[PATCH v2] Preheat CPU in benchtests

[OndÅej BÃlka <neleai at seznam dot cz>]

On Thu, May 02, 2013 at 09:34:04PM -0400, Carlos O'Donell wrote:
> On 05/01/2013 03:23 PM, OndÅej BÃlka wrote:
> > 
> > Ping, 
> > 
> 
> Needs a comment explaining why we don't disable requency
> scaling e.g. needs root (Andi Kleen's comment).
> 
> Needs a comment explaining future work around CPU_CLK_UNHALTED.
This was independent idea, It belongs to hp_timing and so. 
> 
> Needs a comment explaining why we run for a fixed number
> of cycles instead of a fixed amount of time (Petr Baudis' comment).
>
Done. 
> > +  /* This loop should cause CPU switch to maximal freqency.  This makes
> > +      subsequent measurement more accurate.  We need side effect to avoid loop
> > +      being deleted by compiler.  */
> > + for(k = 0; k < 1000000; k++)
> > +   dontoptimize += 23.0 * dontoptimize + 2.1;
> > +
> > +
> 
> Make it a function.
> 
> Can we avoid using the fpu here and avoid possible
> double overflow? Does a volatile unsigned int work just
> as well? If we need to heat the CPU and FPU 
Done.
 
	* benchtests/bench-skeleton.c (main): Preheat CPU.

---
 benchtests/bench-skeleton.c |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/benchtests/bench-skeleton.c b/benchtests/bench-skeleton.c
index 7359184..a0e7bac 100644
--- a/benchtests/bench-skeleton.c
+++ b/benchtests/bench-skeleton.c
@@ -22,6 +22,20 @@
 #include <time.h>
 #include <inttypes.h>
 
+volatile unsigned int = 0;
+void startup ()
+{
+  /* This loop should cause CPU switch to maximal freqency.
+     This makes subsequent measurement more accurate.  We need side effect to
+     avoid loop being deleted by compiler.  
+     This should be enougth to cause switch and it is simpler than run loop
+     for constant time. This is used when user has not root to set
+     constant freqency.  */
+  int k;
+  for (k = 0; k < 10000000; k++)
+    dontoptimize += 23 * dontoptimize + 2;
+}
+
 #define TIMESPEC_AFTER(a, b) \
   (((a).tv_sec == (b).tv_sec) ?						      \
      ((a).tv_nsec > (b).tv_nsec) :					      \
@@ -32,6 +46,8 @@ main (int argc, char **argv)
   unsigned long i, k;
   struct timespec start, end, runtime;
 
+  startup();
+
   memset (&runtime, 0, sizeof (runtime));
   memset (&start, 0, sizeof (start));
   memset (&end, 0, sizeof (end));
-- 
1.7.4.4