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block/io: optimize bdrv_co_pwritev for small requests 

in a read-modify-write cycle a small request might cause
head and tail to fall into the same aligned block. Currently
QEMU reads the same block twice in this case which is
not necessary.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1464607873-28206-1-git-send-email-pl@kamp.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Peter Lieven 2016-05-30 13:31:13 +02:00 committed by Stefan Hajnoczi
parent e3a4f91b4d
commit 117bc3fa22
3 changed files with 9 additions and 37 deletions
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8
block/io.c
View File

@ -1427,6 +1427,14 @@ int coroutine_fn bdrv_co_pwritev(BlockDriverState *bs,
bytes += offset & (align - 1);
offset = offset & ~(align - 1);
/* We have read the tail already if the request is smaller
* than one aligned block.
*/
if (bytes < align) {
qemu_iovec_add(&local_qiov, head_buf + bytes, align - bytes);
bytes = align;
}
}
if ((offset + bytes) & (align - 1)) {

12
tests/qemu-iotests/077
View File

@ -60,7 +60,7 @@ EOF
# Sequential RMW requests on the same physical sector
off=0x1000
for ev in "head" "after_head" "tail" "after_tail"; do
for ev in "head" "after_head"; do
cat <<EOF
break pwritev_rmw_$ev A
aio_write -P 10 $((off + 0x200)) 0x200
@ -211,16 +211,6 @@ function verify_io()
echo read -P 11 0x2400 0x200
echo read -P 0 0x2600 0xa00
echo read -P 0 0x3000 0x200
echo read -P 10 0x3200 0x200
echo read -P 11 0x3400 0x200
echo read -P 0 0x3600 0xa00
echo read -P 0 0x4000 0x200
echo read -P 10 0x4200 0x200
echo read -P 11 0x4400 0x200
echo read -P 0 0x4600 0xa00
# Chained dependencies
echo read -P 10 0x5000 0x200
echo read -P 11 0x5200 0x200

26
tests/qemu-iotests/077.out
View File

@ -19,16 +19,6 @@ wrote XXX/XXX bytes at offset XXX
XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote XXX/XXX bytes at offset XXX
XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
blkdebug: Resuming request 'A'
wrote XXX/XXX bytes at offset XXX
XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote XXX/XXX bytes at offset XXX
XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
blkdebug: Resuming request 'A'
wrote XXX/XXX bytes at offset XXX
XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote XXX/XXX bytes at offset XXX
XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote XXX/XXX bytes at offset XXX
XXX bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote XXX/XXX bytes at offset XXX
@ -114,22 +104,6 @@ read 512/512 bytes at offset 9216
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 2560/2560 bytes at offset 9728
2.500 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 12288
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 12800
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 13312
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 2560/2560 bytes at offset 13824
2.500 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 16384
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 16896
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 17408
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 2560/2560 bytes at offset 17920
2.500 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 20480
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
read 512/512 bytes at offset 20992