From 7e60cc63c33f0c17df36b0ee52ae50a3d04a6697 Mon Sep 17 00:00:00 2001
From: Zeng Jinle <32832641+sneaxiy@users.noreply.github.com>
Date: Wed, 29 Sep 2021 10:13:07 +0800
Subject: [PATCH] refine case when thread_num = 1 (#36201)

---
 .../fast_threaded_ssa_graph_executor.cc       | 20 ++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/paddle/fluid/framework/details/fast_threaded_ssa_graph_executor.cc b/paddle/fluid/framework/details/fast_threaded_ssa_graph_executor.cc
index 120bdd2bc9f..a690b3026db 100644
--- a/paddle/fluid/framework/details/fast_threaded_ssa_graph_executor.cc
+++ b/paddle/fluid/framework/details/fast_threaded_ssa_graph_executor.cc
@@ -47,7 +47,16 @@ FastThreadedSSAGraphExecutor::FastThreadedSSAGraphExecutor(
         << "Change thread number to 1 because the toposort order is unique";
     strategy_.num_threads_ = 1;
   }
-  pool_.reset(new ::ThreadPool(strategy.num_threads_));
+  if (strategy_.num_threads_ > 1) {
+    pool_.reset(new ::ThreadPool(strategy.num_threads_));
+  } else {
+    auto nodes = ir::TopologySortOperations(*graph_);
+    traced_ops_.clear();
+    traced_ops_.reserve(nodes.size());
+    for (auto *node : nodes) {
+      traced_ops_.push_back(&node->Wrapper<OpHandleBase>());
+    }
+  }
   for (auto &op : ir::FilterByNodeWrapper<OpHandleBase>(*graph_)) {
     int dep = static_cast<int>(op->NotReadyInputSize());
     op_deps_.emplace(op, dep);
@@ -228,7 +237,7 @@ void FastThreadedSSAGraphExecutor::RunOpAsync(
     OpHandleBase *op,
     const std::shared_ptr<BlockingQueue<size_t>> &complete_q) {
   ++remaining_;
-  this->pool_->enqueue([=] {
+  auto func = [=] {
     std::deque<OpHandleBase *> op_queue;
     op_queue.push_front(op);
 
@@ -287,7 +296,12 @@ void FastThreadedSSAGraphExecutor::RunOpAsync(
     }
     --remaining_;
     complete_q->Push(complete);
-  });
+  };
+  if (pool_) {
+    pool_->enqueue(func);
+  } else {
+    func();
+  }
 }
 
 void FastThreadedSSAGraphExecutor::PrepareAtomicOpDeps() {
-- 
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