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