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


From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH] qcow2: do not allocate extra memory
Date: Tue, 12 Jul 2016 22:19:04 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

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 <address@hidden>
---

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




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