[pve-devel] [PATCH qemu 02/11] PVE: block/pbs: fast-path reads without allocation if possible

Stefan Reiter s.reiter at proxmox.com
Mon Jan 11 12:14:00 CET 2021


...and switch over to g_malloc/g_free while at it to align with other
QEMU code.

Tracing shows the fast-path is taken almost all the time, though not
100% so the slow one is still necessary.

Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
---

For that slight unnoticable performance boost :)

 block/pbs.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/block/pbs.c b/block/pbs.c
index 1481a2bfd1..fbf0d8d845 100644
--- a/block/pbs.c
+++ b/block/pbs.c
@@ -200,7 +200,16 @@ static coroutine_fn int pbs_co_preadv(BlockDriverState *bs,
     BDRVPBSState *s = bs->opaque;
     int ret;
     char *pbs_error = NULL;
-    uint8_t *buf = malloc(bytes);
+    uint8_t *buf;
+    bool inline_buf = true;
+
+    /* for single-buffer IO vectors we can fast-path the write directly to it */
+    if (qiov->niov == 1 && qiov->iov->iov_len >= bytes) {
+        buf = qiov->iov->iov_base;
+    } else {
+        inline_buf = false;
+        buf = g_malloc(bytes);
+    }
 
     ReadCallbackData rcb = {
         .co = qemu_coroutine_self(),
@@ -218,8 +227,10 @@ static coroutine_fn int pbs_co_preadv(BlockDriverState *bs,
         return -EIO;
     }
 
-    qemu_iovec_from_buf(qiov, 0, buf, bytes);
-    free(buf);
+    if (!inline_buf) {
+        qemu_iovec_from_buf(qiov, 0, buf, bytes);
+        g_free(buf);
+    }
 
     return ret;
 }
-- 
2.20.1





More information about the pve-devel mailing list