Re: [libvirt] [PATCH v4b] Add functions for handling exponential backoff loops.

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(-)

-    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;

+/** + * 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