I tried running the following example program across cachegrind to see metrics
that indicate cache line false-sharing. Here's the example:
#include <pearl/version.h>
#include <boost/thread.hpp>
#include <boost/static_assert.hpp>
#include <iostream>
#define MAX_TESTS 100000
#define MAX_THREADS 100
#define CACHELINE_SIZE_INTS \
(64/sizeof(int))
BOOST_STATIC_ASSERT( (sizeof(int)*CACHELINE_SIZE_INTS)==64 );
struct Work {
Work( int& d) : data( d ) {}
void operator()() {
for(int index=0; index<MAX_TESTS; ++index) {
++data;
}
}
int& data;
};
int main(int argc,const char ** argv) {
boost::thread_group threads;
if( argc>2 )
{
std::cout << "FALSE SHARING version" << std::endl;
int false_sharing[MAX_THREADS] = { 0 };
for (int i = 0; i < MAX_THREADS; ++i) {
int& var=false_sharing[i];
threads.create_thread(Work(var));
}
threads.join_all();
}
else
{
std::cout << "NO FALSE SHARING version" << std::endl;
int no_false_sharing[MAX_THREADS * CACHELINE_SIZE_INTS] = { 0 };
for (int i = 0; i < MAX_THREADS; ++i) {
int& var=no_false_sharing[i * CACHELINE_SIZE_INTS];
threads.create_thread(Work(var));
}
threads.join_all();
}
I compile/link this with:
g++ -fPIC -Wall -Wfloat-equal -Wundef -pipe -Wno-deprecated -ggdb3 -
O0 -DPROFILE -I/usr/local/include -fuse-cxa-atexit -DNDEBUG
-DBUILDINFO="\"09/19/2011 at 06:37:43 AM by zap@iulius\""
-I.. -I../include -Wno-deprecated -fno-strict-aliasing -c -o
false_sharing.o ../src/false_sharing.cpp
g++ -o false_sharing.1.0.28 -Wl,--demangle,-Map,false_sharing.1.0.28.map
false_sharing.o -lrt -lpthread -ldl -lz -lm -lboost_filesystem
-lboost_program_options -lboost_regex -lboost_signals
-lboost_serialization -lboost_thread -lboost_unit_test_framework
-lboost_system
Then run twice with:
valgrind --tool=cachegrind --branch-sim=yes --cache-sim=yes
../profile/false_sharing
and
valgrind --tool=cachegrind --branch-sim=yes --cache-sim=yes
../profile/false_sharing x
The results are what bother me: there is no difference in the stats gathered
between runs. Both show output like:
NO FALSE SHARING version
==13718==
==13718== I refs: 112,888,692
==13718== I1 misses: 3,135
==13718== L2i misses: 2,396
==13718== I1 miss rate: 0.00%
==13718== L2i miss rate: 0.00%
==13718==
==13718== D refs: 65,138,690 (53,817,934 rd + 11,320,756 wr)
==13718== D1 misses: 51,954 ( 47,947 rd + 4,007 wr)
==13718== L2d misses: 20,929 ( 17,626 rd + 3,303 wr)
==13718== D1 miss rate: 0.0% ( 0.0% + 0.0% )
==13718== L2d miss rate: 0.0% ( 0.0% + 0.0% )
==13718==
==13718== L2 refs: 55,089 ( 51,082 rd + 4,007 wr)
==13718== L2 misses: 23,325 ( 20,022 rd + 3,303 wr)
==13718== L2 miss rate: 0.0% ( 0.0% + 0.0% )
==13718==
==13718== Branches: 11,910,054 (11,886,870 cond + 23,184 ind)
==13718== Mispredicts: 55,848 ( 53,014 cond + 2,834 ind)
==13718== Mispred rate: 0.4% ( 0.4% + 12.2% )
And the other:
FALSE SHARING version
==13823==
==13823== I refs: 112,878,279
==13823== I1 misses: 3,117
==13823== L2i misses: 2,395
==13823== I1 miss rate: 0.00%
==13823== L2i miss rate: 0.00%
==13823==
==13823== D refs: 65,131,908 (53,814,295 rd + 11,317,613 wr)
==13823== D1 misses: 51,587 ( 47,744 rd + 3,843 wr)
==13823== L2d misses: 20,896 ( 17,624 rd + 3,272 wr)
==13823== D1 miss rate: 0.0% ( 0.0% + 0.0% )
==13823== L2d miss rate: 0.0% ( 0.0% + 0.0% )
==13823==
==13823== L2 refs: 54,704 ( 50,861 rd + 3,843 wr)
==13823== L2 misses: 23,291 ( 20,019 rd + 3,272 wr)
==13823== L2 miss rate: 0.0% ( 0.0% + 0.0% )
==13823==
==13823== Branches: 11,907,051 (11,883,867 cond + 23,184 ind)
==13823== Mispredicts: 55,946 ( 53,121 cond + 2,825 ind)
==13823== Mispred rate: 0.4% ( 0.4% + 12.1% )
How can the results be virtually identical? I would have thought the "false
sharing" version would be significantly higher than the other, but they are
nearly identical.
Is there something I am doing wrong with the tool? I want to be able to
measure the false-sharing so that I can find and improve it in my target
program.
Many thanks
-z
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