I would like to know how I can program something so that my program runs as long as a second lasts.
I would like to evaluate parts of my code and see where the time is spend most so I am analyzing parts of it.
Here's the interesting part of my code :
int size = 256
clock_t start_benching = clock();
for (uint32_t i = 0;i < size; i+=4)
{
myarray[i];
myarray[i+1];
myarray[i+2];
myarray[i+3];
}
clock_t stop_benching = clock();
This just gives me how long the function needed to perform all the operations.
I want to run the code for one second and see how many operations have been done.
This is the line to print the time measurement:
printf("Walking through buffer took %f seconds\n", (double)(stop_benching - start_benching) / CLOCKS_PER_SEC);
A better approach to benchmarking is to know the % of time spent on each section of the code.
Instead of making your code run for exactly 1 second, make stop_benchmarking - start_benchmarking the total run time - Take the time spent on any part of the code and divide by the total runtime to get a value between 0 and 1. Multiply this value by 100 and you have the % of time consumed at that specific section.
Non-answer advice: Use an actual profiler to profile the performance of code sections.
On *nix you can set an alarm(2) with a signal handler that sets a global flag to indicate the elapsed time. The Windows API provides something similar with SetTimer.
#include <unistd.h>
#include <signal.h>
int time_elapsed = 0;
void alarm_handler(int signal) {
time_elapsed = 1;
}
int main() {
signal(SIGALRM, &alarm_handler);
alarm(1); // set alarm time-out to 1 second
do {
// stuff...
} while (!time_elapsed);
return 0;
}
In more complicated cases you can use setitimer(2) instead of alarm(2), which lets you
use microsecond precision and
choose between counting
wall clock time,
user CPU time, or
user and system CPU time.
For the following code getrusage returning zeros in ru_utime.tv_usec and ru_utime.tv_sec.
Code:
#include "stdlib.h"
#include "stdio.h"
#include "sys/time.h"
#include "sys/resource.h"
int getr_return, who = RUSAGE_SELF;
struct rusage usage;
main()
{
getr_return = getrusage(who, &usage);
printf(" getr_return = %d\n", getr_return);
printf(" time taken in seconds = %.61f\n", usage.ru_utime.tv_sec);
printf(" time taken in seconds = %.61f\n", usage.ru_utime.tv_usec);
Some_Mips_consuming_code().
getr_return = getrusage(who, &usage);
printf(" getr_return = %d\n", getr_return);
printf(" time taken in seconds = %.61f\n", usage.ru_utime.tv_sec);
printf(" time taken in seconds = %.61f\n", usage.ru_utime.tv_usec);
exit;
}
output:
getr_return = 0
time taken in seconds = 0.0000000000000000000000000000000000000000000000000000000000000
time taken in seconds = 0.0000000000000000000000000000000000000000000000000000000000000
getr_return = 0
time taken in seconds = 0.0000000000000000000000000000000000000000000000000000000000000
time taken in seconds = 0.0000000000000000000000000000000000000000000000000000000000000
Compiled code on Linux version 2.6.18-308
Ran executable on ARM board and it's Linux version 3.8.1-2.0
The user usage time is calculated between two instants of the logic to find out the time consumed which should have a start and end usage of timeval. Some thing like below sample,
struct timeval start, end;
getrusage(RUSAGE_SELF, &usage);
start = usage.ru_utime;
/* Code to check the usage consumed */
getrusage(RUSAGE_SELF, &usage);
end = usage.ru_utime;
ru_utime & ru_stime are of structures of type timeval. If you look its declaration both members tv_sec & tv_usec are of type long. So change the format specifier when printing to %ld. Note when on success getrusage() returns 0.
Time measurement is limited in precision, resolution and accuracy. Read time(7). Don't expect significant measures for computation less than e.g. half a second.
Often, CPU time measurement is done by counting jiffies or timer interrupts (related to HZ if you have one in your kernel).
Since you call getrusage(2) at the beginning of your main, not much computing has happened before it (basically, only pre-main initialization in e.g. crt0.o). So you should expect it to be near zero.
You might try using clock_gettime(2) with CLOCK_REALTIME or CLOCK_PROCESS_CPUTIME_ID.
I'm relatively new to C programming and I'm working on a project which needs to be very time accurate; therefore I tried to write something to create a timestamp with milliseconds precision.
It seems to work but my question is whether this way is the right way, or is there a much easier way? Here is my code:
#include<stdio.h>
#include<time.h>
void wait(int milliseconds)
{
clock_t start = clock();
while(1) if(clock() - start >= milliseconds) break;
}
int main()
{
time_t now;
clock_t milli;
int waitMillSec = 2800, seconds, milliseconds = 0;
struct tm * ptm;
now = time(NULL);
ptm = gmtime ( &now );
printf("time before: %d:%d:%d:%d\n",ptm->tm_hour,ptm->tm_min,ptm->tm_sec, milliseconds );
/* wait until next full second */
while(now == time(NULL));
milli = clock();
/* DO SOMETHING HERE */
/* for testing wait a user define period */
wait(waitMillSec);
milli = clock() - milli;
/*create timestamp with milliseconds precision */
seconds = milli/CLOCKS_PER_SEC;
milliseconds = milli%CLOCKS_PER_SEC;
now = now + seconds;
ptm = gmtime( &now );
printf("time after: %d:%d:%d:%d\n",ptm->tm_hour,ptm->tm_min,ptm->tm_sec, milliseconds );
return 0;
}
The following code seems likely to provide millisecond granularity:
#include <windows.h>
#include <stdio.h>
int main(void) {
SYSTEMTIME t;
GetSystemTime(&t); // or GetLocalTime(&t)
printf("The system time is: %02d:%02d:%02d.%03d\n",
t.wHour, t.wMinute, t.wSecond, t.wMilliseconds);
return 0;
}
This is based on http://msdn.microsoft.com/en-us/library/windows/desktop/ms724950%28v=vs.85%29.aspx. The above code snippet was tested with CYGWIN on Windows 7.
For Windows 8, there is GetSystemTimePreciseAsFileTime, which "retrieves the current system date and time with the highest possible level of precision (<1us)."
Your original approach would probably be ok 99.99% of the time (ignoring one minor bug, described below). Your approach is:
Wait for the next second to start, by repeatedly calling time() until the value changes.
Save that value from time().
Save the value from clock().
Calculate all subsequent times using the current value of clock() and the two saved values.
Your minor bug was that you had the first two steps reversed.
But even with this fixed, this is not guaranteed to work 100%, because there is no atomicity. Two problems:
Your code loops time() until you are into the next second. But how far are you into it? It could be 1/2 a second, or even several seconds (e.g. if you are running a debugger with a breakpoint).
Then you call clock(). But this saved value has to 'match' the saved value of time(). If these two calls are almost instantaneous, as they usually are, then this is fine. But Windows (and Linux) time-slice, and so there is no guarantee.
Another issue is the granularity of clock. If CLOCKS_PER_SEC is 1000, as seems to be the case on your system, then of course the best you can do is 1 msec. But it can be worse than that: on Unix systems it is typically 15 msecs. You could improve this by replacing clock with QueryPerformanceCounter(), as in the answer to timespec equivalent for windows, but this may be otiose, given the first two problems.
Clock periods are not at all guaranteed to be in milliseconds. You need to explicitly convert the output of clock() to milliseconds.
t1 = clock();
// do something
t2 = clock();
long millis = (t2 - t1) * (1000.0 / CLOCKS_PER_SEC);
Since you are on Windows, why don't you just use Sleep()?
I am using the time.h lib in c to find the time taken to run an algorithm. The code structure is somewhat as follows :-
#include <time.h>
int main()
{
time_t start,end,diff;
start = clock();
//ALGORITHM COMPUTATIONS
end = clock();
diff = end - start;
printf("%d",diff);
return 0;
}
The values for start and end are always zero. Is it that the clock() function does't work? Please help.
Thanks in advance.
Not that it doesn't work. In fact, it does. But it is not the right way to measure time as the clock () function returns an approximation of processor time used by the program. I am not sure about other platforms, but on Linux you should use clock_gettime () with CLOCK_MONOTONIC flag - that will give you the real wall time elapsed. Also, you can read TSC, but be aware that it won't work if you have a multi-processor system and your process is not pinned to a particular core. If you want to analyze and optimize your algorithm, I'd recommend you use some performance measurement tools. I've been using Intel's vTune for a while and am quite happy. It will show you not only what part uses the most cycles, but highlight memory problems, possible parallelism issues etc. You may be very surprised with the results. For example, most of the CPU cycles might be spent waiting for memory bus. Hope it helps!
UPDATE: Actually, if you run later versions of Linux, it might provide CLOCK_MONOTONIC_RAW, which is a hardware-based clock that is not a subject to NTP adjustments. Here is a small piece of code you can use:
stopwatch.hpp
stopwatch.cpp
Note that clock() returns the execution time in clock ticks, as opposed to wall clock time. Divide a difference of two clock_t values by CLOCKS_PER_SEC to convert the difference to seconds. The actual value of CLOCKS_PER_SEC is a quality-of-implementation issue. If it is low (say, 50), your process would have to run for 20ms to cause a nonzero return value from clock(). Make sure your code runs long enough to see clock() increasing.
I usually do it this way:
clock_t start = clock();
clock_t end;
//algo
end = clock();
printf("%f", (double)(end - start));
Consider the code below:
#include <stdio.h>
#include <time.h>
int main()
{
clock_t t1, t2;
t1 = t2 = clock();
// loop until t2 gets a different value
while(t1 == t2)
t2 = clock();
// print resolution of clock()
printf("%f ms\n", (double)(t2 - t1) / CLOCKS_PER_SEC * 1000);
return 0;
}
Output:
$ ./a.out
10.000000 ms
Might be that your algorithm runs for a shorter amount of time than that.
Use gettimeofday for higher resolution timer.
I have a C program that aims to be run in parallel on several processors. I need to be able to record the execution time (which could be anywhere from 1 second to several minutes). I have searched for answers, but they all seem to suggest using the clock() function, which then involves calculating the number of clocks the program took divided by the Clocks_per_second value.
I'm not sure how the Clocks_per_second value is calculated?
In Java, I just take the current time in milliseconds before and after execution.
Is there a similar thing in C? I've had a look, but I can't seem to find a way of getting anything better than a second resolution.
I'm also aware a profiler would be an option, but am looking to implement a timer myself.
Thanks
CLOCKS_PER_SEC is a constant which is declared in <time.h>. To get the CPU time used by a task within a C application, use:
clock_t begin = clock();
/* here, do your time-consuming job */
clock_t end = clock();
double time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
Note that this returns the time as a floating point type. This can be more precise than a second (e.g. you measure 4.52 seconds). Precision depends on the architecture; on modern systems you easily get 10ms or lower, but on older Windows machines (from the Win98 era) it was closer to 60ms.
clock() is standard C; it works "everywhere". There are system-specific functions, such as getrusage() on Unix-like systems.
Java's System.currentTimeMillis() does not measure the same thing. It is a "wall clock": it can help you measure how much time it took for the program to execute, but it does not tell you how much CPU time was used. On a multitasking systems (i.e. all of them), these can be widely different.
If you are using the Unix shell for running, you can use the time command.
doing
$ time ./a.out
assuming a.out as the executable will give u the time taken to run this
In plain vanilla C:
#include <time.h>
#include <stdio.h>
int main()
{
clock_t tic = clock();
my_expensive_function_which_can_spawn_threads();
clock_t toc = clock();
printf("Elapsed: %f seconds\n", (double)(toc - tic) / CLOCKS_PER_SEC);
return 0;
}
You functionally want this:
#include <sys/time.h>
struct timeval tv1, tv2;
gettimeofday(&tv1, NULL);
/* stuff to do! */
gettimeofday(&tv2, NULL);
printf ("Total time = %f seconds\n",
(double) (tv2.tv_usec - tv1.tv_usec) / 1000000 +
(double) (tv2.tv_sec - tv1.tv_sec));
Note that this measures in microseconds, not just seconds.
Most of the simple programs have computation time in milli-seconds. So, i suppose, you will find this useful.
#include <time.h>
#include <stdio.h>
int main(){
clock_t start = clock();
// Execuatable code
clock_t stop = clock();
double elapsed = (double)(stop - start) * 1000.0 / CLOCKS_PER_SEC;
printf("Time elapsed in ms: %f", elapsed);
}
If you want to compute the runtime of the entire program and you are on a Unix system, run your program using the time command like this time ./a.out
(All answers here are lacking, if your sysadmin changes the systemtime, or your timezone has differing winter- and sommer-times. Therefore...)
On linux use: clock_gettime(CLOCK_MONOTONIC_RAW, &time_variable);
It's not affected if the system-admin changes the time, or you live in a country with winter-time different from summer-time, etc.
#include <stdio.h>
#include <time.h>
#include <unistd.h> /* for sleep() */
int main() {
struct timespec begin, end;
clock_gettime(CLOCK_MONOTONIC_RAW, &begin);
sleep(1); // waste some time
clock_gettime(CLOCK_MONOTONIC_RAW, &end);
printf ("Total time = %f seconds\n",
(end.tv_nsec - begin.tv_nsec) / 1000000000.0 +
(end.tv_sec - begin.tv_sec));
}
man clock_gettime states:
CLOCK_MONOTONIC
Clock that cannot be set and represents monotonic time since some unspecified starting point. This clock is not affected by discontinuous jumps in the system time
(e.g., if the system administrator manually changes the clock), but is affected by the incremental adjustments performed by adjtime(3) and NTP.
Thomas Pornin's answer as macros:
#define TICK(X) clock_t X = clock()
#define TOCK(X) printf("time %s: %g sec.\n", (#X), (double)(clock() - (X)) / CLOCKS_PER_SEC)
Use it like this:
TICK(TIME_A);
functionA();
TOCK(TIME_A);
TICK(TIME_B);
functionB();
TOCK(TIME_B);
Output:
time TIME_A: 0.001652 sec.
time TIME_B: 0.004028 sec.
A lot of answers have been suggesting clock() and then CLOCKS_PER_SEC from time.h. This is probably a bad idea, because this is what my /bits/time.h file says:
/* ISO/IEC 9899:1990 7.12.1: <time.h>
The macro `CLOCKS_PER_SEC' is the number per second of the value
returned by the `clock' function. */
/* CAE XSH, Issue 4, Version 2: <time.h>
The value of CLOCKS_PER_SEC is required to be 1 million on all
XSI-conformant systems. */
# define CLOCKS_PER_SEC 1000000l
# if !defined __STRICT_ANSI__ && !defined __USE_XOPEN2K
/* Even though CLOCKS_PER_SEC has such a strange value CLK_TCK
presents the real value for clock ticks per second for the system. */
# include <bits/types.h>
extern long int __sysconf (int);
# define CLK_TCK ((__clock_t) __sysconf (2)) /* 2 is _SC_CLK_TCK */
# endif
So CLOCKS_PER_SEC might be defined as 1000000, depending on what options you use to compile, and thus it does not seem like a good solution.
#include<time.h>
#include<stdio.h>
int main(){
clock_t begin=clock();
int i;
for(i=0;i<100000;i++){
printf("%d",i);
}
clock_t end=clock();
printf("Time taken:%lf",(double)(end-begin)/CLOCKS_PER_SEC);
}
This program will work like charm.
You have to take into account that measuring the time that took a program to execute depends a lot on the load that the machine has in that specific moment.
Knowing that, the way of obtain the current time in C can be achieved in different ways, an easier one is:
#include <time.h>
#define CPU_TIME (getrusage(RUSAGE_SELF,&ruse), ruse.ru_utime.tv_sec + \
ruse.ru_stime.tv_sec + 1e-6 * \
(ruse.ru_utime.tv_usec + ruse.ru_stime.tv_usec))
int main(void) {
time_t start, end;
double first, second;
// Save user and CPU start time
time(&start);
first = CPU_TIME;
// Perform operations
...
// Save end time
time(&end);
second = CPU_TIME;
printf("cpu : %.2f secs\n", second - first);
printf("user : %d secs\n", (int)(end - start));
}
Hope it helps.
Regards!
ANSI C only specifies second precision time functions. However, if you are running in a POSIX environment you can use the gettimeofday() function that provides microseconds resolution of time passed since the UNIX Epoch.
As a side note, I wouldn't recommend using clock() since it is badly implemented on many(if not all?) systems and not accurate, besides the fact that it only refers to how long your program has spent on the CPU and not the total lifetime of the program, which according to your question is what I assume you would like to measure.
I've found that the usual clock(), everyone recommends here, for some reason deviates wildly from run to run, even for static code without any side effects, like drawing to screen or reading files. It could be because CPU changes power consumption modes, OS giving different priorities, etc...
So the only way to reliably get the same result every time with clock() is to run the measured code in a loop multiple times (for several minutes), taking precautions to prevent the compiler from optimizing it out: modern compilers can precompute the code without side effects running in a loop, and move it out of the loop., like i.e. using random input for each iteration.
After enough samples are collected into an array, one sorts that array, and takes the middle element, called median. Median is better than average, because it throws away extreme deviations, like say antivirus taking up all CPU up or OS doing some update.
Here is a simple utility to measure execution performance of C/C++ code, averaging the values near median: https://github.com/saniv/gauge
I'm myself still looking for a more robust and faster way to measure code. One could probably try running the code in controlled conditions on bare metal without any OS, but that will give unrealistic result, because in reality OS does get involved.
x86 has these hardware performance counters, which including the actual number of instructions executed, but they are tricky to access without OS help, hard to interpret and have their own issues ( http://archive.gamedev.net/archive/reference/articles/article213.html ). Still they could be helpful investigating the nature of the bottle neck (data access or actual computations on that data).
Every solution's are not working in my system.
I can get using
#include <time.h>
double difftime(time_t time1, time_t time0);
Some might find a different kind of input useful: I was given this method of measuring time as part of a university course on GPGPU-programming with NVidia CUDA (course description). It combines methods seen in earlier posts, and I simply post it because the requirements give it credibility:
unsigned long int elapsed;
struct timeval t_start, t_end, t_diff;
gettimeofday(&t_start, NULL);
// perform computations ...
gettimeofday(&t_end, NULL);
timeval_subtract(&t_diff, &t_end, &t_start);
elapsed = (t_diff.tv_sec*1e6 + t_diff.tv_usec);
printf("GPU version runs in: %lu microsecs\n", elapsed);
I suppose you could multiply with e.g. 1.0 / 1000.0 to get the unit of measurement that suits your needs.
If you program uses GPU or if it uses sleep() then clock() diff gives you smaller than actual duration. It is because clock() returns the number of CPU clock ticks. It only can be used to calculate CPU usage time (CPU load), but not the execution duration. We should not use clock() to calculate duration. We still should use gettimeofday() or clock_gettime() for duration in C.
perf tool is more accurate to be used in order to collect and profile the running program. Use perf stat to show all information related to the program being executed.
As simple as possible by using function-like macro
#include <stdio.h>
#include <time.h>
#define printExecTime(t) printf("Elapsed: %f seconds\n", (double)(clock()-(t)) / CLOCKS_PER_SEC)
int factorialRecursion(int n) {
return n == 1 ? 1 : n * factorialRecursion(n-1);
}
int main()
{
clock_t t = clock();
int j=1;
for(int i=1; i <10; i++ , j*=i);
printExecTime(t);
// compare with recursion factorial
t = clock();
j = factorialRecursion(10);
printExecTime(t);
return 0;
}
Comparison of execution time of bubble sort and selection sort
I have a program which compares the execution time of bubble sort and selection sort.
To find out the time of execution of a block of code compute the time before and after the block by
clock_t start=clock();
…
clock_t end=clock();
CLOCKS_PER_SEC is constant in time.h library
Example code:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main()
{
int a[10000],i,j,min,temp;
for(i=0;i<10000;i++)
{
a[i]=rand()%10000;
}
//The bubble Sort
clock_t start,end;
start=clock();
for(i=0;i<10000;i++)
{
for(j=i+1;j<10000;j++)
{
if(a[i]>a[j])
{
int temp=a[i];
a[i]=a[j];
a[j]=temp;
}
}
}
end=clock();
double extime=(double) (end-start)/CLOCKS_PER_SEC;
printf("\n\tExecution time for the bubble sort is %f seconds\n ",extime);
for(i=0;i<10000;i++)
{
a[i]=rand()%10000;
}
clock_t start1,end1;
start1=clock();
// The Selection Sort
for(i=0;i<10000;i++)
{
min=i;
for(j=i+1;j<10000;j++)
{
if(a[min]>a[j])
{
min=j;
}
}
temp=a[min];
a[min]=a[i];
a[i]=temp;
}
end1=clock();
double extime1=(double) (end1-start1)/CLOCKS_PER_SEC;
printf("\n");
printf("\tExecution time for the selection sort is %f seconds\n\n", extime1);
if(extime1<extime)
printf("\tSelection sort is faster than Bubble sort by %f seconds\n\n", extime - extime1);
else if(extime1>extime)
printf("\tBubble sort is faster than Selection sort by %f seconds\n\n", extime1 - extime);
else
printf("\tBoth algorithms have the same execution time\n\n");
}