07-05-2005
fwrite takes extremely long time
After my previous thread, I think I found out what causes the long delays.
I run this program on several Linux computers, and the sometimes (after the file with the arrays becomes big) the fwrite takes between 100 ms to 900 ms.
This is very bad for me, as I want a timer to halt each 30 ms....
The simplified code (without the timer) is:
#include <sys/time.h>
#include <signal.h>
#include <sched.h>
#include <sys/mman.h>
#include <iostream.h>
int main()
{
FILE* stream = fopen ("log.txt","w");
double data[100][100];
struct timeval tv1,tv2;
struct timezone tz1;
int diff;
for (int i=0;i<10000;i++)
{
gettimeofday(&tv1, &tz1);
fwrite(data, sizeof(double), 100 * 100, stream);
fflush(stream);
gettimeofday(&tv2, &tz1);
if (tv2.tv_usec >= tv1.tv_usec)
diff = (tv2.tv_usec - tv1.tv_usec)/1000;
else
diff = (tv2.tv_usec + (1000000-tv1.tv_usec))/1000;
if (diff>1)
cout << diff << endl;
}
fclose(stream);
return 1;
}
Waiting for you proposals how to fix this.
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LEARN ABOUT CENTOS
timeradd
TIMERADD(3) Linux Programmer's Manual TIMERADD(3)
NAME
timeradd, timersub, timercmp, timerclear, timerisset - timeval operations
SYNOPSIS
#include <sys/time.h>
void timeradd(struct timeval *a, struct timeval *b,
struct timeval *res);
void timersub(struct timeval *a, struct timeval *b,
struct timeval *res);
void timerclear(struct timeval *tvp);
int timerisset(struct timeval *tvp);
int timercmp(struct timeval *a, struct timeval *b, CMP);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
All functions shown above: _BSD_SOURCE
DESCRIPTION
The macros are provided to operate on timeval structures, defined in <sys/time.h> as:
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
timeradd() adds the time values in a and b, and places the sum in the timeval pointed to by res. The result is normalized such that
res->tv_usec has a value in the range 0 to 999,999.
timersub() subtracts the time value in b from the time value in a, and places the result in the timeval pointed to by res. The result is
normalized such that res->tv_usec has a value in the range 0 to 999,999.
timerclear() zeros out the timeval structure pointed to by tvp, so that it represents the Epoch: 1970-01-01 00:00:00 +0000 (UTC).
timerisset() returns true (nonzero) if either field of the timeval structure pointed to by tvp contains a nonzero value.
timercmp() compares the timer values in a and b using the comparison operator CMP, and returns true (nonzero) or false (0) depending on the
result of the comparison. Some systems (but not Linux/glibc), have a broken timercmp() implementation, in which CMP of >=, <=, and == do
not work; portable applications can instead use
!timercmp(..., <)
!timercmp(..., >)
!timercmp(..., !=)
RETURN VALUE
timerisset() and timercmp() return true (nonzero) or false (0).
ERRORS
No errors are defined.
CONFORMING TO
Not in POSIX.1-2001. Present on most BSD derivatives.
SEE ALSO
gettimeofday(2), time(7)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2010-02-25 TIMERADD(3)