On 07/04/2016 10:38 AM, Denis V. Lunev wrote:
> From: Evgeny Yakovlev <eyakov...@virtuozzo.com>
>
> Some guests (win2008 server for example) do a lot of unnecessary
> flushing when underlying media has not changed. This adds additional
> overhead on host when calling fsync/fdatasync.
>
> This change introduces a write generation scheme in BlockDriverState.
> Current write generation is checked against last flushed generation to
> avoid unnessesary flushes.
>
> The problem with excessive flushing was found by a performance test
> which does parallel directory tree creation (from 2 processes).
> Results improved from 0.424 loops/sec to 0.432 loops/sec.
> Each loop creates 10^3 directories with 10 files in each.
>
> Signed-off-by: Evgeny Yakovlev <eyakov...@virtuozzo.com>
> Signed-off-by: Denis V. Lunev <d...@openvz.org>
> CC: Kevin Wolf <kw...@redhat.com>
> CC: Max Reitz <mre...@redhat.com>
> CC: Stefan Hajnoczi <stefa...@redhat.com>
> CC: Fam Zheng <f...@redhat.com>
> CC: John Snow <js...@redhat.com>
> ---
> block.c | 3 +++
> block/io.c | 18 ++++++++++++++++++
> include/block/block_int.h | 5 +++++
> 3 files changed, 26 insertions(+)
>
> diff --git a/block.c b/block.c
> index f4648e9..366fad6 100644
> --- a/block.c
> +++ b/block.c
> @@ -234,6 +234,8 @@ BlockDriverState *bdrv_new(void)
> bs->refcnt = 1;
> bs->aio_context = qemu_get_aio_context();
>
> + qemu_co_queue_init(&bs->flush_queue);
> +
> QTAILQ_INSERT_TAIL(&all_bdrv_states, bs, bs_list);
>
> return bs;
> @@ -2582,6 +2584,7 @@ int bdrv_truncate(BlockDriverState *bs, int64_t offset)
> ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
> bdrv_dirty_bitmap_truncate(bs);
> bdrv_parent_cb_resize(bs);
> + ++bs->write_gen;
> }
> return ret;
> }
> diff --git a/block/io.c b/block/io.c
> index 7cf3645..a5451b6 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1294,6 +1294,7 @@ static int coroutine_fn
> bdrv_aligned_pwritev(BlockDriverState *bs,
> }
> bdrv_debug_event(bs, BLKDBG_PWRITEV_DONE);
>
> + ++bs->write_gen;
> bdrv_set_dirty(bs, start_sector, end_sector - start_sector);
>
> if (bs->wr_highest_offset < offset + bytes) {
> @@ -2211,6 +2212,7 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
> {
> int ret;
> BdrvTrackedRequest req;
> + int current_gen = bs->write_gen;
>
> if (!bs || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs) ||
> bdrv_is_sg(bs)) {
> @@ -2219,6 +2221,12 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
>
> tracked_request_begin(&req, bs, 0, 0, BDRV_TRACKED_FLUSH);
>
> + /* Wait until any previous flushes are completed */
> + while (bs->flush_started_gen != bs->flushed_gen) {
> + qemu_co_queue_wait(&bs->flush_queue);
> + }
> + bs->flush_started_gen = current_gen;
> +
> /* Write back all layers by calling one driver function */
> if (bs->drv->bdrv_co_flush) {
> ret = bs->drv->bdrv_co_flush(bs);
> @@ -2239,6 +2247,11 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
> goto flush_parent;
> }
>
> + /* Check if we really need to flush anything */
> + if (bs->flushed_gen == current_gen) {
> + goto flush_parent;
> + }
> +
> BLKDBG_EVENT(bs->file, BLKDBG_FLUSH_TO_DISK);
> if (bs->drv->bdrv_co_flush_to_disk) {
> ret = bs->drv->bdrv_co_flush_to_disk(bs);
> @@ -2279,6 +2292,10 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
> flush_parent:
> ret = bs->file ? bdrv_co_flush(bs->file->bs) : 0;
> out:
> + /* Notify any pending flushes that we have completed */
> + bs->flushed_gen = current_gen;
> + qemu_co_queue_restart_all(&bs->flush_queue);
> +
> tracked_request_end(&req);
> return ret;
> }
> @@ -2402,6 +2419,7 @@ int coroutine_fn bdrv_co_discard(BlockDriverState *bs,
> int64_t sector_num,
> }
> ret = 0;
> out:
> + ++bs->write_gen;
> bdrv_set_dirty(bs, req.offset >> BDRV_SECTOR_BITS,
> req.bytes >> BDRV_SECTOR_BITS);
> tracked_request_end(&req);
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 2057156..8543daf 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -420,6 +420,11 @@ struct BlockDriverState {
> note this is a reference count */
> bool probed;
>
> + CoQueue flush_queue; /* Serializing flush queue */
> + unsigned int write_gen; /* Current data generation */
> + unsigned int flush_started_gen; /* Generation for which flush has
> started */
> + unsigned int flushed_gen; /* Flushed write generation */
> +
> BlockDriver *drv; /* NULL means no media */
> void *opaque;
>
>
Breaks qcow2 iotests 026 089 141 144
--js
