[PATCH] Run benchmarks for constant time instead of constant iterations

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

Hi,

The idea to run benchmarks for a constant number of iterations is
problematic.  While the benchmarks may run for 10 seconds on x86_64,
they could run for about 30 seconds on powerpc and worse, over 3
minutes on arm.  Besides that, adding a new benchmark is cumbersome
since one needs to find out the number of iterations needed for a
sufficient runtime.

A better idea would be to run each benchmark for a specific amount of
time.  This patch does just that.  The run time defaults to 10 seconds
and it is configurable at command line:

  make BENCH_DURATION=5 bench

I have also adjusted the modf benchmark to reflect this change.
Verified that the benchmarks run for the specified amount of time on
x86_64.  OK to commit?

Siddhesh

	* benchtests/Makefile: Remove *-ITER.  Define BENCH_DURATION
	in CPPFLAGS.
	($(objpfx)bench-%.c): Remove *-ITER.
	* benchtests/bench-modf.c: Remove definition of ITER.
	* benchtests/bench-skeleton.c (TIMESPEC_AFTER): New macro.
	(main): Loop for DURATION seconds instead of fixed number of
	iterations.
	* scripts/bench.pl: Don't expect iterations in parameters.

diff --git a/benchtests/Makefile b/benchtests/Makefile
index 387fde4..9d25d69 100644
--- a/benchtests/Makefile
+++ b/benchtests/Makefile
@@ -23,12 +23,6 @@
 
 # - Append the function name to the bench variable
 
-# - Define foo-ITER with the number of iterations you want to run.  Keep it
-#   high enough that the overhead of clock_gettime is only a small fraction of
-#   the total run time of the test.  A good idea would be to keep the run time
-#   of each test at around 10 seconds for x86_64.  That is just a guideline,
-#   since some scenarios may require higher run times.
-
 # - Define foo-ARGLIST as a colon separated list of types of the input
 #   arguments.  Use `void` if function does not take any inputs.  Put in quotes
 #   if the input argument is a pointer, e.g.:
@@ -49,83 +43,70 @@ bench := exp pow rint sin cos tan atan modf \
 	 slowexp slowpow slowsin slowcos slowtan slowatan
 
 # exp function fast path: sysdeps/ieee754/dbl-64/e_exp.c
-exp-ITER = 5e8
 exp-ARGLIST = double
 exp-RET = double
 LDFLAGS-bench-exp = -lm
 
 # pow function fast path: sysdeps/ieee754/dbl-64/e_pow.c
-pow-ITER = 2e8
 pow-ARGLIST = double:double
 pow-RET = double
 LDFLAGS-bench-pow = -lm
 
-rint-ITER = 250000000
 rint-ARGLIST = double
 rint-RET = double
 LDFLAGS-bench-rint = -lm
 
 # exp function slowest path: sysdeps/ieee754/dbl-64/mpexp.c
-slowexp-ITER = 3e5
 slowexp-ARGLIST = double
 slowexp-RET = double
 slowexp-INCLUDE = slowexp.c
 LDFLAGS-bench-slowexp = -lm
 
 # sin function fast path: sysdeps/ieee754/dbl-64/s_sin.c
-sin-ITER = 3e9
 sin-ARGLIST = double
 sin-RET = double
 LDFLAGS-bench-sin = -lm
 
 # cos function fast path: sysdeps/ieee754/dbl-64/s_sin.c
-cos-ITER = 3e9
 cos-ARGLIST = double
 cos-RET = double
 LDFLAGS-bench-cos = -lm
 
 # tan function fast path: sysdeps/ieee754/dbl-64/s_tan.c
-tan-ITER = 3e9
 tan-ARGLIST = double
 tan-RET = double
 LDFLAGS-bench-tan = -lm
 
 # atan function fast path: sysdeps/ieee754/dbl-64/s_atan.c
-atan-ITER = 6e9
 atan-ARGLIST = double
 atan-RET = double
 LDFLAGS-bench-atan = -lm
 
 # pow function slowest path: sysdeps/ieee754/dbl-64/slowpow.c
-slowpow-ITER = 1e5
 slowpow-ARGLIST = double:double
 slowpow-RET = double
 slowpow-INCLUDE = slowpow.c
 LDFLAGS-bench-slowpow = -lm
 
 # sin function slowest path: sysdeps/ieee754/dbl-64/sincos32.c
-slowsin-ITER = 3e7
 slowsin-ARGLIST = double
 slowsin-RET = double
 slowsin-INCLUDE = slowsin.c
 LDFLAGS-bench-slowsin = -lm
 
 # cos function slowest path: sysdeps/ieee754/dbl-64/sincos32.c
-slowcos-ITER = 3e7
 slowcos-ARGLIST = double
 slowcos-RET = double
 slowcos-INCLUDE = slowcos.c
 LDFLAGS-bench-slowcos = -lm
 
 # tan function slowest path: sysdeps/ieee754/dbl-64/mptan.c
-slowtan-ITER = 3e7
 slowtan-ARGLIST = double
 slowtan-RET = double
 slowtan-INCLUDE = slowtan.c
 LDFLAGS-bench-slowtan = -lm
 
 # atan function slowest path: sysdeps/ieee754/dbl-64/mpatan.c
-slowatan-ITER = 3e8
 slowatan-ARGLIST = double
 slowatan-RET = double
 slowatan-INCLUDE = slowatan.c
@@ -141,6 +122,13 @@ include ../Rules
 
 binaries-bench := $(addprefix $(objpfx)bench-,$(bench))
 
+# The default duration: 10 seconds.
+ifndef BENCH_DURATION
+BENCH_DURATION := 10
+endif
+
+CPPFLAGS-nonlib = -DDURATION=$(BENCH_DURATION)
+
 # This makes sure CPPFLAGS-nonlib and CFLAGS-nonlib are passed
 # for all these modules.
 cpp-srcs-left := $(binaries-bench:=.c)
@@ -176,5 +164,5 @@ $(objpfx)bench-%.c: %-inputs $(bench-deps)
 	  cat $($*-INCLUDE); \
 	fi; \
 	$(..)scripts/bench.pl $(patsubst %-inputs,%,$<) \
-	  $($*-ITER) $($*-ARGLIST) $($*-RET); } > $@-tmp
+	  $($*-ARGLIST) $($*-RET); } > $@-tmp
 	mv -f $@-tmp $@
diff --git a/benchtests/bench-modf.c b/benchtests/bench-modf.c
index 975a29f..90a5255 100644
--- a/benchtests/bench-modf.c
+++ b/benchtests/bench-modf.c
@@ -33,7 +33,6 @@ struct args
 static volatile double ret = 0.0;
 #define BENCH_FUNC(j) ({double iptr; ret =  CALL_BENCH_FUNC (j, iptr);})
 
-#define ITER 250000000
 #define FUNCNAME "modf"
 
 #include "bench-skeleton.c"
diff --git a/benchtests/bench-skeleton.c b/benchtests/bench-skeleton.c
index 13f986d..bbd151b 100644
--- a/benchtests/bench-skeleton.c
+++ b/benchtests/bench-skeleton.c
@@ -22,13 +22,18 @@
 #include <time.h>
 #include <inttypes.h>
 
+#define TIMESPEC_AFTER(a, b) \
+  (((a).tv_sec == (b).tv_sec) ?						      \
+     ((a).tv_nsec > (b).tv_nsec) :					      \
+        ((a).tv_sec > (b).tv_sec))
 int
 main (int argc, char **argv)
 {
-  unsigned long i, j, k;
+  unsigned long i, k;
   uint64_t total = 0, max = 0, min = 0x7fffffffffffffff;
-  struct timespec start, end;
+  struct timespec start, end, runtime;
 
+  memset (&runtime, 0, sizeof (runtime));
   memset (&start, 0, sizeof (start));
   memset (&end, 0, sizeof (end));
 
@@ -39,11 +44,15 @@ main (int argc, char **argv)
      Measurements close to the minimum clock resolution won't make much sense,
      but it's better than having nothing at all.  */
   unsigned long iters = 1000 * start.tv_nsec;
-  unsigned long total_iters = ITER / iters;
 
-  for (i = 0; i < NUM_SAMPLES; i++)
+  /* Run for approxmately DURATION seconds.  */
+  clock_gettime (CLOCK_MONOTONIC_RAW, &runtime);
+  runtime.tv_sec += DURATION;
+
+  double d_total_i = 0;
+  while (1)
     {
-      for (j = 0; j < total_iters; j ++)
+      for (i = 0; i < NUM_SAMPLES; i++)
 	{
 	  clock_gettime (CLOCK_PROCESS_CPUTIME_ID, &start);
 	  for (k = 0; k < iters; k++)
@@ -61,12 +70,25 @@ main (int argc, char **argv)
 	    min = cur;
 
 	  total += cur;
+
+	  d_total_i += iters;
 	}
+
+      struct timespec curtime;
+
+      memset (&curtime, 0, sizeof (curtime));
+      clock_gettime (CLOCK_MONOTONIC_RAW, &curtime);
+      if (TIMESPEC_AFTER (curtime, runtime))
+	goto done;
     }
 
-  double d_total_s = total * 1e-9;
-  double d_iters = iters;
-  double d_total_i = (double)ITER * NUM_SAMPLES;
+  double d_total_s;
+  double d_iters;
+
+ done:
+  d_total_s = total * 1e-9;
+  d_iters = iters;
+
   printf (FUNCNAME ": ITERS:%g: TOTAL:%gs, MAX:%gns, MIN:%gns, %g iter/s\n",
 	  d_total_i, d_total_s, max / d_iters, min / d_iters,
 	  d_total_i / d_total_s);
diff --git a/scripts/bench.pl b/scripts/bench.pl
index bb7f648..5856cfa 100755
--- a/scripts/bench.pl
+++ b/scripts/bench.pl
@@ -22,23 +22,22 @@ use warnings;
 # Generate a benchmark source file for a given input.
 
 if (@ARGV < 2) {
-  die "Usage: bench.pl <function> <iterations> [parameter types] [return type]"
+  die "Usage: bench.pl <function> [parameter types] [return type]"
 }
 
 my $arg;
 my $func = $ARGV[0];
-my $iters = $ARGV[1];
 my @args;
 my $ret = "void";
 my $getret = "";
 my $retval = "";
 
-if (@ARGV >= 3) {
-  @args = split(':', $ARGV[2]);
+if (@ARGV >= 2) {
+  @args = split(':', $ARGV[1]);
 }
 
-if (@ARGV == 4) {
-  $ret = $ARGV[3];
+if (@ARGV == 3) {
+  $ret = $ARGV[2];
 }
 
 my $decl = "extern $ret $func (";
@@ -88,6 +87,5 @@ if ($ret ne "void") {
 
 print "#define BENCH_FUNC(j) ({$getret CALL_BENCH_FUNC (j);})\n";
 
-print "#define ITER $iters\n";
 print "#define FUNCNAME \"$func\"\n";
 print "#include \"bench-skeleton.c\"\n";