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

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Date:   Tue, 22 May 2018 11:46:11 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Holger Hoffstätte <holger@...lied-asynchrony.com>,
        Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: kyber: make kyber more friendly with merging

On 5/22/18 10:20 AM, Jens Axboe wrote:
> On 5/22/18 10:17 AM, Holger Hoffstätte wrote:
>> On 05/22/18 16:48, Jianchao Wang wrote:
>>> Currently, kyber is very unfriendly with merging. kyber depends
>>> on ctx rq_list to do merging, however, most of time, it will not
>>> leave any requests in ctx rq_list. This is because even if tokens
>>> of one domain is used up, kyber will try to dispatch requests
>>> from other domain and flush the rq_list there.
>>>
>>> To improve this, we setup kyber_ctx_queue (kcq) which is similar
>>> with ctx, but it has rq_lists for different domain and build same
>>> mapping between kcq and khd as the ctx & hctx. Then we could merge,
>>> insert and dispatch for different domains separately. If one domain
>>> token is used up, the requests could be left in the rq_list of
>>> that domain and maybe merged with following io.
>>>
>>> Following is my test result on machine with 8 cores and NVMe card
>>> INTEL SSDPEKKR128G7
>>>
>>> fio size=256m ioengine=libaio iodepth=64 direct=1 numjobs=8
>>> seq/random
>>> +------+---------------------------------------------------------------+
>>> |patch?| bw(MB/s) |   iops    | slat(usec) |    clat(usec)   |  merge  |
>>> +----------------------------------------------------------------------+
>>> | w/o  |  606/612 | 151k/153k |  6.89/7.03 | 3349.21/3305.40 |   0/0   |
>>> +----------------------------------------------------------------------+
>>> | w/   | 1083/616 | 277k/154k |  4.93/6.95 | 1830.62/3279.95 | 223k/3k |
>>> +----------------------------------------------------------------------+
>>> When set numjobs to 16, the bw and iops could reach 1662MB/s and 425k
>>> on my platform.
>>>
>>> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
>>
>> <snip>
>>
>> This looks great but prevents kyber from being built as module,
>> which is AFAIK supposed to work (and works now):
>>
>> ..
>>   CC [M]  block/kyber-iosched.o
>>   Building modules, stage 2.
>>   MODPOST 313 modules
>> ERROR: "bio_attempt_back_merge" [block/kyber-iosched.ko] undefined!
>> ERROR: "bio_attempt_front_merge" [block/kyber-iosched.ko] undefined!
>> ERROR: "bio_attempt_discard_merge" [block/kyber-iosched.ko] undefined!
>> ERROR: "blk_try_merge" [block/kyber-iosched.ko] undefined!
>> ERROR: "blk_rq_merge_ok" [block/kyber-iosched.ko] undefined!
>> ..
>>
>> It does build fine when compiled in, obviously. :)
> 
> It's basically duplicating the contents of blk_attempt_plug_merge().
> I would suggest abstracting out the list loop and merge check
> into a helper, that could then both be called from kyber and the
> plug merge function.

See attached, prep patch and yours rebased on top of it.

-- 
Jens Axboe


View attachment "0002-block-kyber-make-kyber-more-friendly-with-merging.patch" of type "text/x-patch" (13143 bytes)

View attachment "0001-blk-mq-abstract-out-blk-mq-sched-rq-list-iteration-b.patch" of type "text/x-patch" (3038 bytes)