Re: [PATCH v2 1/2] tests/qht-bench: Adjust testing rate by -1

[Philippe Mathieu-Daudé]

On 6/26/20 10:09 PM, Richard Henderson wrote:
> Since the seed must be non-zero, subtracting 1 means puts the
> rate in 0..UINT64_MAX-1, which allows the 0 and UINT64_MAX
> thresholds to corrspond to 0% (never) and 100% (always).
> 
> Suggested-by: Emilio G. Cota <cota@braap.org>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  tests/qht-bench.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/tests/qht-bench.c b/tests/qht-bench.c
> index eb88a90137..ad885d89d0 100644
> --- a/tests/qht-bench.c
> +++ b/tests/qht-bench.c
> @@ -25,7 +25,13 @@ struct thread_stats {
>  struct thread_info {
>      void (*func)(struct thread_info *);
>      struct thread_stats stats;
> -    uint64_t r;
> +    /*
> +     * Seed is in the range [1..UINT64_MAX], because the RNG requires
> +     * a non-zero seed.  To use, subtract 1 and compare against the
> +     * threshold with </>=.  This lets threshold = 0 never match (0% hit),
> +     * and threshold = UINT64_MAX always match (100% hit).
> +     */
> +    uint64_t seed;
>      bool write_op; /* writes alternate between insertions and removals */
>      bool resize_down;
>  } QEMU_ALIGNED(64); /* avoid false sharing among threads */
> @@ -131,8 +137,9 @@ static uint64_t xorshift64star(uint64_t x)
>  static void do_rz(struct thread_info *info)
>  {
>      struct thread_stats *stats = &info->stats;
> +    uint64_t r = info->seed - 1;
>  
> -    if (info->r < resize_threshold) {
> +    if (r < resize_threshold) {
>          size_t size = info->resize_down ? resize_min : resize_max;
>          bool resized;
>  
> @@ -151,13 +158,14 @@ static void do_rz(struct thread_info *info)
>  static void do_rw(struct thread_info *info)
>  {
>      struct thread_stats *stats = &info->stats;
> +    uint64_t r = info->seed - 1;
>      uint32_t hash;
>      long *p;
>  
> -    if (info->r >= update_threshold) {
> +    if (r >= update_threshold) {
>          bool read;
>  
> -        p = &keys[info->r & (lookup_range - 1)];
> +        p = &keys[r & (lookup_range - 1)];
>          hash = hfunc(*p);
>          read = qht_lookup(&ht, p, hash);
>          if (read) {
> @@ -166,7 +174,7 @@ static void do_rw(struct thread_info *info)
>              stats->not_rd++;
>          }
>      } else {
> -        p = &keys[info->r & (update_range - 1)];
> +        p = &keys[r & (update_range - 1)];
>          hash = hfunc(*p);
>          if (info->write_op) {
>              bool written = false;
> @@ -208,7 +216,7 @@ static void *thread_func(void *p)
>  
>      rcu_read_lock();
>      while (!atomic_read(&test_stop)) {
> -        info->r = xorshift64star(info->r);
> +        info->seed = xorshift64star(info->seed);
>          info->func(info);
>      }
>      rcu_read_unlock();
> @@ -221,7 +229,7 @@ static void *thread_func(void *p)
>  static void prepare_thread_info(struct thread_info *info, int i)
>  {
>      /* seed for the RNG; each thread should have a different one */
> -    info->r = (i + 1) ^ time(NULL);
> +    info->seed = (i + 1) ^ time(NULL);
>      /* the first update will be a write */
>      info->write_op = true;
>      /* the first resize will be down */
> 

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>