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Re: [Qemu-block] [PATCH] qemu-img: optimize is_allocated_sectors_min


From: Peter Lieven
Subject: Re: [Qemu-block] [PATCH] qemu-img: optimize is_allocated_sectors_min
Date: Thu, 19 Jan 2017 17:31:04 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

Please ignore this one The code was suboptimal. Will send v2.

Peter

Am 19.01.2017 um 16:56 schrieb Peter Lieven:
the current implementation always splits requests if a buffer
begins or ends with zeroes independent of the length of the
zero area. Change this to really only split off zero areas
that have at least a length of 'min' bytes.

Signed-off-by: Peter Lieven <address@hidden>
---
  qemu-img.c | 38 ++++++++++++--------------------------
  1 file changed, 12 insertions(+), 26 deletions(-)

diff --git a/qemu-img.c b/qemu-img.c
index 5df66fe..046696f 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1010,45 +1010,31 @@ static int is_allocated_sectors(const uint8_t *buf, int 
n, int *pnum)
  }
/*
- * Like is_allocated_sectors, but if the buffer starts with a used sector,
- * up to 'min' consecutive sectors containing zeros are ignored. This avoids
- * breaking up write requests for only small sparse areas.
+ * Like is_allocated_sectors, but only at least 'min' consecutive sectors
+ * containing zeros are considered unallocated. This avoids breaking up write
+ * requests for only small sparse areas.
   */
  static int is_allocated_sectors_min(const uint8_t *buf, int n, int *pnum,
-    int min)
+                                    int min)
  {
-    int ret;
-    int num_checked, num_used;
-
-    if (n < min) {
-        min = n;
-    }
-
-    ret = is_allocated_sectors(buf, n, pnum);
-    if (!ret) {
-        return ret;
-    }
-
-    num_used = *pnum;
-    buf += BDRV_SECTOR_SIZE * *pnum;
-    n -= *pnum;
-    num_checked = num_used;
+    int ret, num_checked = 0, num_used = 0;
while (n > 0) {
          ret = is_allocated_sectors(buf, n, pnum);
-
          buf += BDRV_SECTOR_SIZE * *pnum;
          n -= *pnum;
          num_checked += *pnum;
-        if (ret) {
-            num_used = num_checked;
-        } else if (*pnum >= min) {
+        if (!ret && *pnum >= min) {
              break;
          }
+        num_used = num_checked;
      }
- *pnum = num_used;
-    return 1;
+    if (num_used) {
+        *pnum = num_used;
+    }
+
+    return !!num_used;
  }
/*





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