Re: Why is java consumer/producer so much faster than C++
On Thu, 26 Jul 2012 23:12:54 +0200
Luca Risolia <luca.risolia@studio.unibo.it> wrote:
On 26/07/2012 23:01, Melzzzzz wrote:
On Thu, 26 Jul 2012 22:15:33 +0200
Luca Risolia <luca.risolia@studio.unibo.it> wrote:
On 26/07/2012 21:56, Melzzzzz wrote:
f = ++f % cap_;
compiler warns about undefined behavior here.
Ouch..split that:
++f;
f %= cap_;
I corrected that already ;)
I have installed gcc-snapshot compiler
gcc version 4.8.0 20120314 (experimental) [trunk revision 185382]
(Ubuntu/Linaro 20120314-0ubuntu2)
Your timings with array assignment and rand included, are much worse
than gcc 4.6.3. Definitely something is wrong if
other thread does something else besides waiting on
spin lock.
Yes, it's worth to strace std::rand() to see what it does exactly.
Out of my curiosity, try to yield() after releasing the lock in both
put() and take():
m_.unlock();
boost::this_thread::yield(); // add this line
do you see any improvements?
Finally got it ;)
Problem was that producer thread constantly spins, but consumer does
some work. Queue was constantly full and that slowed it down
significantly.
I have used Howards trick, and this time correctly ;)
now times are fastest (queue size was 1000):
(with rand)
bmaxa@maxa:~/examples$ time ./consprod5
real 0m6.215s
user 0m6.440s
sys 0m5.108s
(without rand)
bmaxa@maxa:~/examples$ time ./consprod5
real 0m3.232s
user 0m3.024s
sys 0m2.968s
code:
void put(T t) {
for(;;)
{
m_.lock();
if(n < cap_)
{
int retry = 0;
while(n > 3 * cap_ / 4 && ++retry < 3000)
{
m_.unlock();
std::this_thread::yield();
m_.lock();
}
break;
}
m_.unlock();
}
alloc_.construct(&queue_[f], std::move(t));
++f;
if(f == cap_)f = 0;
++n;
m_.unlock();
}
T take() {
for(;;)
{
m_.lock();
if(n > 0)
{
int retry = 0;
while(n < cap_ / 4 && ++retry < 3000)
{
m_.unlock();
std::this_thread::yield();
m_.lock();
}
break;
}
m_.unlock();
}
T t = std::move(queue_[b]);
alloc_.destroy(&queue_[b]);
++b;
if(b == cap_) b = 0;
--n;
m_.unlock();
return t;
}