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

[Ming Lei]

On Thu, Sep 17, 2015 at 11:19 PM, Jens Axboe <axboe@xxxxxxxxx> 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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