From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752944AbaEBQlm (ORCPT <rfc822;w@1wt.eu>);
	Fri, 2 May 2014 12:41:42 -0400
Received: from mail-pa0-f49.google.com ([209.85.220.49]:64676 "EHLO
	mail-pa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752726AbaEBQll (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 2 May 2014 12:41:41 -0400
Message-ID: <5363CAC4.4040306@kernel.dk>
Date: Fri, 02 May 2014 10:41:40 -0600
From: Jens Axboe <axboe@kernel.dk>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0
MIME-Version: 1.0
To: Christoph Hellwig <hch@infradead.org>
CC: Kent Overstreet <kmo@daterainc.com>, Ming Lei <tom.leiming@gmail.com>,
        Alexander Gordeev <agordeev@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Shaohua Li <shli@kernel.org>, Nicholas Bellinger <nab@linux-iscsi.org>,
        Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH RFC 0/2] percpu_ida: Take into account CPU topology when
 stealing tags
References: <CACVXFVMXh5ZP+ZsO2fV2mdKJrJF9AHg58v5KWmt_ZbEWvszsLw@mail.gmail.com> <535716A5.6050108@kernel.dk> <CACVXFVN-roORYH8b-qZ10e2_wrL1W5-xO2P=A_ARPFL2GqDn2Q@mail.gmail.com> <535AD235.90604@kernel.dk> <CACVXFVPkiQT6yxse=sX1yuX8wLbr1sgbT4chQycbbPDR_A6hqA@mail.gmail.com> <535B13D7.4050202@kernel.dk> <CACVXFVORX_qgJNXcB3OFpscVx0eQAbLPv2Jr1N3_yBKrPsGXFA@mail.gmail.com> <53601602.5060306@kernel.dk> <20140501224744.GA2285@kmo-pixel> <536300BB.5060906@kernel.dk> <20140502050523.GA21708@infradead.org>
In-Reply-To: <20140502050523.GA21708@infradead.org>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 05/01/2014 11:05 PM, Christoph Hellwig wrote:
> On Thu, May 01, 2014 at 08:19:39PM -0600, Jens Axboe wrote:
>> I've taken the consequence of this and implemented another tagging
>> scheme that blk-mq will use if it deems that percpu_ida isn't going
>> to be effective for the device being initialized. But I really hate
>> to have both of them in there. Unfortunately I have no devices
>> available that have a tag space that will justify using percu_ida,
>> so comparisons are a bit hard at the moment. NVMe should change
>> that, though, so decision will have to be deferred until that is
>> tested.
> 
> At least for SCSI devices _tag space_ is plenty, it's just the we
> artifically limit our tag space to the queue depth to avoid having to
> track that one separately.  In addition we also preallocaste a request
> for each tag, so even if we would track the queue depth separately
> we would waste a lot of memory.

In practice it comes out to the same, it's not feasible to run a much
larger space and track on queue depth. So I don't think that changes the
conclusion for SCSI.


-- 
Jens Axboe