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[PULL 02/31] aio-posix: avoid reacquiring rcu_read_lock() when polling
From: |
Stefan Hajnoczi |
Subject: |
[PULL 02/31] aio-posix: avoid reacquiring rcu_read_lock() when polling |
Date: |
Sat, 22 Feb 2020 08:50:01 +0000 |
The first rcu_read_lock/unlock() is expensive. Nested calls are cheap.
This optimization increases IOPS from 73k to 162k with a Linux guest
that has 2 virtio-blk,num-queues=1 and 99 virtio-blk,num-queues=32
devices.
Signed-off-by: Stefan Hajnoczi <address@hidden>
Reviewed-by: Paolo Bonzini <address@hidden>
Message-id: address@hidden
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
util/aio-posix.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/util/aio-posix.c b/util/aio-posix.c
index a4977f538e..f67f5b34e9 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -15,6 +15,7 @@
#include "qemu/osdep.h"
#include "block/block.h"
+#include "qemu/rcu.h"
#include "qemu/rcu_queue.h"
#include "qemu/sockets.h"
#include "qemu/cutils.h"
@@ -514,6 +515,16 @@ static bool run_poll_handlers_once(AioContext *ctx,
int64_t *timeout)
bool progress = false;
AioHandler *node;
+ /*
+ * Optimization: ->io_poll() handlers often contain RCU read critical
+ * sections and we therefore see many rcu_read_lock() -> rcu_read_unlock()
+ * -> rcu_read_lock() -> ... sequences with expensive memory
+ * synchronization primitives. Make the entire polling loop an RCU
+ * critical section because nested rcu_read_lock()/rcu_read_unlock() calls
+ * are cheap.
+ */
+ RCU_READ_LOCK_GUARD();
+
QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
if (!node->deleted && node->io_poll &&
aio_node_check(ctx, node->is_external) &&
--
2.24.1
- [PULL 00/31] Block patches, Stefan Hajnoczi, 2020/02/22
- [PULL 01/31] virtio: increase virtqueue size for virtio-scsi and virtio-blk, Stefan Hajnoczi, 2020/02/22
- [PULL 02/31] aio-posix: avoid reacquiring rcu_read_lock() when polling,
Stefan Hajnoczi <=
- [PULL 03/31] rcu_queue: add QSLIST functions, Stefan Hajnoczi, 2020/02/22
- [PULL 04/31] util/async: make bh_aio_poll() O(1), Stefan Hajnoczi, 2020/02/22
- [PULL 05/31] aio-posix: fix use after leaving scope in aio_poll(), Stefan Hajnoczi, 2020/02/22
- [PULL 06/31] aio-posix: don't pass ns timeout to epoll_wait(), Stefan Hajnoczi, 2020/02/22
- [PULL 07/31] qemu/queue.h: add QLIST_SAFE_REMOVE(), Stefan Hajnoczi, 2020/02/22
- [PULL 08/31] aio-posix: make AioHandler deletion O(1), Stefan Hajnoczi, 2020/02/22
- [PULL 09/31] aio-posix: make AioHandler dispatch O(1) with epoll, Stefan Hajnoczi, 2020/02/22
- [PULL 10/31] softmmu: move vl.c to softmmu/, Stefan Hajnoczi, 2020/02/22
- [PULL 11/31] softmmu: split off vl.c:main() into main.c, Stefan Hajnoczi, 2020/02/22
- [PULL 12/31] module: check module wasn't already initialized, Stefan Hajnoczi, 2020/02/22