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[Qemu-devel] [PULL 34/38] block: align bounce buffers to page
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] [PULL 34/38] block: align bounce buffers to page |
Date: |
Fri, 22 May 2015 10:02:06 +0100 |
From: "Denis V. Lunev" <address@hidden>
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 <address@hidden>
Reviewed-by: Kevin Wolf <address@hidden>
Message-id: address@hidden
CC: Paolo Bonzini <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
block.c | 8 ++++----
block/io.c | 2 +-
block/raw-posix.c | 13 +++++++------
3 files changed, 12 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 e6c3639..b3ea95c 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..2990e95 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 +727,7 @@ 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;
+ bs->bl.opt_mem_alignment = MAX(s->buf_align, getpagesize());
}
static int check_for_dasd(int fd)
--
2.1.0
- [Qemu-devel] [PULL 22/38] block/parallels: improve image reading performance, (continued)
- [Qemu-devel] [PULL 22/38] block/parallels: improve image reading performance, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 23/38] block/parallels: create bat_entry_off helper, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 26/38] block/parallels: optimize linear image expansion, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 28/38] configure: handle clang -nopie argument warning, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 27/38] block/parallels: improve image writing performance further, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 29/38] configure: factor out supported flag check, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 25/38] block/parallels: add prealloc-mode and prealloc-size open paramemets, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 30/38] configure: silence glib unknown attribute __alloc_size__, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 31/38] configure: Add workaround for ccache and clang, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 33/38] block: minimal bounce buffer alignment, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 34/38] block: align bounce buffers to page,
Stefan Hajnoczi <=
- [Qemu-devel] [PULL 35/38] Revert "block: Fix unaligned zero write", Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 32/38] block: return EPERM on writes or discards to read-only devices, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 38/38] block: get_block_status: use "else" when testing the opposite condition, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 37/38] qemu-iotests: Test unaligned sub-block zero write, Stefan Hajnoczi, 2015/05/22
- [Qemu-devel] [PULL 36/38] block: Fix NULL deference for unaligned write if qiov is NULL, Stefan Hajnoczi, 2015/05/22
- Re: [Qemu-devel] [PULL 00/38] Block patches, Peter Maydell, 2015/05/22