Hi all,
Someone recently mentioned that osl_increment/decrementInterlockedCount
would show up as top scorers with certain profiling tools (vtune?).
That got me thinking. On both Linux x86 and Windows x86, those
functions are implemented in assembler, effectively consisting of a
LOCK-prefixed XADD. Now, I thought that, at least on a uniprocessor
machine, the LOCK would probably not be that expensive, but that the
profiling tool in question might be confused by it and present bogus
results.
However, the following little program on Linux x86 (where incLocked is a
copy of osl_incrementInterlockedCount, and incUnlocked is the same,
without the LOCK prefix) told a different story:
// lock.c
#include <stdio.h>
int incLocked(int * p) {
int n;
__asm__ __volatile__ (
"movl $1, %0\n\t"
"lock\n\t"
"xaddl %0, %2\n\t"
"incl %0" :
"=&r" (n), "=m" (*p) :
"m" (*p) :
"memory");
return n;
}
int incUnlocked(int * p) {
int n;
__asm__ __volatile__ (
"movl $1, %0\n\t"
"xaddl %0, %2\n\t"
"incl %0" :
"=&r" (n), "=m" (*p) :
"m" (*p) :
"memory");
return n;
}
int main(int argc, char ** argv) {
int i;
int n = 0;
if (argv[1][0] == 'l') {
puts("locked version");
for (i = 0; i < 100000000; ++i) {
incLocked(&n);
}
} else {
puts("unlocked version");
for (i = 0; i < 100000000; ++i) {
incUnlocked(&n);
}
}
return 0;
}
m1> cat /proc/cpuinfo
processor : 0
model name: Intel(R) Pentium(R) 4 CPU 1.80GHz
...
m1> time ./lock l
locked version
11.868u 0.000s 0:12.19 97.2% 0+0k 0+0io 0pf+0w
m1> time ./lock u
unlocked version
1.516u 0.000s 0:01.57 96.1% 0+0k 0+0io 0pf+0w
m2> cat /proc/cpuinfo
processor : 0
model name: AMD Opteron(tm) Processor 242
processor : 1
model name: AMD Opteron(tm) Processor 242
...
m2> time ./lock l
locked version
1.863u 0.000s 0:01.86 100.0% 0+0k 0+0io 0pf+0w
m2> time ./lock u
unlocked version
0.886u 0.000s 0:00.89 98.8% 0+0k 0+0io 0pf+0w
So, depending on CPU type, the version with LOCK is 2--8 times slower
than the version without LOCK. Would be interesting to see whether this
has any actual impact on overall OOo performance. (But first, I'm off
on vacation...)
-Stephan
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