On Sat, Apr 09, 2016 at 08:39:58PM +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 a macro VIR_TIME_WHILE_WITH_BACKOFF 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 | 10 +++++---- > src/qemu/qemu_monitor.c | 10 +++++---- > src/util/virtime.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++ > src/util/virtime.h | 34 ++++++++++++++++++++++++++++++ > 6 files changed, 109 insertions(+), 12 deletions(-) > > diff --git a/src/util/virtime.h b/src/util/virtime.h > index 8ebad38..b0d9f89 100644 > --- a/src/util/virtime.h > +++ b/src/util/virtime.h > @@ -64,4 +64,38 @@ char *virTimeStringThen(unsigned long long when); > int virTimeLocalOffsetFromUTC(long *offset) > ATTRIBUTE_NONNULL(1) ATTRIBUTE_RETURN_CHECK; > > +/** > + * VIR_TIME_WHILE_WITH_BACKOFF: > + * @var: Timeout variable (with type virTimeBackOffVar). > + * > + * You must initialize @var first by calling this function, which > + * also starts the timer: > + * > + * virTimeBackOffStart(&var, first, timeout); > + * > + * This macro is a 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). > + */ > +#define VIR_TIME_WHILE_WITH_BACKOFF(var) \ > + while (virTimeBackOffCondition(&(var))) This still unnecessarily hides the while keyword. ACK to the rest. Jan -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list