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SubjectRe: [PATCH v1 1/5] block: blk-mq: avoid initializing request during its completion
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Hi Jens,

On Wed, Jan 1, 2014 at 12:38 AM, Jens Axboe <axboe@kernel.dk> wrote:
> On Thu, Dec 26 2013, Ming Lei wrote:
>> One problem is that request->start_time/start_time_ns could be
>> set as wrong.
>>
>> Also it is normal to intialize one data structure just after its
>> allocation.
>>
>> So move the initialization out of its completion path.
>
> It's done that way because of presumed cache hotness on completion,
> since we just touched a lot of the members. Lets just fix the start time
> issue by itself.

But some members of request are already touched in allocation
path(blk-mq core) too, and lots of members will be touched by
driver for starting the transfer.

Also I didn't observe obvious effect on L1 dcache load/store miss rate
after applying the patch when reading/writing on null_blk device.

Considered the theoretic advantage of reinitializing in free path, I
will just fix the start time issue by itself.

Thanks,
--
Ming Lei


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