I found a benchmark program for threads and found a major difference between a single CPU and dual CPU system.
Here's the code:
http://www-124.ibm.com/pipermail/pthreads-users/2002-April/000176.html
Is this showing context switches going between CPUs??
Wouldn't one expect the dual CPU to run twice as fast instead of ten times slower?
As you can see from the timing user time increases by a factor of 4 and system time by a factor of 10.
I seem to remember something about gettimeofday() possibly being a problem but couldn't find a reference to it.
Anybody have an explanation/fix for this?
I'm running 2.4.21 with glibc-2.3.2
Single CPU (Athlon 2400):
Thread Count Start Time End Time Reverses
------ ------- ---------- -------- --------
0 100000 0.00014 0.55257 99998
1 100000 0.00018 0.44839 99997
2 100000 0.00018 0.57385 100000
3 100000 0.00026 0.37884 99999
4 100000 0.00026 0.50911 100000
5 100000 0.00028 0.49115 99998
6 100000 0.00035 0.53783 100000
7 100000 0.00036 0.60039 100000
8 100000 0.00038 0.60346 81847
9 100000 0.45934 0.59906 99998
Total elapsed time is 0.60348 seconds
0.080u 0.530s 0:00.60 101.6% 0+0k 0+0io 136pf+0w
Dual CPU (Athlon 2000MP):
Thread Count Start Time End Time Reverses
------ ------- ---------- -------- --------
0 100000 0.00024 2.35316 100000
1 100000 0.00031 2.65760 100000
2 100000 0.00036 2.61471 100000
3 100000 0.00048 2.59529 100000
4 100000 0.00055 2.65601 100000
5 100000 0.00060 2.70938 98040
6 100000 0.00065 2.68379 100000
7 100000 0.00075 2.71154 95341
8 100000 0.00082 2.69375 100000
9 100000 0.00089 2.71491 78796
Total elapsed time is 2.71494 seconds
0.380u 5.040s 0:02.71 200.0% 0+0k 0+0io 133pf+0w
Michael D. Black [email protected]
http://www.csi-inc.com/
http://www.csi-inc.com/~mike
Melbourne FL
Michael D. Black [email protected]
http://www.csi-inc.com/
http://www.csi-inc.com/~mike
321-676-2923, x203
Melbourne FL
> I found a benchmark program for threads and found a major
> difference between a single CPU and dual CPU system.
> Here's the code:
> http://www-124.ibm.com/pipermail/pthreads-users/2002-April/000176.html
> Is this showing context switches going between CPUs??
No.
> Wouldn't one expect the dual CPU to run twice as fast instead of
> ten times slower?
Not necessarily. The code doesn't do much other than modified shared data
without a lock. Not only does this invoke undefined behavior, but it
penalizes SMP machines because only they are subject to cache ping-ponging.
> As you can see from the timing user time increases by a factor of
> 4 and system time by a factor of 10.
> I seem to remember something about gettimeofday() possibly being
> a problem but couldn't find a reference to it.
> Anybody have an explanation/fix for this?
The program doesn't seem to measure anything useful and invokes undefined
behavior.
If you would like a fix, try this:
#define sched_yield() while(false)
DS