From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753234AbdKJPnT (ORCPT ); Fri, 10 Nov 2017 10:43:19 -0500 Received: from mail-qt0-f174.google.com ([209.85.216.174]:56350 "EHLO mail-qt0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751885AbdKJPnS (ORCPT ); Fri, 10 Nov 2017 10:43:18 -0500 X-Google-Smtp-Source: AGs4zMbo0Ys87KUSTdNHAEelXWeIFmlPtUFkN/eJVUZ3C/njVBsU/WErqUeBYKNwZzEMwJxeZnUkDA== Date: Fri, 10 Nov 2017 07:43:14 -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: <20171110154314.GE983427@devbig577.frc2.facebook.com> References: <20171109221924.GB983427@devbig577.frc2.facebook.com> <20171109231212.mbqwyzpmciyshxov@kernel.org> <20171109234258.GD983427@devbig577.frc2.facebook.com> <20171110042713.quytcwbu6g6xwvpt@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171110042713.quytcwbu6g6xwvpt@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 08:27:13PM -0800, Shaohua Li wrote: > I think the absolute latency would only work for HD. For a SSD, a 4k latency > probably is 60us and 1M latency is 500us. The disk must be very contended to > make 4k latency reach 500us. Not sensitive doesn't mean no protection. If the > use case sets rough latency, say 1ms, there will be no protection for 4k IO at > all. The baseline latency is pretty reliable for SSD actually. So I'd rather I don't understand how that would mean no protection. The latency naturally includes the queueing time on the host side and, even for a fast SSD device, it isn't too difficult to saturate the device to the point where the host-side waiting time becomes pretty long. All that's necessary is IOs being issued faster than completed and we can almost always do that. > keeping the baseline latency for SSD but using absolute latency for HD, which > can be done easily by setting DFL_HD_BASELINE_LATENCY to 0. I don't think that'd be a good interface choice. It's too misleading. If we actually need to specify baseline + margin, it'd probably be better to add another notation - say, "+N" - than overloading the meaning of "N". Thanks. -- tejun