Hi,
I did some measurements with a copy of SRC680 m164 and one of the more
pathological calc documents, and found that the "lock" prefix indeed
imposes a significant overhead of about 8% on a non HT 1.8 GHz Pentium IV.
(The tests included starting StarOffice, loading the document and
closing the application as soon as the document is loaded).
$ time ./soffice numbers_large.ods
With "lock": w/o "lock"
user time: 41.474s 38.379s
user time: 41.611s 38.676s
user time: 41.796s 38.397s
user time: 41.623s 38.412s
user time: 41.696s 38.742s
mean: 41.64s 38.52s
Comparing the wall clock times showed essentially the same value of 8%
overhead for the "lock" case.
Heiner
Stephan Bergmann wrote:
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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