Re: [Qemu-devel] [PATCH v2] mirror: double performance of the bulk stage if the disc is full

[Jeff Cody]

On Thu, Jul 14, 2016 at 08:19:01PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Mirror can do up to 16 in-flight requests, but actually on full copy
> (the whole source disk is non-zero) in-flight is always 1. This happens
> as the request is not limited in size: the data occupies maximum available
> capacity of s->buf.
> 
> The patch limits the size of the request to some artificial constant
> (1 Mb here), which is not that big or small. This effectively enables
> back parallelism in mirror code as it was designed.
> 
> The result is important: the time to migrate 10 Gb disk is reduced from
> ~350 sec to 170 sec.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Fam Zheng <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Max Reitz <address@hidden>
> CC: Jeff Cody <address@hidden>
> CC: Eric Blake <address@hidden>
> ---
> 
> v2: in case of s->buf_size larger than default use it to limit io_sectors
> 
>  block/mirror.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index b1e633e..3ac3b4d 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -23,7 +23,9 @@
>  
>  #define SLICE_TIME    100000000ULL /* ns */
>  #define MAX_IN_FLIGHT 16
> -#define DEFAULT_MIRROR_BUF_SIZE   (10 << 20)
> +#define MAX_IO_SECTORS ((1 << 20) >> BDRV_SECTOR_BITS) /* 1 Mb */
> +#define DEFAULT_MIRROR_BUF_SIZE \
> +    (MAX_IN_FLIGHT * MAX_IO_SECTORS * BDRV_SECTOR_SIZE)
>  
>  /* The mirroring buffer is a list of granularity-sized chunks.
>   * Free chunks are organized in a list.
> @@ -322,6 +324,8 @@ static uint64_t coroutine_fn 
> mirror_iteration(MirrorBlockJob *s)
>      int nb_chunks = 1;
>      int64_t end = s->bdev_length / BDRV_SECTOR_SIZE;
>      int sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
> +    int max_io_sectors = MAX((s->buf_size >> BDRV_SECTOR_BITS) / 
> MAX_IN_FLIGHT,
> +                             MAX_IO_SECTORS);
>  
>      sector_num = hbitmap_iter_next(&s->hbi);
>      if (sector_num < 0) {
> @@ -385,7 +389,9 @@ static uint64_t coroutine_fn 
> mirror_iteration(MirrorBlockJob *s)
>                                            nb_chunks * sectors_per_chunk,
>                                            &io_sectors, &file);
>          if (ret < 0) {
> -            io_sectors = nb_chunks * sectors_per_chunk;
> +            io_sectors = MIN(nb_chunks * sectors_per_chunk, max_io_sectors);
> +        } else if (ret & BDRV_BLOCK_DATA) {
> +            io_sectors = MIN(io_sectors, max_io_sectors);
>          }
>  
>          io_sectors -= io_sectors % sectors_per_chunk;
> -- 
> 1.8.3.1
> 

Thanks,

Applied to my block branch:

git://github.com/codyprime/qemu-kvm-jtc.git block

-Jeff