Re: [Qemu-devel] [RFC PATCH V3 4/6] cpu_exec: Add sleeping algorithm

[Paolo Bonzini]

Il 30/06/2014 15:59, Sebastian Tanase ha scritto:
> 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: Sebastian Tanase <address@hidden>
> Tested-by: Camille Bégué <address@hidden>
> ---
>  cpu-exec.c | 112 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 112 insertions(+)
> 
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 38e5f02..ac741b7 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -22,6 +22,102 @@
>  #include "tcg.h"
>  #include "qemu/atomic.h"
>  #include "sysemu/qtest.h"
> +#include "qemu/timer.h"
> +
> +/* Structs and function pointers for delaying the host */
> +typedef struct SyncClocks SyncClocks;
> +typedef void (*init_delay_func)(SyncClocks *sc,
> +                                const CPUState *cpu);
> +typedef void (*perform_align_func)(SyncClocks *sc,
> +                                   const CPUState *cpu);
> +struct SyncClocks {
> +    int64_t diff_clk;
> +    int64_t original_instr_counter;
> +    init_delay_func init_delay;
> +    perform_align_func perform_align;
> +};
> +
> +#if !defined(CONFIG_USER_ONLY)
> +/* Allow the guest to have a max 3ms advance.
> + * The difference between the 2 clocks could therefore
> + * oscillate around 0.
> + */
> +#define VM_CLOCK_ADVANCE 3000000
> +
> +static int64_t delay_host(int64_t diff_clk)
> +{
> +    struct timespec sleep_delay, rem_delay;
> +    if (diff_clk > VM_CLOCK_ADVANCE) {
> +        sleep_delay.tv_sec = diff_clk / 1000000000LL;
> +        sleep_delay.tv_nsec = diff_clk % 1000000000LL;
> +        if (nanosleep(&sleep_delay, &rem_delay) < 0) {
> +            diff_clk -= (sleep_delay.tv_sec - rem_delay.tv_sec) * 
> 1000000000LL;
> +            diff_clk -= sleep_delay.tv_nsec - rem_delay.tv_nsec;
> +        } else {
> +            diff_clk = 0;
> +        }
> +    }
> +    return diff_clk;
> +}
> +
> +static int64_t instr_to_vtime(int64_t instr_counter, const CPUState *cpu)
> +{
> +    int64_t instr_exec_time;
> +    instr_exec_time = instr_counter -
> +                      (cpu->icount_extra +
> +                       cpu->icount_decr.u16.low);
> +    instr_exec_time = instr_exec_time << icount_time_shift;
> +
> +    return instr_exec_time;
> +}
> +
> +static void align_clocks(SyncClocks *sc, const CPUState *cpu)
> +{
> +    if (!icount_align_option) {
> +        return;
> +    }
> +    sc->diff_clk += instr_to_vtime(sc->original_instr_counter, cpu);
> +    sc->original_instr_counter = cpu->icount_extra + 
> cpu->icount_decr.u16.low;
> +    sc->diff_clk = delay_host(sc->diff_clk);
> +}
> +
> +static void init_delay_params(SyncClocks *sc,
> +                              const CPUState *cpu)
> +{
> +    static int64_t clocks_offset = -1;
> +    int64_t realtime_clock_value, virtual_clock_value;
> +    if (!icount_align_option) {
> +        return;
> +    }
> +    /* On x86 target architecture, the PIT reset function (called
> +       by qemu_system_reset) will end up calling qemu_clock_warp
> +       and then icount_warp_rt changing vm_clock_warp_start from 0 (initial
> +       value) to -1. This in turn will make us skip the initial offset
> +       between the real and virtual clocks (initially virtual clock is 0).
> +       Therefore we impose that the first time we run the cpu
> +       the host and virtual clocks should be aligned; we don't alter any of
> +       the clocks, we just calculate the difference between them. */

I'm not sure if these gory details are really relevant.  The point, I 
think, is basically that the bases of QEMU_CLOCK_REALTIME and 
QEMU_CLOCK_VIRTUAL differ.  QEMU_CLOCK_REALTIME is based at the Unix epoch,
QEMU_CLOCK_VIRTUAL is based at the time QEMU starts.


> +    realtime_clock_value = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> +    virtual_clock_value = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +    if (clocks_offset == -1) {
> +        clocks_offset = realtime_clock_value - virtual_clock_value;
> +    }
> +    sc->diff_clk = virtual_clock_value - realtime_clock_value + 
> clocks_offset;
> +    sc->original_instr_counter = cpu->icount_extra + 
> cpu->icount_decr.u16.low;
> +}
> +#else
> +/* We don't use the align feature for User emulation
> +   thus we add empty functions which shall be ignored
> +   by the compiler */
> +static void align_clocks(SyncClocks *sc, const CPUState *cpu)
> +{
> +}
> +
> +static void init_delay_params(SyncClocks *sc,
> +                              const CPUState *cpu)
> +{
> +}
> +#endif /* CONFIG USER ONLY */
>  
>  void cpu_loop_exit(CPUState *cpu)
>  {
> @@ -227,6 +323,11 @@ int cpu_exec(CPUArchState *env)
>      TranslationBlock *tb;
>      uint8_t *tc_ptr;
>      uintptr_t next_tb;
> +    /* Delay algorithm */
> +    static SyncClocks sc = {

This need not be static.

> +        .init_delay = init_delay_params,
> +        .perform_align = align_clocks
> +    };
>      /* This must be volatile so it is not trashed by longjmp() */
>      volatile bool have_tb_lock = false;
>  
> @@ -283,6 +384,11 @@ int cpu_exec(CPUArchState *env)
>  #endif
>      cpu->exception_index = -1;
>  
> +    /* Calculate difference between guest clock and host clock.
> +       This delay includes the delay of the last cycle, so
> +       what we have to do is sleep until it is 0. As for the
> +       advance/delay we gain here, we try to fix it next time. */
> +    sc.init_delay(&sc, cpu);
>      /* prepare setjmp context for exception handling */
>      for(;;) {
>          if (sigsetjmp(cpu->jmp_env, 0) == 0) {
> @@ -672,6 +778,9 @@ int cpu_exec(CPUArchState *env)
>                              if (insns_left > 0) {
>                                  /* Execute remaining instructions.  */
>                                  cpu_exec_nocache(env, insns_left, tb);
> +                               /* Try to align the host and virtual clocks
> +                                  if the guest is in advance. */
> +                                sc.perform_align(&sc, cpu);
>                              }
>                              cpu->exception_index = EXCP_INTERRUPT;
>                              next_tb = 0;
> @@ -684,6 +793,9 @@ int cpu_exec(CPUArchState *env)
>                      }
>                  }
>                  cpu->current_tb = NULL;
> +                /* Try to align the host and virtual clocks
> +                   if the guest is in advance */
> +                sc.perform_align(&sc, cpu);
>                  /* reset soft MMU for next block (it can currently
>                     only be set by a memory fault) */
>              } /* for(;;) */
>