linux-kernel - Re: [PATCH RFC 0/2] percpu_ida: Take into account CPU topology when stealing tags

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Date:	Fri, 02 May 2014 10:41:40 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Kent Overstreet <kmo@...erainc.com>,
	Ming Lei <tom.leiming@...il.com>,
	Alexander Gordeev <agordeev@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Shaohua Li <shli@...nel.org>,
	Nicholas Bellinger <nab@...ux-iscsi.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC 0/2] percpu_ida: Take into account CPU topology when
 stealing tags

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

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