thread_migrate strategy changes from random to round-robin (#163)

thread/workerpool.cpp

@@ -24,6 +24,7 @@ limitations under the License.
 #include <algorithm>
 #include <random>
 #include <thread>
+#include <atomic>
 
 namespace photon {
 
@@ -34,19 +35,17 @@ class WorkPool::impl {
  photon::mutex worker_mtx;
  std::vector<std::thread> owned_std_threads;
  std::vector<photon::vcpu_base *> vcpus;
+ std::atomic<uint64_t> vcpu_index{0};
  photon::semaphore queue_sem;
  photon::semaphore ready_vcpu;
  photon::condition_variable exit_cv;
  photon::common::RingChannel<
  LockfreeMPMCRingQueue<Delegate<void>, RING_SIZE>>
  ring;
-
- std::random_device rd;
- std::mt19937 gen;
  int mode;
 
  impl(size_t vcpu_num, int ev_engine, int io_engine, int mode)
- : queue_sem(0), ready_vcpu(0), gen(rd()), mode(mode) {
+ : queue_sem(0), ready_vcpu(0), mode(mode) {
  for (size_t i = 0; i < vcpu_num; ++i) {
  owned_std_threads.emplace_back(
  &WorkPool::impl::worker_thread_routine, this, ev_engine,
@@ -134,8 +133,9 @@ class WorkPool::impl {
  }
 
  photon::vcpu_base *get_vcpu_in_pool(size_t index) {
- if (index >= vcpus.size()) {
- index = gen() % vcpus.size();
+ auto size = vcpus.size();
+ if (index >= size) {
+ index = vcpu_index++ % size;
  }
  return vcpus[index];
  }

thread/workerpool.h

@@ -99,7 +99,7 @@ class WorkPool {
  *
  * @param th Photon thread that goint to migrate
  * @param index Which vcpu in pool to migrate to. if index is not in range
- * [0, vcpu_num), it will choose random one in pool.
+ * [0, vcpu_num), it will choose the next one in pool (round-robin).
  * @return int 0 for success, and <0 means failed to migrate.
  */
  int thread_migrate(photon::thread* th = CURRENT, size_t index = -1UL) {