Re: dm-writecache

[Dan Williams <dan.j.williams@intel.com>]

On Fri, May 18, 2018 at 3:00 PM, Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
> On Fri, 18 May 2018, Dan Williams wrote:
>
>> >> ...and I wonder what the benefit is of the 16-byte case? I would
>> >> assume the bulk of the benefit is limited to the 4 and 8 byte copy
>> >> cases.
>> >
>> > dm-writecache uses 16-byte writes frequently, so it is needed for that.
>> >
>> > If we split 16-byte write to two 8-byte writes, it would degrade
>> > performance for architectures where memcpy_flushcache needs to flush the
>> > cache.
>>
>> My question was how measurable it is to special case 16-byte
>> transfers? I know Ingo is going to ask this question, so it would
>> speed things along if this patch included performance benefit numbers
>> for each special case in the changelog.
>
> I tested it some times ago - and the movnti instruction has 2% better
> throughput than the existing memcpy_flushcache function.
>
> It is doing one 16-byte write for every sector written and one 8-byte
> write for every sector clean-up. So, the overhead is measurable.

Awesome, include those measured numbers in the changelog for the next
spin of the patch.
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