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

From: Jens Axboe
Date: Tue Dec 09 2014 - 11:10:58 EST

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

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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