[Qemu-devel] [PATCH] block: Let bdrv_drain_all() to call aio_poll() for each AioContext

[Alexander Yarygin]

After the commit 9b536adc ("block: acquire AioContext in
bdrv_drain_all()") the aio_poll() function got called for every
BlockDriverState, in assumption that every device may have its own
AioContext. The bdrv_drain_all() function is called in each
virtio_reset() call, which in turn is called for every virtio-blk
device on initialization, so we got aio_poll() called
'length(device_list)^2' times.

If we have thousands of disks attached, there are a lot of
BlockDriverStates but only a few AioContexts, leading to tons of
unnecessary aio_poll() calls. For example, startup times with 1000 disks
takes over 13 minutes.

This patch changes the bdrv_drain_all() function allowing it find shared
AioContexts and to call aio_poll() only for unique ones. This results in
much better startup times, e.g. 1000 disks do come up within 5 seconds.

Cc: Christian Borntraeger <address@hidden>
Cc: Cornelia Huck <address@hidden>
Cc: Kevin Wolf <address@hidden>
Cc: Paolo Bonzini <address@hidden>
Cc: Stefan Hajnoczi <address@hidden>
Signed-off-by: Alexander Yarygin <address@hidden>
---
 block.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index f2f8ae7..7414815 100644
--- a/block.c
+++ b/block.c
@@ -1994,7 +1994,6 @@ static bool bdrv_drain_one(BlockDriverState *bs)
     bdrv_flush_io_queue(bs);
     bdrv_start_throttled_reqs(bs);
     bs_busy = bdrv_requests_pending(bs);
-    bs_busy |= aio_poll(bdrv_get_aio_context(bs), bs_busy);
     return bs_busy;
 }
 
@@ -2010,8 +2009,12 @@ static bool bdrv_drain_one(BlockDriverState *bs)
  */
 void bdrv_drain(BlockDriverState *bs)
 {
-    while (bdrv_drain_one(bs)) {
+    bool busy = true;
+
+    while (busy) {
         /* Keep iterating */
+        busy = bdrv_drain_one(bs);
+        busy |= aio_poll(bdrv_get_aio_context(bs), busy);
     }
 }
 
@@ -2032,6 +2035,7 @@ void bdrv_drain_all(void)
     /* Always run first iteration so any pending completion BHs run */
     bool busy = true;
     BlockDriverState *bs;
+    GList *aio_ctxs = NULL;
 
     while (busy) {
         busy = false;
@@ -2041,9 +2045,14 @@ void bdrv_drain_all(void)
 
             aio_context_acquire(aio_context);
             busy |= bdrv_drain_one(bs);
+            if (!aio_ctxs || !g_list_find(aio_ctxs, aio_context)) {
+                busy |= aio_poll(aio_context, busy);
+                aio_ctxs = g_list_append(aio_ctxs, aio_context);
+            }
             aio_context_release(aio_context);
         }
     }
+    g_list_free(aio_ctxs);
 }
 
 /* make a BlockDriverState anonymous by removing from bdrv_state and
-- 
1.9.1