• Sebastian Tanase's avatar
    cpu-exec: Add sleeping algorithm · c2aa5f81
    Sebastian Tanase authored
    The goal is to sleep qemu whenever the guest clock
    is in advance compared to the host clock (we use
    the monotonic clocks). The amount of time to sleep
    is calculated in the execution loop in cpu_exec.
    
    At first, we tried to approximate at each for loop the real time elapsed
    while searching for a TB (generating or retrieving from cache) and
    executing it. We would then approximate the virtual time corresponding
    to the number of virtual instructions executed. The difference between
    these 2 values would allow us to know if the guest is in advance or delayed.
    However, the function used for measuring the real time
    (qemu_clock_get_ns(QEMU_CLOCK_REALTIME)) proved to be very expensive.
    We had an added overhead of 13% of the total run time.
    
    Therefore, we modified the algorithm and only take into account the
    difference between the 2 clocks at the begining of the cpu_exec function.
    During the for loop we try to reduce the advance of the guest only by
    computing the virtual time elapsed and sleeping if necessary. The overhead
    is thus reduced to 3%. Even though this method still has a noticeable
    overhead, it no longer is a bottleneck in trying to achieve a better
    guest frequency for which the guest clock is faster than the host one.
    
    As for the the alignement of the 2 clocks, with the first algorithm
    the guest clock was oscillating between -1 and 1ms compared to the host clock.
    Using the second algorithm we notice that the guest is 5ms behind the host, which
    is still acceptable for our use case.
    
    The tests where conducted using fio and stress. The host machine in an i5 CPU at
    3.10GHz running Debian Jessie (kernel 3.12). The guest machine is an arm versatile-pb
    built with buildroot.
    
    Currently, on our test machine, the lowest icount we can achieve that is suitable for
    aligning the 2 clocks is 6. However, we observe that the IO tests (using fio) are
    slower than the cpu tests (using stress).
    Signed-off-by: default avatarSebastian Tanase <sebastian.tanase@openwide.fr>
    Tested-by: default avatarCamille Bégué <camille.begue@openwide.fr>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    c2aa5f81
cpu-exec.c 32 KB