linux-kernel - Re: [PATCH 5/6] blk-mq: Use all available hardware queues

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Date:	Tue, 09 Dec 2014 09:10:51 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Bart Van Assche <bvanassche@....org>
CC:	Christoph Hellwig <hch@....de>, Robert Elliott <elliott@...com>,
	Ming Lei <ming.lei@...onical.com>,
	Alexander Gordeev <agordeev@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] blk-mq: Use all available hardware queues

On 12/09/2014 08:59 AM, Bart Van Assche wrote:
> Suppose that a system has two CPU sockets, three cores per socket,
> that it does not support hyperthreading and that four hardware
> queues are provided by a block driver. With the current algorithm
> this will lead to the following assignment of CPU cores to hardware
> queues:
>
>    HWQ 0: 0 1
>    HWQ 1: 2 3
>    HWQ 2: 4 5
>    HWQ 3: (none)
>
> This patch changes the queue assignment into:
>
>    HWQ 0: 0 1
>    HWQ 1: 2
>    HWQ 2: 3 4
>    HWQ 3: 5
>
> In other words, this patch has the following three effects:
> - All four hardware queues are used instead of only three.
> - CPU cores are spread more evenly over hardware queues. For the
>    above example the range of the number of CPU cores associated
>    with a single HWQ is reduced from [0..2] to [1..2].
> - If the number of HWQ's is a multiple of the number of CPU sockets
>    it is now guaranteed that all CPU cores associated with a single
>    HWQ reside on the same CPU socket.

I have thought about this since your last posting, and I think it should 
be a win for most cases to do this, even if we end up with asymmetric 
queue <-> cpu mappings.

-- 
Jens Axboe

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