Re: [Qemu-devel] [PATCH 2/2] block: align bounce buffers to page

["Denis V. Lunev" <den@openvz.org>]

On 16/02/15 13:59, Kevin Wolf wrote:
> Am 06.02.2015 um 18:37 hat Denis V. Lunev geschrieben:
>> The following sequence
>>      int fd = open(argv[1], O_RDWR | O_CREAT | O_DIRECT, 0644);
>>      for (i = 0; i < 100000; i++)
>>              write(fd, buf, 4096);
>> performs 5% better if buf is aligned to 4096 bytes.
>>
>> The difference is quite reliable.
>>
>> On the other hand we do not want at the moment to enforce bounce
>> buffering if guest request is aligned to 512 bytes.
>>
>> The patch introduces new concept: minimal memory alignment for bounce
>> buffers. Original so called "optimal" value is actually minimal required
>> value for aligment. Optimal should be set to page size by default.
>> There is no driver which should change this default at the moment.
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Paolo Bonzini <pbonzini@redhat.com>
>> CC: Kevin Wolf <kwolf@redhat.com>
> I don't think the subject lines describes the patch correctly. IIUC,
> patch 1 introduces a hard enforcement of page size alignment, and this
> patch only loosens it again.
>
> Perhaps the order of patches should be changed so that min_mem_alignment
> is introduced first (without changes in the behaviour), and only then
> opt_mem_alignment is increased.
>
> Kevin
ok :)