Re: [PATCH RFC 00/21] blk-mq: Introduce combined hardware queues

From: Bart Van Assche
Date: Mon Sep 19 2016 - 09:48:02 EST

On 09/19/16 03:38, Alexander Gordeev wrote:
> On Fri, Sep 16, 2016 at 05:04:48PM -0400, Keith Busch wrote:
> CC-ing linux-block@xxxxxxxxxxxxxxx
>> I'm not sure I see how this helps. That probably means I'm not considering
>> the right scenario. Could you elaborate on when having multiple hardware
>> queues to choose from a given CPU will provide a benefit?
> No, I do not keep in mind any particular scenario besides common
> sense. Just an assumption deeper queues are better (in this RFC
> a virtual combined queue consisting of multipe h/w queues).
> Apparently, there could be positive effects only in systems where
> # of queues / # of CPUs > 1 or # of queues / # of cores > 1. But
> I do not happen to have ones. If I had numbers this would not be
> the RFC and I probably would not have posted in the first place ;)
> Would it be possible to give it a try on your hardware?

Hello Alexander,

It is your task to measure the performance impact of these patches and
not Keith's task. BTW, I'm not convinced that multiple hardware queues
per CPU will result in a performance improvement. I have not yet seen
any SSD for which a queue depth above 512 results in better performance
than queue depth equal to 512. Which applications do you think will
generate and sustain a queue depth above 512? Additionally, my
experience from another high performance context (RDMA) is that reducing
the number of queues can result in higher IOPS due to fewer interrupts
per I/O.