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| author | Stefan Hajnoczi <stefanha@redhat.com> | 2020-02-18 18:27:08 +0000 |
|---|---|---|
| committer | Stefan Hajnoczi <stefanha@redhat.com> | 2020-02-22 08:26:47 +0000 |
| commit | f25c0b547916962d0b1be260b5b643287bea0851 (patch) | |
| tree | 15de99bb92f45f7ffd006085ccb8f5e6fcd40427 /util/aio-posix.c | |
| parent | c9b7d9ec21dfca716f0bb3b68dee75660d86629c (diff) | |
| download | focaccia-qemu-f25c0b547916962d0b1be260b5b643287bea0851.tar.gz focaccia-qemu-f25c0b547916962d0b1be260b5b643287bea0851.zip | |
aio-posix: avoid reacquiring rcu_read_lock() when polling
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 <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20200218182708.914552-1-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'util/aio-posix.c')
| -rw-r--r-- | util/aio-posix.c | 11 |
1 files changed, 11 insertions, 0 deletions
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) && |