linux-kernel - Re: [PATCH] block: blk-merge: fast-clone bio when splitting rw bios

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Date:	Thu, 17 Sep 2015 10:01:16 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Ming Lin <ming.l@....samsung.com>,
	Dongsu Park <dpark@...teo.net>
Subject: Re: [PATCH] block: blk-merge: fast-clone bio when splitting rw bios

On 09/17/2015 09:55 AM, Jens Axboe wrote:
> On 09/17/2015 09:50 AM, Ming Lei wrote:
>> On Thu, Sep 17, 2015 at 11:19 PM, Jens Axboe <axboe@...nel.dk> wrote:
>>> On 09/17/2015 09:13 AM, Ming Lei wrote:
>>>>
>>>> biovecs has become immutable since v3.13, so it isn't necessary
>>>> to allocate biovecs for the new cloned bios, then we can save
>>>> one extra biovecs allocation/copy, and the allocation is often
>>>> not fixed-length and a bit more expensive.
>>>>
>>>> For example, if the 'max_sectors_kb' of null blk's queue is set
>>>> as 16(32 sectors) via sysfs just for making more splits, this patch
>>>> can increase throught about ~70% in the sequential read test over
>>>> null_blk(direct io, bs: 1M).
>>>
>>>
>>> I'd be curious how this compares to before we did the splitting, not
>>> exceeding the limits through bio_add_page() instead?
>>
>> Let me show these test results:
>>
>> ----------------------------------------------------------------------------------
>>
>> kernel                                                    | throught
>> ----------------------------------------------------------------------------------
>>
>> 4.3.0-rc1-next-20150916                   | bw=12227MB/s, iops=12227
>> ----------------------------------------------------------------------------------
>>
>> 4.3.0-rc1-next-20150916 with patch | bw=21011MB/s, iops=21011
>> ----------------------------------------------------------------------------------
>>
>> v4.2                                                       |
>> bw=18959MB/s, iops=18958
>> ----------------------------------------------------------------------------------
>>
>>
>> So from the above, looks this patch is kind of fix for performance
>> regression
>> introduced by 54efd50bfd(block: make generic_make_request handle
>> arbitrarily sized bios), :-)
>
> So that's 1MB user IO, and 16KB device limit, correct? If that is the
> case, then the results make sense. And looks like we're still ahead of
> the older bio_add_page() approach, which is what I mostly cared about.
> Thanks! I'll apply this for -rc2.

Hand applied, as it didn't apply with the blk-merge.c warning fix at all 
(against for-linus). Please double check:

http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=52cc6eead9095e2faf2ec7afc013aa3af1f01ac5

-- 
Jens Axboe

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