Re: [Qemu-devel] [PATCH] qcow2: do not allocate extra memory

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

add Fabrice Bellard

On 12.07.2016 20:43, Vladimir Sementsov-Ogievskiy wrote:
> There are no needs to allocate more than one cluster, as we set
> avail_out for deflate to one cluster.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>
> Hi all!
>
> Please, can anybody say me what I'm missing?
>
> I've looked through deflate documentation at
> http://www.zlib.net/manual.html, and I didn't find anything about
> allocating more memory for out buffer than specified in avail_out
> field.. What is this magic formula?
>
> ========
>
> All uses of out_buf in the function:
>
> uint8_t *out_buf;
> ...
> out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
> ...
> strm.avail_out = s->cluster_size;
> strm.next_out = out_buf;
>
> ret = deflate(&strm, Z_FINISH);
> ...
> out_len = strm.next_out - out_buf;
> ...
> ret = bdrv_pwrite(bs->file, cluster_offset, out_buf, out_len);
> ...
> g_free(out_buf);
>
>   block/qcow.c  | 2 +-
>   block/qcow2.c | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/block/qcow.c b/block/qcow.c
> index ac849bd..d8826f3 100644
> --- a/block/qcow.c
> +++ b/block/qcow.c
> @@ -983,7 +983,7 @@ static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
>           return ret;
>       }
>   
> -    out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
> +    out_buf = g_malloc(s->cluster_size);
>   
>       /* best compression, small window, no zlib header */
>       memset(&strm, 0, sizeof(strm));
> diff --git a/block/qcow2.c b/block/qcow2.c
> index a5ea19b..b1c90ae 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2612,7 +2612,7 @@ static int qcow2_write_compressed(BlockDriverState *bs, int64_t sector_num,
>           return ret;
>       }
>   
> -    out_buf = g_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
> +    out_buf = g_malloc(s->cluster_size);
>   
>       /* best compression, small window, no zlib header */
>       memset(&strm, 0, sizeof(strm));


-- 
Best regards,
Vladimir