Commit 117bc3fa authored by Peter Lieven's avatar Peter Lieven Committed by Stefan Hajnoczi
Browse files

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: default avatarPeter Lieven <pl@kamp.de>
Message-id: 1464607873-28206-1-git-send-email-pl@kamp.de
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
parent e3a4f91b

block/io.c

+8 −0
Original line number Diff line number Diff line
@@ -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)) {

tests/qemu-iotests/077

+1 −11
Original line number Diff line number Diff line
@@ -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

tests/qemu-iotests/077.out

+0 −26
Original line number Diff line number Diff line
@@ -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