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    SubjectRe: [PATCH] block: blk-merge: fast-clone bio when splitting rw bios
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    On 09/17/2015 09:50 AM, Ming Lei wrote:
    > On Thu, Sep 17, 2015 at 11:19 PM, Jens Axboe <axboe@kernel.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.

    --
    Jens Axboe



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