I have an application where I have to increment some statistics counters in a multi-threaded method. The incrementing has to be thread-safe, so I decided to use the gcc atomic builtins __sync_add_and_fetch() function. Just to get an idea of their impact, I did some simple performance testing and noticed that these functions are much slower than simple pre/post incrementing.
Here is the test program that I created:
#include <iostream>
#include <pthread.h>
#include <time.h>
using namespace std;
uint64_t diffTimes(struct timespec &start, struct timespec &end)
{
if(start.tv_sec == end.tv_sec)
{
return end.tv_nsec - start.tv_nsec;
}
else if(start.tv_sec < end.tv_sec)
{
uint64_t nsecs = (end.tv_sec - start.tv_sec) * 1000000000;
return nsecs + end.tv_nsec - start.tv_nsec;
}
else
{
// this is actually an error
return 0;
}
}
void outputResult(const char *msg, struct timespec &start, struct timespec &end, uint32_t numIterations, uint64_t val)
{
uint64_t diff = diffTimes(start, end);
cout << msg << ": "
<< "\n\t iterations: " << numIterations
<< ", result: " << val
<< "\n\t times [start, end] = [" << start.tv_sec << ", " << start.tv_nsec << "]"
<< "\n\t [" << end.tv_sec << ", " << end.tv_nsec << "]"
<< "\n\t [total, avg] = [" << diff
<< ", " << (diff/numIterations) << "] nano seconds"
<< endl;
}
int main(int argc, char **argv)
{
struct timespec start, end;
uint64_t val = 0;
uint32_t numIterations = 1000000;
//
// NON ATOMIC pre increment
//
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
for(uint32_t i = 0; i < numIterations; ++i)
{
++val;
}
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
outputResult("Non-Atomic pre-increment", start, end, numIterations, val);
val = 0;
//
// NON ATOMIC post increment
//
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
for(uint32_t i = 0; i < numIterations; ++i)
{
val++;
}
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
outputResult("Non-Atomic post-increment", start, end, numIterations, val);
val = 0;
//
// ATOMIC add and fetch
//
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
for(uint32_t i = 0; i < numIterations; ++i)
{
__sync_add_and_fetch(&val, 1);
}
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
outputResult("Atomic add and fetch", start, end, numIterations, val);
val = 0;
//
// ATOMIC fetch and add
//
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
for(uint32_t i = 0; i < numIterations; ++i)
{
__sync_fetch_and_add(&val, 1);
}
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
outputResult("Atomic fetch and add", start, end, numIterations, val);
val = 0;
//
// Mutex protected post-increment
//
pthread_mutex_t mutex;
pthread_mutex_init(&mutex, NULL);
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
for(uint32_t i = 0; i < numIterations; ++i)
{
pthread_mutex_lock(&mutex);
val++;
pthread_mutex_unlock(&mutex);
}
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
outputResult("Mutex post-increment", start, end, numIterations, val);
val = 0;
//
// RWlock protected post-increment
//
pthread_rwlock_t rwlock;
pthread_rwlock_init(&rwlock, NULL);
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
for(uint32_t i = 0; i < numIterations; ++i)
{
pthread_rwlock_wrlock(&rwlock);
val++;
pthread_rwlock_unlock(&rwlock);
}
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
outputResult("RWlock post-increment", start, end, numIterations, val);
val = 0;
return 0;
}
And here are the results:
# ./atomicVsNonAtomic
Non-Atomic pre-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 1585375]
[0, 1586185]
[total, avg] = [810, 0] nano seconds
Non-Atomic post-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 1667489]
[0, 1667920]
[total, avg] = [431, 0] nano seconds
Atomic add and fetch:
iterations: 1000000, result: 1000000
times [start, end] = [0, 1682037]
[0, 16595016]
[total, avg] = [14912979, 14] nano seconds
Atomic fetch and add:
iterations: 1000000, result: 1000000
times [start, end] = [0, 16617178]
[0, 31499571]
[total, avg] = [14882393, 14] nano seconds
Mutex post-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 31526810]
[0, 68515763]
[total, avg] = [36988953, 36] nano seconds
RWlock post-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 68547649]
[0, 110877351]
[total, avg] = [42329702, 42] nano seconds
Here is the gcc compilation:
g++ -o atomicVsNonAtomic.o -c -march=i686 -O2 -I. atomicVsNonAtomic.cc
g++ -o atomicVsNonAtomic atomicVsNonAtomic.o -lrt -lpthread
And related info and versions:
# gcc --version
gcc (GCC) 4.3.2
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
# uname -a
Linux gtcba2v1 2.6.32.12-0.7-default #1 SMP 2010-05-20 11:14:20 +0200 i686 i686 i386 GNU/Linux
And now for the actual question :) Is it normal that the atomic operations are so much slower?
The difference for one million iterations is:
- simple post-increment: 431 nano seconds
- atomic fetch and add operation: 14882393 nano seconds
Of course I understand that an atomic operation should be more costly, but this seems exaggerated. Just for completeness, I also checked a pthread mutex and rwlock. At least the atomic operations are faster then the pthread operations, but Im still wondering if I've done something wrong. I couldnt get it to link without specifying the -march=i686 option, maybe this has an impact?
UPDATE:
I took out the -O2 compiler optimization and was able to get more coherent results as follows:
# ./atomicVsNonAtomic
Non-Atomic pre-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 1647303]
[0, 4171164]
[total, avg] = [2523861, 2] nano seconds
Non-Atomic post-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 4310230]
[0, 7262704]
[total, avg] = [2952474, 2] nano seconds
Atomic add and fetch:
iterations: 1000000, result: 1000000
times [start, end] = [0, 7285996]
[0, 25919067]
[total, avg] = [18633071, 18] nano seconds
Atomic fetch and add:
iterations: 1000000, result: 1000000
times [start, end] = [0, 25941677]
[0, 44544234]
[total, avg] = [18602557, 18] nano seconds
Mutex post-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 44573933]
[0, 82318615]
[total, avg] = [37744682, 37] nano seconds
RWlock post-increment:
iterations: 1000000, result: 1000000
times [start, end] = [0, 82344866]
[0, 125124498]
[total, avg] = [42779632, 42] nano seconds
The answer is that GCC optimizes your non-atomic increments away. When it sees a loop like:
for (int i=0; i<N; i++) x++;
it replaces it with:
x += N;
This can be seen in the generated assembly, which contains:
call clock_gettime
leal -32(%ebp), %edx
addl $1000000, -40(%ebp) <- increment by 1000000
adcl $0, -36(%ebp)
movl %edx, 4(%esp)
movl $2, (%esp)
call clock_gettime
So you are not measuring what you think you are.
You can make your variable volatile to prevent this optimization. On my computer, after doing this, non-atomic access is about 8 times as fast as atomic access. When using a 32-bit variable instead of 64-bit (I'm compiling as 32-bit), the difference drops to about a factor of 3.
I'm guessing that gcc is optimizing your non-atomic increment operation to something like
val += numIterations;
You say that 10^6 increments are taking 431 nanoseconds, which works out to 0.000431 ns per loop iteration. On a 4 GHz processor, a clock cycle is 0.25 ns, so it's pretty obvious the loop is being optimized away. This explains the big performance difference you're seeing.
Edit: You measured an atomic operation as taking 14 ns -- assuming a 4 GHz processor again, that works out to 56 cycles, which is pretty decent!
The slowness of any synchronization mechanism cannot be measured by a single thread. Single-process sync objects like POSIX mutexes/Windows critical sections only really cost time when they are contested.
You would have to introduce several threads- doing other work which mirrors the time of your real application- for the synchronized methods to gain a real idea of how long it takes.
来源：https://stackoverflow.com/questions/11608869/is-it-normal-that-the-gcc-atomic-builtins-are-so-slow