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Re: [PATCH] tests/qht-bench: Adjust rate computation and comparisons
From: |
Emilio G. Cota |
Subject: |
Re: [PATCH] tests/qht-bench: Adjust rate computation and comparisons |
Date: |
Sun, 21 Jun 2020 17:28:25 -0400 |
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