Re: [FFmpeg-devel] [PATCH 1/2] avutil/random_seed: Speed up fate test

[Anton Khirnov <anton@khirnov.net>]

Quoting Michael Niedermayer (2022-01-25 11:21:08)
> This decreases the quality of the seeds during the test, it does not affect
> the seeds outside the test.
> There is a small chance that this causes test failures, if that happens
> the threshold needs adjusting
> 
> Testing on an idle x86_64 system shows that this passes even with the test strength
> increased to 32768 from 256 with 0 retries
> So test failures are not anticipated
> Lowering the threshold from 3 to 2 causes failure at test strength of 32768
> 
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavutil/random_seed.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
> index 70dc509d2f..509b74936c 100644
> --- a/libavutil/random_seed.c
> +++ b/libavutil/random_seed.c
> @@ -95,7 +95,7 @@ static uint32_t get_generic_seed(void)
>              last_td = t - last_t;
>              buffer[++i & 511] += last_td % 3294638521U;
>              if ((t - init_t) >= CLOCKS_PER_SEC>>5)
> -                if (last_i && i - last_i > 4 || i - last_i > 64 || TEST && i - last_i > 8)
> +                if (last_i && i - last_i > 4 || i - last_i > 64 || TEST && i - last_i > 3)

On my Ryzen 5950x, this brings the test runtime from > 60s to ~30s,
which is a good improvement, but still quite a lot (the entire FATE run
with -j32 and without the random-seed test is ~40s).

How about something like this instead (takes about 8s on my machines, observed
no test failures):
diff --git a/libavutil/random_seed.c b/libavutil/random_seed.c
index 70dc509d2f..b006a75c39 100644
--- a/libavutil/random_seed.c
+++ b/libavutil/random_seed.c
@@ -73,6 +73,7 @@ static uint32_t get_generic_seed(void)
     static uint32_t buffer[512] = { 0 };
     unsigned char digest[20];
     uint64_t last_i = i;
+    uint64_t cnt = 0;
 
     av_assert0(sizeof(tmp) >= av_sha_size);
 
@@ -88,9 +89,11 @@ static uint32_t get_generic_seed(void)
 
     for (;;) {
         clock_t t = clock();
-        if (last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >= t) {
+        if (last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >= t &&
+            !(TEST && cnt > (1 << 15))) {
             last_td = t - last_t;
             buffer[i & 511] = 1664525*buffer[i & 511] + 1013904223 + (last_td % 3294638521U);
+            cnt++;
         } else {
             last_td = t - last_t;
             buffer[++i & 511] += last_td % 3294638521U;

-- 
Anton Khirnov
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