ClockServices.h
/* |
File: ClockServices.h |
Abstract: |
This header declares MeasureNetTimeInCPUCycles, which measures the |
execution time of a routine. Comments below describe the driver routine |
that must be passed to MeasureNetTimeInCPUCyles and the other parameters |
to MeasureNetTimeInCPUCycles. |
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*/ |
#if !defined(apple_com_Accelerate_ClockServices_h) |
#define apple_com_Accelerate_ClockServices_h |
#if defined(__cplusplus) |
extern "C" { |
#endif |
/* Define a type describing a routine whose performance will be measured. |
The routine must take a parameter which is the number of iterations to |
execute and another parameter which is a pointer to any type of data. To |
time any routine, write a small driver routine. For example: |
struct MyParameters |
{ |
int Argument0; |
float *Argument1; |
double Argument2; |
}; |
void MyDriver(unsigned int iterations, void *data) |
{ |
struct MyParameters *p = (struct MyParameters *) data; |
while (--iterations) |
MyRoutine(p->Argument0, p->Argument1, p->Argument2); |
} |
Then use MyDriver as the routine to be measured. |
*/ |
typedef void (*RoutineToBeTimedType)(unsigned int iterations, void *data); |
/* MeasureNetTimeInCPUCycles. |
This routine measures the net amount of time it takes to execute an |
arbitrary routine. The net time excludes overhead such as reading the |
clock value and loading the routine into cache. |
The time for multiple iterations (and multiple samples, see below) is |
measured. This time is divided to return the time for a single iteration. |
Input: |
RoutineToBeTimedType routine. |
Address of a routine to measure. (See typedef for this type, |
above.) |
unsigned int iterations. |
Number of iterations to be measured. |
void *data. |
Pointer to be passed to the routine. |
unsigned int samples. |
Number of samples to take. The time a routine takes to perform |
varies due to many factors, so multiple samples are taken in an |
attempt to find the "true" time taken by the routine itself. |
On Mac OS X, the largest variance is often due to interrupts or |
process context switching. There is no good way to exclude these |
or filter them out. When interrupts occur, a routine takes longer |
than is required for the routine alone. Thus, the minimum time |
observed in the samples may be the time the routine takes if no |
interrupts occur. |
By itself, this could produce an incorrectly low time. For example, |
we might happen to start measuring just after a clock tick and |
stop just before a clock tick. However, some fine-grain clocks are |
available, so the size of a clock tick should not bias the |
measurement too much. Also, the measuring routine makes a control |
measurement the same way. When N iterations are requested, |
MeasureNetTime measures the subject routine for 2 iterations and |
for N+2 iterations. Factors that may make one measurement too low |
may also make the other measurement too low. The measuring |
routine returns the difference, so errors should cancel to some |
extent. |
Output: |
Return value. |
Number of CPU cycles to execute one iteration. |
*/ |
double MeasureNetTimeInCPUCycles( |
RoutineToBeTimedType routine, |
unsigned int iterations, |
void *data, |
unsigned int samples |
); |
#if defined(__cplusplus) |
} // extern "C" |
#endif |
#endif // !defined(apple_com_Accelerate_ClockServices_h) |
Copyright © 2008 Apple Inc. All Rights Reserved. Terms of Use | Privacy Policy | Updated: 2008-06-06