On 18/02/20 19:27, Stefan Hajnoczi wrote:
> 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 <stefa...@redhat.com>
> ---
> 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) &&
>
Reviewed-by: Paolo Bonzini <pbonz...@redhat.com>
