Re: [Qemu-devel] [PATCH v4 00/10] main-loop: switch to g_poll(3) on POSIX hosts

[Laurent Desnogues]

On Wed, Feb 20, 2013 at 11:28 AM, Stefan Hajnoczi <address@hidden> wrote:
> Amos Kong <address@hidden> reported that file descriptors numbered higher
> than 1024 could crash QEMU.  This is due to the fixed size of the fd_set type
> used for select(2) event polling.
>
> This series converts the main-loop.c and aio-posix.c select(2) calls to
> g_poll(3).  This eliminates the fd_set type and allows QEMU to scale to high
> numbers of file descriptors.
>
> The g_poll(3) interface is a portable version of the poll(2) system call.  The
> difference to select(2) is that fine-grained events (G_IO_IN, G_IO_OUT,
> G_IO_HUP, G_IO_ERR, G_IO_PRI) can be monitored instead of just
> read/write/exception.  Also, there is no limit to the file descriptor numbers
> that may be used, allowing applications to scale to many file descriptors.  
> See
> the documentation for details:
>
>   http://developer.gnome.org/glib/2.28/glib-The-Main-Event-Loop.html#g-poll

g_poll isn't available in older glib versions used by CentOS (2.12
on CentOS 5.6).

Thanks,

Laurent

> The QEMU main loop works as follows today:
>
> 1. Call out to slirp, iohandlers, and glib sources to fill rfds/wfds/xfds with
>    the file descriptors to select(2).
> 2. Perform the select(2) call.
> 3. Call out to slirp, iohandlers, and glib sources to handle events polled in
>    rfds/wfds/xfds.
>
> The plan of attack is as follows:
>
> 1. Replace select(2) with g_poll(3).  Use glue that converts between
>    rfds/wfds/xfds and GPollFD so that the unconverted QEMU components still
>    work.
>
> 2. Convert slirp, iohandlers, and glib source fill/poll functions to use
>    GPollFD directly instead of rfds/wfds/xfds.
>
> 3. Drop the glue since all components now natively use GPollFD.
>
> 4. Convert aio-posix.c to g_poll(3) by reusing GPollFD.
>
> I have tested that the series builds and is bisectable on Linux and Windows
> hosts.  But I have not done extensive testing on other host platforms or with
> long-term guests to check for performance regressions.
>
> v4:
>  * Assign revents instead of bitwise OR to make code clearer [Laszlo]
>  * Invoke pollfds_poll() only when select(2) succeeds [Laszlo]
>  * Fix gpollfds_to_select(select_ret || g_poll_ret) call [Laszlo]
>
> v3:
>  * Convert slirp/slirp.c to tabs to spaces [Blue Swirl]
>
> v2:
>  * Replace custom Poller type with GArray [aliguori]
>
> Stefan Hajnoczi (10):
>   main-loop: fix select_ret uninitialized variable warning
>   main-loop: switch to g_poll() on POSIX hosts
>   main-loop: switch POSIX glib integration to GPollFD
>   slirp: slirp/slirp.c coding style cleanup
>   slirp: switch to GPollFD
>   iohandler: switch to GPollFD
>   main-loop: drop rfds/wfds/xfds for good
>   aio: extract aio_dispatch() from aio_poll()
>   aio: convert aio_poll() to g_poll(3)
>   aio: support G_IO_HUP and G_IO_ERR
>
>  aio-posix.c              | 130 ++++-----
>  async.c                  |   2 +
>  include/block/aio.h      |   3 +
>  include/qemu/main-loop.h |   4 +-
>  iohandler.c              |  40 ++-
>  main-loop.c              | 158 ++++++-----
>  slirp/libslirp.h         |   6 +-
>  slirp/main.h             |   1 -
>  slirp/slirp.c            | 673 
> +++++++++++++++++++++++++----------------------
>  slirp/socket.c           |   9 -
>  slirp/socket.h           |   2 +
>  stubs/slirp.c            |   6 +-
>  12 files changed, 547 insertions(+), 487 deletions(-)
>
> --
> 1.8.1.2
>
>