[Qemu-devel] Re: [PATCH v2] Improve accuracy of block migration bandwidth calculation

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On 04/03/2011 03:31 AM, Avishay Traeger wrote:
>
> Revised patch for improving the accuracy of the block migration bandwidth
> calculation.  Thanks a lot to Michael Roth for the input.
>
> For those that missed the original patch, here is the description:
> block_mig_state.total_time is currently the sum of the read request
> latencies.  This is not very accurate because block migration uses aio and
> so several requests can be submitted at once.  Bandwidth should be computed
> with wall-clock time, not by adding the latencies.  In this case,
> "total_time" has a higher value than it should, and so the computed
> bandwidth is lower than it is in reality.  This means that migration can
> take longer than it needs to.
> However, we don't want to use pure wall-clock time here.  We are computing
> bandwidth in the asynchronous phase, where the migration repeatedly wakes
> up and sends some aio requests.  The computed bandwidth will be used for
> synchronous transfer.
>
>
> Avishay
>
>
> Signed-off-by: Avishay Traeger<avishay@il.ibm.com>
> ---
>   block-migration.c |   21 +++++++++------------
>   1 files changed, 9 insertions(+), 12 deletions(-)
>
> diff --git a/block-migration.c b/block-migration.c
> index 8218bac..47296ca 100644
> --- a/block-migration.c
> +++ b/block-migration.c
> @@ -62,7 +62,6 @@ typedef struct BlkMigBlock {
>       QEMUIOVector qiov;
>       BlockDriverAIOCB *aiocb;
>       int ret;
> -    int64_t time;
>       QSIMPLEQ_ENTRY(BlkMigBlock) entry;
>   } BlkMigBlock;
>
> @@ -78,6 +77,7 @@ typedef struct BlkMigState {
>       int prev_progress;
>       int bulk_completed;
>       long double total_time;
> +    long double prev_time_offset;
>       int reads;
>   } BlkMigState;
>
> @@ -131,12 +131,6 @@ uint64_t blk_mig_bytes_total(void)
>       return sum<<  BDRV_SECTOR_BITS;
>   }
>
> -static inline void add_avg_read_time(int64_t time)
> -{
> -    block_mig_state.reads++;
> -    block_mig_state.total_time += time;
> -}
> -
>   static inline long double compute_read_bwidth(void)
>   {
>       assert(block_mig_state.total_time != 0);
> @@ -191,13 +185,14 @@ static void alloc_aio_bitmap(BlkMigDevState *bmds)
>
>   static void blk_mig_read_cb(void *opaque, int ret)
>   {
> +    long double curr_time = qemu_get_clock_ns(rt_clock);
>       BlkMigBlock *blk = opaque;
>
>       blk->ret = ret;
>
> -    blk->time = qemu_get_clock_ns(rt_clock) - blk->time;
> -
> -    add_avg_read_time(blk->time);
> +    block_mig_state.reads++;
> +    block_mig_state.total_time += (curr_time -
> block_mig_state.prev_time_offset);
> +    block_mig_state.prev_time_offset = curr_time;
>
>       QSIMPLEQ_INSERT_TAIL(&block_mig_state.blk_list, blk, entry);
>       bmds_set_aio_inflight(blk->bmds, blk->sector, blk->nr_sectors, 0);
> @@ -250,7 +245,8 @@ static int mig_save_device_bulk(Monitor *mon, QEMUFile
> *f,
>       blk->iov.iov_len = nr_sectors * BDRV_SECTOR_SIZE;
>       qemu_iovec_init_external(&blk->qiov,&blk->iov, 1);
>
> -    blk->time = qemu_get_clock_ns(rt_clock);
> +    if (block_mig_state.submitted == 0)
> +        block_mig_state.prev_time_offset = qemu_get_clock_ns(rt_clock);
>
>       blk->aiocb = bdrv_aio_readv(bs, cur_sector,&blk->qiov,
>                                   nr_sectors, blk_mig_read_cb, blk);
> @@ -409,7 +405,8 @@ static int mig_save_device_dirty(Monitor *mon, QEMUFile
> *f,
>                   blk->iov.iov_len = nr_sectors * BDRV_SECTOR_SIZE;
>                   qemu_iovec_init_external(&blk->qiov,&blk->iov, 1);
>
> -                blk->time = qemu_get_clock_ns(rt_clock);
> +                if (block_mig_state.submitted == 0)
> +                    block_mig_state.prev_time_offset = qemu_get_clock_ns
> (rt_clock);
>
>                   blk->aiocb = bdrv_aio_readv(bmds->bs, sector,&blk->qiov,
>                                               nr_sectors, blk_mig_read_cb,
> blk);
> --
> 1.7.0.4
>

Looks good. This addresses the concerns I had.

Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>