Re: [PATCH 2/2] oslat: Don't take trace_threshold into account during preheat

[John Kacur <jkacur@redhat.com>]

On Wed, 7 Jul 2021, Nicolas Saenz Julienne wrote:

> The point of preheat is to make sure CPUs are out of idle and running at
> max frequency by the time the real test starts. So it's expected to
> incur into extra latencies we don't really mean to measure. With this in
> mind, it doesn't make sense to take into account the trace threshold
> during that run. So don't do it.
> 
> Note that this has been observed in practice. The threshold would be hit
> during preheat but not during the real test.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
> ---
>  src/oslat/oslat.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/src/oslat/oslat.c b/src/oslat/oslat.c
> index 1cba6fc..6ff5ba8 100644
> --- a/src/oslat/oslat.c
> +++ b/src/oslat/oslat.c
> @@ -29,6 +29,7 @@
>  #include <numa.h>
>  #include <math.h>
>  #include <limits.h>
> +#include <stdbool.h>
>  
>  #include <sys/prctl.h>
>  #include <sys/stat.h>
> @@ -150,6 +151,7 @@ struct thread {
>  struct global {
>  	/* Configuration. */
>  	unsigned int          runtime_secs;
> +	bool		      preheat;
>  	/*
>  	 * Number of threads running for current test
>  	 * (either pre heat or real run)
> @@ -299,7 +301,7 @@ static void insert_bucket(struct thread *t, stamp_t value)
>  	us = index + 1;
>  	assert(us > 0);
>  
> -	if (g.trace_threshold && us >= g.trace_threshold) {
> +	if (!g.preheat && g.trace_threshold && us >= g.trace_threshold) {
>  		char *line = "%s: Trace threshold (%d us) triggered with %u us!\n"
>  		    "Stopping the test.\n";
>  		tracemark(line, g.app_name, g.trace_threshold, us);
> @@ -515,11 +517,12 @@ static void write_summary_json(FILE *f, void *data)
>  	fprintf(f, "  }\n");
>  }
>  
> -static void run_expt(struct thread *threads, int runtime_secs)
> +static void run_expt(struct thread *threads, int runtime_secs, bool preheat)
>  {
>  	int i;
>  
>  	g.runtime_secs = runtime_secs;
> +	g.preheat = preheat;
>  	g.n_threads_started = 0;
>  	g.n_threads_running = 0;
>  	g.n_threads_finished = 0;
> @@ -846,14 +849,14 @@ int main(int argc, char *argv[])
>  		g.n_threads = 1;
>  	else
>  		g.n_threads = g.n_threads_total;
> -	run_expt(threads, 1);
> +	run_expt(threads, 1, true);
>  	record_bias(threads);
>  
>  	if (!g.quiet)
>  		printf("Test starts...\n");
>  	/* Reset n_threads to always run on all the cores */
>  	g.n_threads = g.n_threads_total;
> -	run_expt(threads, g.runtime);
> +	run_expt(threads, g.runtime, false);
>  
>  	if (!g.quiet)
>  		printf("Test completed.\n\n");
> -- 
> 2.31.1
> 
> 
Signed-off-by: John Kacur <jkacur@redhat.com>