Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751966AbaFEOQT (ORCPT ); Thu, 5 Jun 2014 10:16:19 -0400 Received: from mail-ve0-f177.google.com ([209.85.128.177]:60016 "EHLO mail-ve0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751438AbaFEOQR (ORCPT ); Thu, 5 Jun 2014 10:16:17 -0400 MIME-Version: 1.0 In-Reply-To: <53907895.8090102@kernel.dk> 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> Date: Thu, 5 Jun 2014 22:16:16 +0800 Message-ID: Subject: Re: blk-mq: bitmap tag: performance degradation? From: Ming Lei To: Jens Axboe Cc: Alexander Gordeev , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanks, -- Ming Lei -- 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/