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

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

On 12/05/15 16:08, Kevin Wolf wrote:
> Am 12.05.2015 um 12:36 hat Denis V. Lunev geschrieben:
>> On 12/05/15 13:27, Kevin Wolf wrote:
>>> Am 12.05.2015 um 07:47 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 | 14 ++++++++------
>>>>   3 files changed, 13 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) {
>>> I think it would make more sense to keep this specific to the raw-posix
>>> driver. After all, it's only the kernel page cache that we optimise
>>> here. Other backends probably don't take advantage of page alignment.
>>>
>>>> diff --git a/block/raw-posix.c b/block/raw-posix.c
>>>> index 7083924..04f3d4e 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 = bs->bl.min_mem_alignment;
>>>> +    }
>>> Or, if you want to keep the getpagesize() initialisation as a generic
>>> fallback just in case, I would still suggest to be explicit here instead
>>> of relying on the default, like this:
>>>
>>>      bs->bl.opt_mem_alignment = MAX(s->buf_align, getpagesize()).
>>>
>>> Kevin
>>>
>> definitely I can do this if this is a strict requirement and I have
>> not performed any real testing on Windows and other platforms
>> but from my point of view we will be on a safe side with this
>> alignment.
> Yes, it certainly won't hurt as a default, so I'm okay with keeping it
> in block.c. I would only like to have it explicit in raw-posix, too,
> because the justification you use in the commit message is specific to
> raw-posix (or, to be more precise, specific to raw-posix on Linux).
>
> Paolo is right that I missed that the page cache isn't involved, but
> then it must be the Linux block layer that splits the requests as you
> reported. That's still raw-posix only.
>
> For other backends (like network protocols), defaulting to pagesize
> shouldn't hurt and possibly there are some effects that make it an
> improvement there as well, but for raw-posix we actually have a good
> reason to do so and to be explicit about it in the driver.

ok, makes sense.

>> Pls note, that I do not make any new allocation and any new
>> alignment check. The patch just forces alignment of the
>> allocation which will be performed in any case. And this
>> approach just matches IO coming from guest with IO initiated
>> by the qemu-img/io. All guest operations (both Windows and
>> Linux) are really page aligned by address and offset
>> nowadays.
>>
>> This approach is safe. It does not bring any additional
>> (significant) overhead.
> Yes, I understand that. :-)
>
> Kevin