ppoll(2) doesn't scale as well as epoll: The elapsed time of the syscall is linear to the number of fd's we poll, which hurts performance a bit when the number of devices are many, or when a virtio device registers many virtqueues (virtio-serial, for instance).
To show some data from my test on current master: - As a base point (10~20 fd's), it takes 22000 ns for each qemu_poll_ns. - Add 10 virtio-serial, which adds some 6 hundreds of fd's in the main loop. The time spent in qemu_poll_ns goes up to 75000 ns. This series introduces qemu_poll, which is implemented with g_poll and epoll, decided at configure time with CONFIG_EPOLL. After this change, the times to do the same thing with qemu_poll (more precisely, with a sequence of qemu_poll_set_fds(), qemu_poll(), qemu_poll_get_events() followed by syncing back to gpollfds), are reduced to 21000 ns and 25000 ns, respectively. We are still not O(1) because as a transition, the qemu_poll_set_fds before qemu_poll is not optimized out yet. Fam Fam Zheng (5): poll: Introduce QEMU Poll API posix-aio: Use QEMU poll interface poll: Add epoll implementation for qemu_poll main-loop: Replace qemu_poll_ns with qemu_poll tests: Add test case for qemu_poll Makefile.objs | 2 + aio-posix.c | 52 ++++----- async.c | 5 +- include/block/aio.h | 7 +- include/qemu/poll.h | 40 +++++++ include/qemu/timer.h | 13 --- include/qemu/typedefs.h | 2 + main-loop.c | 35 ++++++- poll-glib.c | 130 +++++++++++++++++++++++ poll-linux.c | 216 ++++++++++++++++++++++++++++++++++++++ qemu-timer.c | 21 ---- tests/Makefile | 2 + tests/test-poll.c | 272 ++++++++++++++++++++++++++++++++++++++++++++++++ 13 files changed, 723 insertions(+), 74 deletions(-) create mode 100644 include/qemu/poll.h create mode 100644 poll-glib.c create mode 100644 poll-linux.c create mode 100644 tests/test-poll.c -- 1.9.3
