boost asio thread_group only run one thread - c++

Question

I am studing boost library. I want to use fiber with blocking api so I try to use a thread pool to run blocking api. When the blocking api is finished, the fiber get the result. I have created a thread pool of 4 threads, but I found that only one thread is running. Why?
Here is my code.
#include <boost/asio.hpp>
#include <boost/fiber/all.hpp>
#include <boost/fiber/fiber.hpp>
#include <boost/fiber/future/future.hpp>
#include <boost/fiber/future/promise.hpp>
#include <boost/fiber/operations.hpp>
#include <chrono>
#include <ctime>
#include <iostream>
#include <string>
#include <thread>
#include <unistd.h>
using namespace std;
using namespace boost;
time_t block_api() {
std::this_thread::sleep_for(std::chrono::seconds(1));
return clock();
}
class AsyncWrapper {
public:
AsyncWrapper(asio::thread_pool &pool) : pool_(&pool) {}
~AsyncWrapper() {}
time_t async_api() {
fibers::promise<time_t> promise;
boost::fibers::future<time_t> future(promise.get_future());
auto func = [&]() mutable {
auto ret = block_api();
std::cout << "run on:"<<getpid()<<std::endl;
promise.set_value(ret);
};
asio::post(pool_->get_executor(), func);
return future.get();
}
private:
asio::thread_pool *pool_ = nullptr;
};
static asio::thread_pool g_pool(4);
int main(int argc, char **argv) {
if (argc != 2) {
return -1;
}
std::string arg = argv[1];
std::cout << "press enter to start" << std::endl;
getchar();
for (int32_t i = 0; i < stoi(arg); i++) {
fibers::fiber f([]() {
AsyncWrapper async_wrapper(g_pool);
auto ret = async_wrapper.async_api();
});
f.detach();
}
this_fiber::yield();
return 0;
}
When I use command to analysis my program ,I saw that
top - 17:32:03 up 5:29, 0 users, load average: 0.19, 0.17, 0.20
Threads: 5 total, 1 running, 4 sleeping, 0 stopped, 0 zombie
%Cpu(s): 8.3 us, 0.0 sy, 0.0 ni, 91.6 id, 0.0 wa, 0.0 hi, 0.1 si, 0.0 st
MiB Mem : 7931.7 total, 5037.4 free, 2405.4 used, 488.9 buff/cache
MiB Swap: 2048.0 total, 1259.0 free, 789.0 used. 5229.7 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
10338 wuhaotao 20 0 431880 15732 3424 R 99.7 0.2 0:03.63 with_blocking_a
10339 wuhaotao 20 0 431880 15732 3424 S 0.0 0.2 0:00.00 with_blocking_a
10340 wuhaotao 20 0 431880 15732 3424 S 0.0 0.2 0:00.00 with_blocking_a
10341 wuhaotao 20 0 431880 15732 3424 S 0.0 0.2 0:00.00 with_blocking_a
10342 wuhaotao 20 0 431880 15732 3424 S 0.0 0.2 0:00.00 with_blocking_a
Only the 10338 thread is running......