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authorDmitry Konishchev <konishchev@gmail.com>2011-05-18 15:03:59 +0400
committerKevin Wolf <kwolf@redhat.com>2011-05-18 14:39:15 +0200
commitf6a00aa150ec685966ca9453601328fdb2f87f34 (patch)
tree0a1ed191b8a845f9583132d1f68c13abdcff8f5d /qemu-img.c
parent6f321e93abb27b4e7ceb228b4204aa304e95daad (diff)
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qemu_img: is_not_zero() optimization
I run qemu-img under profiler and realized, that most of CPU time is consumed by is_not_zero() function. I had made a couple of optimizations on it and got the following output for `time qemu-img convert -O qcow2 volume.qcow2 snapshot.qcow2`: Original qemu-img: real 0m56.159s user 0m34.670s sys 0m12.079s Patched qemu-img: real 0m34.805s user 0m18.445s sys 0m12.552s Signed-off-by: Dmitry Konishchev <konishchev@gmail.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'qemu-img.c')
-rw-r--r--qemu-img.c29
1 files changed, 26 insertions, 3 deletions
diff --git a/qemu-img.c b/qemu-img.c
index 1da5484..4f162d1 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -496,14 +496,37 @@ static int img_commit(int argc, char **argv)
return 0;
}
+/*
+ * Checks whether the sector is not a zero sector.
+ *
+ * Attention! The len must be a multiple of 4 * sizeof(long) due to
+ * restriction of optimizations in this function.
+ */
static int is_not_zero(const uint8_t *sector, int len)
{
+ /*
+ * Use long as the biggest available internal data type that fits into the
+ * CPU register and unroll the loop to smooth out the effect of memory
+ * latency.
+ */
+
int i;
- len >>= 2;
- for(i = 0;i < len; i++) {
- if (((uint32_t *)sector)[i] != 0)
+ long d0, d1, d2, d3;
+ const long * const data = (const long *) sector;
+
+ len /= sizeof(long);
+
+ for(i = 0; i < len; i += 4) {
+ d0 = data[i + 0];
+ d1 = data[i + 1];
+ d2 = data[i + 2];
+ d3 = data[i + 3];
+
+ if (d0 || d1 || d2 || d3) {
return 1;
+ }
}
+
return 0;
}