Abramo Bagnara wrote:
>
> However if I interpret right the web page:
>
> "The benchmark times how long it takes to go from one process to another
> when the context switch is done by reading and writing data."
>
> Then I suppose this is not the pure context switch time, but it's
> cumulated with read/write syscall and copy time that seems to be
> significative. Note also that in current kernel code pipe needs a double
> copy (Manfred Spraul is improving that in recent past days).
>
> If (I underline "if") the cost is ~2 microseconds this means 0.1538% of
> CPU use per context switch.
>
> Do you think I'm missing something?
>
> I'll try to do some tests here (with a k6 II @ 333Mhz: a little machine)
> using sched_yield and rdtsc. Probably I can obtain more data.
I attach my test program (don't try it on SMP).
Results are very interesting:
$ cat /proc/cpuinfo
processor : 0
vendor_id : AuthenticAMD
cpu family : 5
model : 8
model name : AMD-K6(tm) 3D processor
stepping : 0
cpu MHz : 333.113
cache size : 64 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr mce cx8 mmx syscall 3dnow
bogomips : 663.55
$ cat /proc/version
Linux version 2.4.4 (root@igor) (gcc version 2.96 20000731 (Red Hat
Linux 7.1 2.96-81)) #5 SMP sab mag 5 19:00:39 CEST 2001
$ ./ctx 2
0: 499998 iterations
1: 500002 iterations
min=893 max=397782 avg=918
$ ./ctx 2
0: 499992 iterations
1: 500008 iterations
min=886 max=85798 avg=917
$ ./ctx 2
0: 499997 iterations
1: 500003 iterations
min=893 max=85635 avg=918
$ bc
bc 1.06
Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
1000000000*918/333113000
2755
Average time for a context switch:
2755ns = 2.755us
Another machine:
$ cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 8
model name : Pentium III (Coppermine)
stepping : 6
cpu MHz : 797.995
cache size : 256 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca
cmov pat pse36 mmx fxsr sse
bogomips : 1592.52
$ cat /proc/version
Linux version 2.4.4-xfs (root@linux1) (gcc version 2.96 20000731 (Red
Hat Linux 7.1 2.96-84)) #12 mer mag 9 11:47:14 CEST 2001
$ ./ctx 2
0: 499999 iterations
1: 500001 iterations
min=556 max=57779 avg=570
$ ./ctx 2
0: 499999 iterations
1: 500001 iterations
min=556 max=42148 avg=570
$ ./ctx 2
0: 500001 iterations
1: 499999 iterations
min=559 max=183607 avg=571
$ bc
bc 1.06
Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
1000000000*571/797995000
715
Average time for a context switch:
715ns = 0.715us
I don't know if to use fork would change significatively this numbers.
--
Abramo Bagnara mailto:[EMAIL PROTECTED]
Opera Unica Phone: +39.546.656023
Via Emilia Interna, 140
48014 Castel Bolognese (RA) - Italy
ALSA project http://www.alsa-project.org
It sounds good!
#include <limits.h>
#include <pthread.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <asm/msr.h>
unsigned long long old;
unsigned long min_diff = LONG_MAX, max_diff = 0;
unsigned long long sum_diff = 0;
unsigned int count = 0;
unsigned int start = 0;
unsigned int distr[100];
void *thread(void *data)
{
long long prev, diff;
unsigned int k = (unsigned int) data;
volatile int *s = &start;
while (!*s);
while (count < 1000000) {
prev = old;
rdtscll(old);
diff = old - prev;
if (diff < min_diff) {
min_diff = diff;
}
if (diff > max_diff) {
max_diff = diff;
}
sum_diff += diff;
count++;
distr[k]++;
sched_yield();
}
return 0;
}
int main(int argc, char **argv)
{
int num_threads = atoi(argv[1]);
pthread_t threads[num_threads];
int k;
for (k = 0; k < num_threads; ++k)
pthread_create(&threads[k], NULL, thread, (void*) k);
rdtscll(old);
start = 1;
for (k = 0; k < num_threads; ++k) {
pthread_join(threads[k], 0);
printf("%d: %d iterations\n", k, distr[k]);
}
printf("min=%ld max=%ld avg=%lld\n", min_diff, max_diff, sum_diff / count);
return 0;
}
