The best practice is not to reuse the pool (what would be the use of pooling, if you keep creating new pools?).

If you want to be sure you “time” the batches together, I’d suggest using when_all on futures:

Live On Coliru

#define BOOST_THREAD_PROVIDES_FUTURE_WHEN_ALL_WHEN_ANY
#include <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/thread.hpp>

uint64_t foo(uint64_t begin) {
    uint64_t prev[] = {begin, 0};
    for (uint64_t i = 0; i < 1000000000; ++i) {
        const auto tmp = (prev[0] + prev[1]) % 1000;
        prev[1] = prev[0];
        prev[0] = tmp;
    }
    return prev[0];
}

void batch(boost::asio::thread_pool &pool, const uint64_t a[2])
{
    using T = boost::packaged_task<uint64_t>;

    T tasks[] {
        T(boost::bind(foo, a[0])),
        T(boost::bind(foo, a[1])),
    };

    auto all = boost::when_all(
        tasks[0].get_future(),
        tasks[1].get_future());

    for (auto& t : tasks)
        post(pool, std::move(t));

    auto [r0, r1] = all.get();
    std::cerr << "foo(" << a[0] << "): " << r0.get() << " foo(" << a[1] << "): " << r1.get() << std::endl;
}

int main() {
    boost::asio::thread_pool pool(2);

    const uint64_t a[] = {2, 4};
    batch(pool, a);

    const uint64_t b[] = {3, 5};
    batch(pool, b);

    const uint64_t c[] = {7, 9};
    batch(pool, c);
}

Prints

foo(2): 2 foo(4): 4
foo(3): 503 foo(5): 505
foo(7): 507 foo(9): 509

I would consider

• generalizing
• message queuing

Generalized

Make it somewhat more flexible by not hardcoding batch sizes. After all, the pool size is already fixed, we don’t need to “make sure batches fit” or something:

Live On Coliru

#define BOOST_THREAD_PROVIDES_FUTURE_WHEN_ALL_WHEN_ANY
#include <iostream>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <boost/thread/future.hpp>

struct Result { uint64_t begin, result; };

Result foo(uint64_t begin) {
    uint64_t prev[] = {begin, 0};
    for (uint64_t i = 0; i < 1000000000; ++i) {
        const auto tmp = (prev[0] + prev[1]) % 1000;
        prev[1] = prev[0];
        prev[0] = tmp;
    }
    return { begin, prev[0] };
}

void batch(boost::asio::thread_pool &pool, std::vector<uint64_t> const a)
{
    using T = boost::packaged_task<Result>;
    std::vector<T> tasks;
    tasks.reserve(a.size());

    for(auto begin : a)
        tasks.emplace_back(boost::bind(foo, begin));

    std::vector<boost::unique_future<T::result_type> > futures;
    for (auto& t : tasks) {
        futures.push_back(t.get_future());
        post(pool, std::move(t));
    }

    for (auto& fut : boost::when_all(futures.begin(), futures.end()).get()) {
        auto r = fut.get();
        std::cerr << "foo(" << r.begin << "): " << r.result << " ";
    }
    std::cout << std::endl;
}

int main() {
    boost::asio::thread_pool pool(2);

    batch(pool, {2});
    batch(pool, {4, 3, 5});
    batch(pool, {7, 9});
}

Prints

foo(2): 2 
foo(4): 4 foo(3): 503 foo(5): 505 
foo(7): 507 foo(9): 509 

Generalized2: Variadics Simplify

Contrary to popular believe (and honestly, what usually happens) this time we can leverage variadics to get rid of all the intermediate vectors (every single one of them):

Live On Coliru

void batch(boost::asio::thread_pool &pool, T... a)
{
    auto launch = [&pool](uint64_t begin) {
        boost::packaged_task<Result> pt(boost::bind(foo, begin));
        auto fut = pt.get_future();
        post(pool, std::move(pt));
        return fut;
    };

    for (auto& r : {launch(a).get()...}) {
        std::cerr << "foo(" << r.begin << "): " << r.result << " ";
    }

    std::cout << std::endl;
}

If you insist on outputting the results in time, you can still add when_all into the mix (requiring a bit more heroics to unpack the tuple):

Live On Coliru

template <typename...T>
void batch(boost::asio::thread_pool &pool, T... a)
{
    auto launch = [&pool](uint64_t begin) {
        boost::packaged_task<Result> pt(boost::bind(foo, begin));
        auto fut = pt.get_future();
        post(pool, std::move(pt));
        return fut;
    };

    std::apply([](auto&&... rfut) {
        Result results[] {rfut.get()...};
        for (auto& r : results) {
            std::cerr << "foo(" << r.begin << "): " << r.result << " ";
        }
    }, boost::when_all(launch(a)...).get());

    std::cout << std::endl;
}

Both still print the same result

Message Queuing

This is very natural to boost, and sort of skips most complexity. If you also want to report per batched group, you’d have to coordinate:

Live On Coliru

#include <iostream>
#include <boost/asio.hpp>
#include <memory>

struct Result { uint64_t begin, result; };

Result foo(uint64_t begin) {
    uint64_t prev[] = {begin, 0};
    for (uint64_t i = 0; i < 1000000000; ++i) {
        const auto tmp = (prev[0] + prev[1]) % 1000;
        prev[1] = prev[0];
        prev[0] = tmp;
    }
    return { begin, prev[0] };
}

using Group = std::shared_ptr<size_t>;
void batch(boost::asio::thread_pool &pool, std::vector<uint64_t> begins) {
    auto group = std::make_shared<std::vector<Result> >(begins.size());

    for (size_t i=0; i < begins.size(); ++i) {
        post(pool, [i,begin=begins.at(i),group] {
              (*group)[i] = foo(begin);
              if (group.unique()) {
                  for (auto& r : *group) {
                      std::cout << "foo(" << r.begin << "): " << r.result << " ";
                      std::cout << std::endl;
                  }
              }
          });
    }
}

int main() {
    boost::asio::thread_pool pool(2);

    batch(pool, {2});
    batch(pool, {4, 3, 5});
    batch(pool, {7, 9});
    pool.join();
}

Note this is having concurrent access to group, which is safe due to the limitations on element accesses.

Prints:

foo(2): 2 
foo(4): 4 foo(3): 503 foo(5): 505 
foo(7): 507 foo(9): 509