Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752257AbaFERR0 (ORCPT ); Thu, 5 Jun 2014 13:17:26 -0400 Received: from mail-pb0-f48.google.com ([209.85.160.48]:62612 "EHLO mail-pb0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751000AbaFERRZ (ORCPT ); Thu, 5 Jun 2014 13:17:25 -0400 Message-ID: <5390A624.8020308@kernel.dk> Date: Thu, 05 Jun 2014 11:17:24 -0600 From: Jens Axboe User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: Ming Lei CC: Alexander Gordeev , Linux Kernel Mailing List Subject: Re: blk-mq: bitmap tag: performance degradation? References: <20140604103544.GA11350@dhcp-26-207.brq.redhat.com> <538F2AC2.1060904@kernel.dk> <20140605140127.GA22198@dhcp-26-207.brq.redhat.com> <53907895.8090102@kernel.dk> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/05/2014 08:16 AM, Ming Lei wrote: > On Thu, Jun 5, 2014 at 10:03 PM, Jens Axboe wrote: >> On 2014-06-05 08:01, Alexander Gordeev wrote: >>> >>> On Wed, Jun 04, 2014 at 08:18:42AM -0600, Jens Axboe wrote: >>>> >>>> A null_blk test is the absolute best case for percpu_ida, since >>>> there are enough tags and everything is localized. The above test is >>>> more useful for testing blk-mq than any real world application of >>>> the tagging. >>>> >>>> I've done considerable testing on both 2 and 4 socket (32 and 64 >>>> CPUs) and bitmap tagging is better in a much wider range of >>>> applications. This includes even high tag depth devices like nvme, >>>> and more normal ranges like mtip32xx and scsi-mq setups. >>> >>> >>> Just for the record: bitmap tags on a 48 CPU box with NVMe device >>> indeed shows almost the same performance/cache rate as the stock >>> kernel. >> >> >> Thanks for confirming. It's one of the dangers of null_blk, it's not always >> a very accurate simulation of what a real device will do. I think it's >> mostly a completion side thing, would be great with a small device that >> supported msi-x and could be used as an irq trigger :-) > > Maybe null_blk at IRQ_TIMER mode is more close to > a real device, and I guess the result may be different with > mode IRQ_NONE/IRQ_SOFTIRQ. It'd be closer in behavior, but the results might then be skewed by hitting the timer way too hard. And it'd be a general slowdown, again possibly skewing it. But I haven't tried with the timer completion, to see if that yields more accurate modelling for this test, so it might actually be a lot better. -- Jens Axboe -- 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/