On Fri, 2016-04-15 at 09:49 +0100, Richard W.M. Jones wrote:
> In a few places in libvirt we busy-wait for events, for example qemu
> creating a monitor socket. This is problematic because:
>
> - We need to choose a sufficiently small polling period so that
> libvirt doesn't add unnecessary delays.
>
> - We need to choose a sufficiently large polling period so that
> the effect of busy-waiting doesn't affect the system.
>
> The solution to this conflict is to use an exponential backoff.
>
> This patch adds two functions to hide the details, and modifies a few
> places where we currently busy-wait.
> ---
> src/fdstream.c | 10 +++++----
> src/libvirt_private.syms | 2 ++
> src/qemu/qemu_agent.c | 9 ++++----
> src/qemu/qemu_monitor.c | 10 +++++----
> src/util/virtime.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++
> src/util/virtime.h | 38 ++++++++++++++++++++++++++++++++++
> 6 files changed, 111 insertions(+), 12 deletions(-)
Just a couple of comments passing by...
> - do {
> + if (virTimeBackOffStart(&timeout, 1, 3*1000 /* ms */) < 0)
> + goto error;
> + while (virTimeBackOffWhile(&timeout)) {
> ret = connect(fd, (struct sockaddr *)&sa, sizeof(sa));
> if (ret == 0)
> break;
Having two "whiles" like that looks kinda off... I'd rather
have something like
while (!virTimeBackOffHasExpired(&timeout))
or preferably something better than what I can come up with :)
> +/**
> + * virTimeBackOffWhile
> + * @var: Timeout variable (with type virTimeBackOffVar *).
> + *
> + * You must initialize @var first by calling the following function,
> + * which also starts the timer:
> + *
> + * if (virTimeBackOffStart(&var, first, timeout) < 0) {
> + * // handle errors
> + * }
> + *
> + * Then you use a while loop:
> + *
> + * while (virTimeBackOffWhile(&var)) {
> + * //...
> + * }
> + *
> + * The while loop that runs the body of the code repeatedly, with an
> + * exponential backoff. It first waits for first milliseconds, then
> + * runs the body, then waits for 2*first ms, then runs the body again.
> + * Then 4*first ms, and so on.
> + *
> + * When timeout milliseconds is reached, the while loop ends.
> + *
> + * The body should use "break" or "goto" when whatever condition it is
> + * testing for succeeds (or there is an unrecoverable error).
> + */
> +bool virTimeBackOffWhile(virTimeBackOffVar *var);
> +
> #endif
API documentation should live in the .c file, like you did
with virTimeBackOffStart(). I guess it's just a consequence
of the "a" implementation using a macro for this part :)
Cheers.
--
Andrea Bolognani
Software Engineer - Virtualization Team
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