Date: Mon, 28 May 2007 15:47:53 +0900 (GMT-9) From: Akihiro KAYAMA <kayama@personal-media.co.jp> To: freebsd-questions@freebsd.org Subject: wall-clock time profiling Message-ID: <20070528.154753.32719103.kayama@personal-media.co.jp>
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Hi all.
What is the right way to measure wall-clock time in profiling on FreeBSD?
Standard profiling method in UNIX like 'gprof' measures CPU time, but
it doesn't always offer a good indication for tuning if application is
not CPU bound.
For example, the below simple program spend most of the time for disk
I/O, but gprof doesn't tell me that fsync(2) in output() is the
hot-spot which should be removed at first.
------------------------------------------------------------
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
char calc()
{
int i;
char c;
for (i = 0; i < 10000; i++) {
c = c * i;
}
return c;
}
void output(int fd, char c)
{
write(fd, &c, 1);
fsync(fd); /* XXX time consuming system call */
}
int main()
{
int fd;
int i;
char c;
fd = open("testfile", O_CREAT | O_WRONLY);
for (i = 0; i < 10000; i++) {
c = calc(i);
output(fd, c);
}
return 0;
}
------------------------------------------------------------
gprof's output:
% cumulative self self total
time seconds seconds calls ms/call ms/call name
84.2 0.56 0.56 10000 0.06 0.06 calc [3]
13.6 0.65 0.09 10000 0.01 0.01 _fsync [5]
1.4 0.66 0.01 10000 0.00 0.00 write [6]
0.7 0.66 0.00 .mcount (12)
0.2 0.66 0.00 10000 0.00 0.01 output [4]
------------------------------------------------------------
Measuring CPU time has been proper for traditional UNIX as TSS, but I
think wall-clock time is also useful in these days. It is affected by
system load, but it is easy to arrange some dedicated system for
profiling purpose.
Wall-clock time profiling itself could be implemented by simple kernel
hack (i386/i386/trap.c:syscall() and kern/subr_trap.c:userret()), but
if both CPU and wall-clock time profiling are available, I don't know
how users should change the method. sysctl(8) may be inappropriate.
Although I'm not familiar with another analysis tools, like DTrace for
FreeBSD, is there something help for me?
Thanks.
--
Akihiro KAYAMA
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