On Wed, Nov 16, 2016 at 07:14:47PM +0100, Paolo Bonzini wrote: > > > On 16/11/2016 18:47, Stefan Hajnoczi wrote: > > The AioContext event loop uses ppoll(2) or epoll_wait(2) to monitor file > > descriptors or until a timer expires. In cases like virtqueues, Linux > > AIO, and ThreadPool it is technically possible to wait for events via > > polling (i.e. continuously checking for events without blocking). > > > > Polling can be faster than blocking syscalls because file descriptors, > > the process scheduler, and system calls are bypassed. > > > > The main disadvantage to polling is that it increases CPU utilization. > > In classic polling configuration a full host CPU thread might run at > > 100% to respond to events as quickly as possible. This patch implements > > a timeout so we fall back to blocking syscalls if polling detects no > > activity. After the timeout no CPU cycles are wasted on polling until > > the next event loop iteration. > > > > This patch implements an experimental polling mode that can be > > controlled with the QEMU_AIO_POLL_MAX_NS=<nanoseconds> environment > > variable. The aio_poll() event loop function will attempt to poll > > instead of using blocking syscalls. > > > > The run_poll_handlers_begin() and run_poll_handlers_end() trace events > > are added to aid performance analysis and troubleshooting. If you need > > to know whether polling mode is being used, trace these events to find > > out. > > > > Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> > > --- > > aio-posix.c | 107 > > +++++++++++++++++++++++++++++++++++++++++++++++++++- > > async.c | 11 +++++- > > include/block/aio.h | 3 ++ > > trace-events | 4 ++ > > 4 files changed, 123 insertions(+), 2 deletions(-) > > Nice! > > > > diff --git a/aio-posix.c b/aio-posix.c > > index 4379c13..5e5a561 100644 > > --- a/aio-posix.c > > +++ b/aio-posix.c > > @@ -18,6 +18,8 @@ > > #include "block/block.h" > > #include "qemu/queue.h" > > #include "qemu/sockets.h" > > +#include "qemu/cutils.h" > > +#include "trace.h" > > #ifdef CONFIG_EPOLL_CREATE1 > > #include <sys/epoll.h> > > #endif > > @@ -27,12 +29,16 @@ struct AioHandler > > GPollFD pfd; > > IOHandler *io_read; > > IOHandler *io_write; > > + AioPollFn *io_poll; > > int deleted; > > void *opaque; > > bool is_external; > > QLIST_ENTRY(AioHandler) node; > > }; > > > > +/* How long to poll AioPollHandlers before monitoring file descriptors */ > > +static int64_t aio_poll_max_ns; > > + > > #ifdef CONFIG_EPOLL_CREATE1 > > > > /* The fd number threashold to switch to epoll */ > > @@ -206,11 +212,12 @@ void aio_set_fd_handler(AioContext *ctx, > > AioHandler *node; > > bool is_new = false; > > bool deleted = false; > > + int poll_disable_cnt = 0; > > poll_disable_cnt = !io_poll - !node->io_poll > > ? Not the most readable thing, but effective... > > > node = find_aio_handler(ctx, fd);
Taking into account creation of new nodes:
if (node) {
poll_disable_cnt = !io_poll - !node->io_poll;
} else {
poll_disable_cnt = !io_poll;
}
This does shorten the code quite a bit so I'll use it in the next
version.
> > +static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
> > +{
> > + bool progress = false;
> > + int64_t end_time;
> > +
> > + assert(ctx->notify_me);
> > + assert(ctx->walking_handlers > 0);
> > + assert(ctx->poll_disable_cnt == 0);
> > +
> > + trace_run_poll_handlers_begin(ctx, max_ns);
> > +
> > + end_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + max_ns;
> > +
> > + do {
> > + AioHandler *node;
> > +
> > + /* Bail if aio_notify() was called (e.g. BH was scheduled) */
> > + if (atomic_read(&ctx->notified)) {
> > + progress = true;
> > + break;
> > + }
>
> This can be done in event_notifier_dummy_poll.
Nice idea, then the "dummy" function can serve a real purpose!
Stefan
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