-qk0-f170.google.com ([209.85.220.170]:56998 "EHLO mail-qk0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755030AbdKIXnC (ORCPT ); Thu, 9 Nov 2017 18:43:02 -0500 X-Google-Smtp-Source: AGs4zMaQfYxnjIpmhkHXTKcRm6rF6yZLwfjiiq7NcsySm1+klzM9o5pUZI3LMz38YM6BP20ZQLOV9w== Date: Thu, 9 Nov 2017 15:42:58 -0800 From: Tejun Heo To: Shaohua Li Cc: Jens Axboe , linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 1/2] blk-throtl: make latency= absolute Message-ID: <20171109234258.GD983427@devbig577.frc2.facebook.com> References: <20171109221924.GB983427@devbig577.frc2.facebook.com> <20171109231212.mbqwyzpmciyshxov@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171109231212.mbqwyzpmciyshxov@kernel.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Shaohua. On Thu, Nov 09, 2017 at 03:12:12PM -0800, Shaohua Li wrote: > The percentage latency makes sense, but the absolute latency doesn't to me. A > 4k IO latency could be much smaller than 1M IO latency. If we don't add > baseline latency, we can't specify a latency target which works for both 4k and > 1M IO. It isn't adaptive for sure. I think it's still useful for the following reasons. 1. The absolute latency target is by nature both workload and device dependent. For a lot of use cases, coming up with a decent number should be possible. 2. There are many use cases which aren't sensitive to the level where they care much about the different between small and large requests. e.g. protecting a managerial job so that it doesn't completely stall doesn't require tuning things to that level. A value which is comfortably higher than usually expected latencies would often be enough (say 100ms). 3. It's also useful for verification / testing. Thanks. -- tejun