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

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Message-ID: <00e5dfd4-d3a2-b008-3d8b-390a788f61c9@kernel.dk>
Date:   Tue, 22 May 2018 10:20:34 -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 10:17 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.

-- 
Jens Axboe