Re: [PATCH] tests/qht-bench: Adjust rate computation and comparisons

[Emilio G. Cota]

On Sat, Jun 20, 2020 at 14:45:51 -0700, Richard Henderson wrote:
> Use <= comparisons vs the threshold, so that threshold UINT64_MAX
> is always true, corresponding to rate 1.0 being unity.  Simplify
> do_threshold scaling to 2**64, with a special case for 1.0.
> 
> Cc: Emilio G. Cota <cota@braap.org>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  tests/qht-bench.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/tests/qht-bench.c b/tests/qht-bench.c
> index eb88a90137..21b1b7de82 100644
> --- a/tests/qht-bench.c
> +++ b/tests/qht-bench.c
> @@ -132,7 +132,7 @@ static void do_rz(struct thread_info *info)
>  {
>      struct thread_stats *stats = &info->stats;
>  
> -    if (info->r < resize_threshold) {
> +    if (info->r <= resize_threshold) {
>          size_t size = info->resize_down ? resize_min : resize_max;
>          bool resized;

This works, but only because info->r cannot be 0 since xorshift never
returns it. (xorshift returns a random number in the range [1, u64max],
a fact that I missed when I wrote this code.)
If r were 0, then we would resize even if resize_threshold == 0.0.

I think it will be easier to reason about this if we rename info->r
to info->seed, and then have a local r = info->seed - 1. Then we can keep
the "if random < threshold" form (and its negated "if random >= threshold"
as below), which (at least to me) is intuitive provided that random's range
is [0, threshold), e.g. [0.0, 1.0) with drand48(3).

> @@ -154,7 +154,7 @@ static void do_rw(struct thread_info *info)
>      uint32_t hash;
>      long *p;
>  
> -    if (info->r >= update_threshold) {
> +    if (info->r > update_threshold) {
>          bool read;
>  
>          p = &keys[info->r & (lookup_range - 1)];
> @@ -281,11 +281,18 @@ static void pr_params(void)
>  
>  static void do_threshold(double rate, uint64_t *threshold)
>  {
> +    /*
> +     * For 0 <= rate <= 1, scale to fit in a uint64_t.
> +     *
> +     * For rate == 1, returning UINT64_MAX means 100% certainty: all
> +     * uint64_t will match using <=.  The largest representable value
> +     * for rate less than 1 is 0.999999999999999889; scaling that
> +     * by 2**64 results in 0xfffffffffffff800.
> +     */
>      if (rate == 1.0) {
>          *threshold = UINT64_MAX;
>      } else {
> -        *threshold = (rate * 0xffff000000000000ull)
> -                   + (rate * 0x0000ffffffffffffull);
> +        *threshold = rate * 0x1p64;

I'm sorry this caused a breakage for some integration tests; I thought
this was fixed in May with:
  https://lists.nongnu.org/archive/html/qemu-devel/2020-05/msg01477.html

Just for my own education, why isn't nextafter needed here?

Thanks,
                Emilio