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Subject: Re: [boost] [threadpool] version 22 with default pool
From: vicente.botet (vicente.botet_at_[hidden])
Date: 2009-03-09 17:04:48

----- Original Message -----
From: "Edouard A." <edouard_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Monday, March 09, 2009 9:19 PM
Subject: Re: [boost] [threadpool] version 22 with default pool

>> > If you are using a kernel object to wait, you pay the cost of a
>> transition
>> > even if you don't have to wait.
>> Sorry, I'm a little bit lost.
> [Edouard A.]
> My phrasing was confusing. I meant:
> Even if you don't need to wait, you pay the cost of context switch when
> accessing a kernel synchronization object.

The single function accesd will be is_ready, which is not a core function (see below).

>> Boost.Interthreads has already this kind of free functions (wait()).
>> Now that Boost.ThreadPool allows to check if the tread is a worker of
>> a pool we can add a layer doing exactly that.
>> What do you think?
> The principle is good, however I'm afraid that you will add to the cost of
> the waiting a nasty thing: poisoning. The threads that will wait will not be
> able to schedule other useful tasks. Unless you wish to dedicate a pool for
> this purpose.

As I have already explained in other threads the worker thread will not block while waiting, because it is replaced by reschedul_until the condition is satisfyed (see in particular my last post)
> In other news, I've improved the sorting function as you suggested:
> template <typename RandomIterator, typename Pool> void
> rec_pivot_partition(RandomIterator first, RandomIterator last, Pool & p) {
> if (std::distance(first, last) > 1000)
> {
> RandomIterator pivot = pivot_partition(first, last);
> boost::tp::task<void> task =
> p.submit(boost::bind(&rec_pivot_partition<RandomIterator, Pool>,
> boost::ref(first),
> boost::ref(pivot),
> boost::ref(p)));
> rec_pivot_partition(pivot, last, p);
> task.result().wait();
> }
> else
> {
> std::sort(first, last);
> }
> };
> The results are much better, this time on a larger vector to account Phil's
> remark:
> std::fill reverse 0..10000000 elapsed: 0.039 ok : 1
> tbb::parallel_for fill 0..10000000 elapsed: 0.016 ok : 1
> std::sort reverse 0..10000000 elapsed: 1.172 ok : 1
> tbb::parallel_sort reverse 0..10000000 elapsed: 0.269 ok : 1
> boost::tp::sort reverse 0..10000000 elapsed: 0.331 ok : 1
> This should not be seen as a real benchmark for a parallel_sort
> implementation - there are many more cases to evaluate when it comes to
> sorting - but I think it's safe to say that TBB is significantly more
> efficient. That should not discourage anyone of course: TBB is a mature
> library with a lot of skilled people working on it.
> I also increased the size of the block to 10,000 and performance improved a
> little bit (to ~ 0.316). If I increase to 100,000, performance go back to ~
> 0.331.

Good new, isn't it?
> I initially set up the slice to 1,000 because that's the size of a slice in
> TBB. I wanted - first of all - to compare the engines.
> I've run a test with sizes between 100 and 150,000 and it seems to have
> little impact on the outcome. This is strange. It's a bit early to say if
> it's bad news. See csv & graph attached. I've used GetTickCount to measure.

I agree this is suspect.
> We should perhaps do a test where we would inject large amount of tasks of
> precise duration and see how the scheduler behaves. It would also be
> interesting to measure the delay of execution of one of the tasks (if you
> know a task should last 1 s, measure how long it actually took).

Profiling will be welcome.


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