Re: [Qemu-devel] [PATCH 1/1] block: change default memory alignment for block requests to 4096

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

On 28/01/15 23:07, Paolo Bonzini wrote:
>
> On 28/01/2015 19:49, Denis V. Lunev wrote:
>> 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 10% better if buf is aligned to 4096 bytes rather then to
>> 512 bytes on HDD with 512/4096 logical/physical sector size.
>>
>> The difference is quite reliable.
> The 10% difference, however, is probably not enough to cover the cost of
> providing a bounce buffer if a guest is (rightfully) using a 512-byte
> aligned buffer: bs->bl.opt_mem_alignment is in fact badly named and it
> should be bs->bl.min_mem_alignment instead.
>
> Instead, you probably should patch bdrv_opt_mem_align to return at least
> 4096, and leave the detection logic intact.  This will let
> qemu_blockalign return a properly aligned buffer to qemu-img and other
> in-process allocations, without negatively affecting the guest.
>
> Thanks,
>
> Paolo
ok, this looks good to me :)


>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>>   block.c           | 4 ++--
>>   block/raw-posix.c | 4 ++--
>>   2 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index d45e4dd..bc5d1e7 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -543,7 +543,7 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
>>           bs->bl.max_transfer_length = bs->file->bl.max_transfer_length;
>>           bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
>>       } else {
>> -        bs->bl.opt_mem_alignment = 512;
>> +        bs->bl.opt_mem_alignment = 4096;
>>       }
>>   
>>       if (bs->backing_hd) {
>> @@ -966,7 +966,7 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
>>   
>>       bs->open_flags = flags;
>>       bs->guest_block_size = 512;
>> -    bs->request_alignment = 512;
>> +    bs->request_alignment = 4096;
>>       bs->zero_beyond_eof = true;
>>       open_flags = bdrv_open_flags(bs, flags);
>>       bs->read_only = !(open_flags & BDRV_O_RDWR);
>> diff --git a/block/raw-posix.c b/block/raw-posix.c
>> index ec38fee..d1b3388 100644
>> --- a/block/raw-posix.c
>> +++ b/block/raw-posix.c
>> @@ -266,7 +266,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
>>       if (!s->buf_align) {
>>           size_t align;
>>           buf = qemu_memalign(MAX_BLOCKSIZE, 2 * MAX_BLOCKSIZE);
>> -        for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
>> +        for (align = 4096; align <= MAX_BLOCKSIZE; align <<= 1) {
>>               if (pread(fd, buf + align, MAX_BLOCKSIZE, 0) >= 0) {
>>                   s->buf_align = align;
>>                   break;
>> @@ -278,7 +278,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
>>       if (!bs->request_alignment) {
>>           size_t align;
>>           buf = qemu_memalign(s->buf_align, MAX_BLOCKSIZE);
>> -        for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
>> +        for (align = 4096; align <= MAX_BLOCKSIZE; align <<= 1) {
>>               if (pread(fd, buf, align, 0) >= 0) {
>>                   bs->request_alignment = align;
>>                   break;
>>