linux-kernel - Re: [PATCH] block: kyber: make kyber more friendly with merging

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Date:   Wed, 30 May 2018 17:44:28 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     Omar Sandoval <osandov@...ndov.com>, Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: kyber: make kyber more friendly with merging

On Wed, May 30, 2018 at 5:20 PM, jianchao.wang
<jianchao.w.wang@...cle.com> wrote:
> Hi ming
>
> On 05/30/2018 05:13 PM, Ming Lei wrote:
>>> Yes, it maybe good for merging of 'none', because the rq_list is split into 3
>>> lists, and not need to iterate the whole rq_list any more.
>>> But what's about the dispatch when there is no io scheduler.
>> blk_mq_flush_busy_ctxs() and blk_mq_dequeue_from_ctx() should work
>> fine in case of 'none' if per-domain list is added to ctx. Then we can make
>> none to be a bit fair on READ/WRITE.
>>
>
> But how to determine when to dispatch READ, WRITE or other more, when there is no io scheduler ?
>

For blk-mq, no io scheduler means 'none' actually, and it works like a
scheduler too, but just shares driver tags, IMO.

Wrt. the current code of 'none', blk-mq just picks up one request from
ctx->rq_list
directly in FIFO style. If READ/WRITE lists are introduced, only
blk_mq_dequeue_from_ctx() is effected, there are several choices
left for us:

1) keep the FIFO style of current behaviour by using req->start_time_ns

2) READ/WRIRE fair style by picking up request from the lists in round-robin
order

3) or others

It just will make more choices for us, :-)

Thanks,
Ming Lei