[Libguestfs] [PATCH libnbd 2/3] examples: copy-libev: Do not initialize pread buffer
This safety feature is not needed since we handle pread errors, and even
if we did not, we use initialized buffers. Testing shows 10% speedup
when copying a real image.
$ qemu-nbd --read-only --persistent --shared 8 --cache none --aio native \
--socket /tmp/src.sock --format raw fedora-35-data.raw &
$ qemu-nbd --persistent --shared 8 --cache none --aio native --discard unmap \
--socket /tmp/dst.sock --format raw dst.raw &
$ hyperfine -p "sleep 5" "./copy-libev-init $SRC $DST"
"./copy-libev-no-init $SRC $DST"
Benchmark 1: ./copy-libev-init nbd+unix:///?socket=/tmp/src.sock
nbd+unix:///?socket=/tmp/dst.sock
Time (mean ± σ): 2.895 s ± 0.026 s [User: 0.387 s, System: 1.645 s]
Range (min … max): 2.853 s … 2.933 s 10 runs
Benchmark 2: ./copy-libev-no-init nbd+unix:///?socket=/tmp/src.sock
nbd+unix:///?socket=/tmp/dst.sock
Time (mean ± σ): 2.636 s ± 0.034 s [User: 0.052 s, System: 1.628 s]
Range (min … max): 2.557 s … 2.677 s 10 runs
Summary
'./copy-libev-no-init nbd+unix:///?socket=/tmp/src.sock
nbd+unix:///?socket=/tmp/dst.sock' ran
1.10 ± 0.02 times faster than './copy-libev-init nbd+unix:///?socket=/tmp/src.sock
nbd+unix:///?socket=/tmp/dst.sock'
Signed-off-by: Nir Soffer <nsoffer(a)redhat.com>
---
examples/copy-libev.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/examples/copy-libev.c b/examples/copy-libev.c
index ad50b64c..f36939a7 100644
--- a/examples/copy-libev.c
+++ b/examples/copy-libev.c
@@ -647,20 +647,26 @@ main (int argc, char *argv[])
size = nbd_get_size (src.nbd);
if (size > nbd_get_size (dst.nbd))
FAIL ("Destinatio is not large enough\n");
/* Check destination server capabilities. */
dst.can_zero = nbd_can_zero (dst.nbd) > 0;
+ /* Disable pread buffer initialization. This is not needed since we
+ * handle pread errors, and even if we fail to handle errors, we use
+ * initialized buffers. */
+
+ nbd_set_pread_initialize (src.nbd, false);
+
/* Start the copy "loop". When request completes, it starts the
* next request, until entire image was copied. */
for (i = 0; i < MAX_REQUESTS; i++) {
struct request *r = &requests[i];
r->index = i;
/*
* Clear the buffer before starting the copy, so if we fail to
* handle a read error we will not write uninitilized data to
--
2.35.1