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

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Message-ID: <CACVXFVO76w7HwYjY049ZM-5_SG047HnQQPGSmek952-KZp1MgQ@mail.gmail.com>
Date:	Thu, 17 Sep 2015 23:50:05 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...nel.dk>
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 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), :-)

Thanks,
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