linux-kernel - Re: [PATCH 09/16] blk-mq: cache request hardware queue mapping

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Date:   Thu, 1 Nov 2018 06:22:14 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Hannes Reinecke <hare@...e.de>, linux-block@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/16] blk-mq: cache request hardware queue mapping

On 11/1/18 3:27 AM, Hannes Reinecke wrote:
> On 10/30/18 7:32 PM, Jens Axboe wrote:
>> We call blk_mq_map_queue() a lot, at least two times for each
>> request per IO, sometimes more. Since we now have an indirect
>> call as well in that function. cache the mapping so we don't
>> have to re-call blk_mq_map_queue() for the same request
>> multiple times.
>>
>> Signed-off-by: Jens Axboe <axboe@...nel.dk>
>> ---
>>   block/blk-flush.c      | 12 ++++--------
>>   block/blk-mq-debugfs.c |  4 +---
>>   block/blk-mq-sched.c   |  6 ++----
>>   block/blk-mq-tag.c     |  9 +--------
>>   block/blk-mq.c         | 22 +++++++++-------------
>>   block/blk-mq.h         |  5 +----
>>   include/linux/blkdev.h |  1 +
>>   7 files changed, 19 insertions(+), 40 deletions(-)
>>
> Hehe.
> I had been wondering about this since the original mq implementation, 
> but was told not to worry :-)

To be fair, it was just an array index. Now it's got an indirect
call, so it's more expensive.
 > Reviewed-by: Hannes Reinecke <hare@...e.com>

Thanks!

-- 
Jens Axboe