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

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

On 12/05/15 17:08, Kevin Wolf wrote:
> Am 12.05.2015 um 15:41 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 changes default bounce buffer optimal alignment to
>> MAX(page size, 4k). 4k is chosen as maximal known sector size on real
>> HDD.
>>
>> The justification of the performance improve is quite interesting.
>>  From the kernel point of view each request to the disk was split
>> by two. This could be seen by blktrace like this:
>>    9,0   11  1     0.000000000 11151  Q  WS 312737792 + 1023 [qemu-img]
>>    9,0   11  2     0.000007938 11151  Q  WS 312738815 + 8 [qemu-img]
>>    9,0   11  3     0.000030735 11151  Q  WS 312738823 + 1016 [qemu-img]
>>    9,0   11  4     0.000032482 11151  Q  WS 312739839 + 8 [qemu-img]
>>    9,0   11  5     0.000041379 11151  Q  WS 312739847 + 1016 [qemu-img]
>>    9,0   11  6     0.000042818 11151  Q  WS 312740863 + 8 [qemu-img]
>>    9,0   11  7     0.000051236 11151  Q  WS 312740871 + 1017 [qemu-img]
>>    9,0    5  1     0.169071519 11151  Q  WS 312741888 + 1023 [qemu-img]
>> After the patch the pattern becomes normal:
>>    9,0    6  1     0.000000000 12422  Q  WS 314834944 + 1024 [qemu-img]
>>    9,0    6  2     0.000038527 12422  Q  WS 314835968 + 1024 [qemu-img]
>>    9,0    6  3     0.000072849 12422  Q  WS 314836992 + 1024 [qemu-img]
>>    9,0    6  4     0.000106276 12422  Q  WS 314838016 + 1024 [qemu-img]
>> and the amount of requests sent to disk (could be calculated counting
>> number of lines in the output of blktrace) is reduced about 2 times.
>>
>> Both qemu-img and qemu-io are affected while qemu-kvm is not. The guest
>> does his job well and real requests comes properly aligned (to page).
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Paolo Bonzini <pbonzini@redhat.com>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>>   block.c           |  8 ++++----
>>   block/io.c        |  2 +-
>>   block/raw-posix.c | 15 +++++++++------
>>   3 files changed, 14 insertions(+), 11 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index e293907..325f727 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -106,8 +106,8 @@ int is_windows_drive(const char *filename)
>>   size_t bdrv_opt_mem_align(BlockDriverState *bs)
>>   {
>>       if (!bs || !bs->drv) {
>> -        /* 4k should be on the safe side */
>> -        return 4096;
>> +        /* page size or 4k (hdd sector size) should be on the safe side */
>> +        return MAX(4096, getpagesize());
>>       }
>>   
>>       return bs->bl.opt_mem_alignment;
>> @@ -116,8 +116,8 @@ size_t bdrv_opt_mem_align(BlockDriverState *bs)
>>   size_t bdrv_min_mem_align(BlockDriverState *bs)
>>   {
>>       if (!bs || !bs->drv) {
>> -        /* 4k should be on the safe side */
>> -        return 4096;
>> +        /* page size or 4k (hdd sector size) should be on the safe side */
>> +        return MAX(4096, getpagesize());
>>       }
>>   
>>       return bs->bl.min_mem_alignment;
>> diff --git a/block/io.c b/block/io.c
>> index 908a3d1..071652c 100644
>> --- a/block/io.c
>> +++ b/block/io.c
>> @@ -205,7 +205,7 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
>>           bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
>>       } else {
>>           bs->bl.min_mem_alignment = 512;
>> -        bs->bl.opt_mem_alignment = 512;
>> +        bs->bl.opt_mem_alignment = getpagesize();
>>       }
>>   
>>       if (bs->backing_hd) {
>> diff --git a/block/raw-posix.c b/block/raw-posix.c
>> index 7083924..4659552 100644
>> --- a/block/raw-posix.c
>> +++ b/block/raw-posix.c
>> @@ -301,6 +301,7 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
>>   {
>>       BDRVRawState *s = bs->opaque;
>>       char *buf;
>> +    size_t max_align = MAX(MAX_BLOCKSIZE, getpagesize());
>>   
>>       /* For /dev/sg devices the alignment is not really used.
>>          With buffered I/O, we don't have any restrictions. */
>> @@ -330,9 +331,9 @@ static void raw_probe_alignment(BlockDriverState *bs, int fd, Error **errp)
>>       /* If we could not get the sizes so far, we can only guess them */
>>       if (!s->buf_align) {
>>           size_t align;
>> -        buf = qemu_memalign(MAX_BLOCKSIZE, 2 * MAX_BLOCKSIZE);
>> -        for (align = 512; align <= MAX_BLOCKSIZE; align <<= 1) {
>> -            if (raw_is_io_aligned(fd, buf + align, MAX_BLOCKSIZE)) {
>> +        buf = qemu_memalign(max_align, 2 * max_align);
>> +        for (align = 512; align <= max_align; align <<= 1) {
>> +            if (raw_is_io_aligned(fd, buf + align, max_align)) {
>>                   s->buf_align = align;
>>                   break;
>>               }
>> @@ -342,8 +343,8 @@ 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) {
>> +        buf = qemu_memalign(s->buf_align, max_align);
>> +        for (align = 512; align <= max_align; align <<= 1) {
>>               if (raw_is_io_aligned(fd, buf, align)) {
>>                   bs->request_alignment = align;
>>                   break;
>> @@ -726,7 +727,9 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
>>   
>>       raw_probe_alignment(bs, s->fd, errp);
>>       bs->bl.min_mem_alignment = s->buf_align;
>> -    bs->bl.opt_mem_alignment = s->buf_align;
>> +    if (bs->bl.min_mem_alignment > bs->bl.opt_mem_alignment) {
>> +        bs->bl.opt_mem_alignment = MAX(s->buf_align, getpagesize());
>> +    }
>>   }
> I think this should be unconditional now.
>
> Kevin
frankly speaking I am not comfortable with that. With the 'if' in the code
we are protected if for some reason
   bs->bl.opt_mem_alignment
is greater then page size, f.e. if this is a requirement
of underlying backing storage.

Keeping if here is safe IMHO.

Den