• Peter Portante's avatar
    qemu-timer.c: Remove 250us timeouts · 158fd3ce
    Peter Portante authored
    Basically, the main wait loop calls qemu_run_all_timers() unconditionally. The
    first thing this routine used to do is to see if a timer had been serviced,
    and then reset the loop timeout to the next deadline.
    
    However, the new deadlines had not been calculated at that point, as
    qemu_run_timers() had not been called yet for each of the clocks. So
    qemu_rearm_alarm_timer() would end up with a negative or zero deadline, and
    default to setting a 250us timeout for the loop.
    
    As qemu_run_timers() is called for each clock, the real deadlines would be put
    in place, but because a loop timeout was already set, the loop timeout would
    not be changed.
    
    Once that 250us timeout fired, the real deadline would be used for the
    subsequent timeout.
    
    For idle VMs, this effectively doubles the number of times through the loop,
    doubling the number of select() system calls, timer calls, etc. putting added
    scheduling pressure on the kernel. And under cgroups, this really causes a big
    problem because the cgroup code does not scale well.
    
    By simply running the timers before trying to rearm the timer, we always rearm
    with a non-zero deadline, effectively halving the number of system calls.
    Signed-off-by: default avatarPeter Portante <pportant@redhat.com>
    Signed-off-by: default avatarAnthony Liguori <aliguori@us.ibm.com>
    158fd3ce
qemu-timer.c 19.6 KB