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fix custom operation in fork #14451

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merged 6 commits into from
Mar 20, 2019
Merged

fix custom operation in fork #14451

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bbc5414
fix custom operation in fork
arcadiaphy Mar 16, 2019
5bd800c
add test
arcadiaphy Mar 17, 2019
8539e90
fix custom stop
arcadiaphy Mar 17, 2019
69aebdf
swap order
arcadiaphy Mar 17, 2019
8951444
add docs
arcadiaphy Mar 20, 2019
3edd5ad
update doc
arcadiaphy Mar 20, 2019
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5 changes: 5 additions & 0 deletions src/initialize.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@
#include <dmlc/logging.h>
#include <mxnet/engine.h>
#include "./engine/openmp.h"
#include "./operator/custom/custom-inl.h"
#if MXNET_USE_OPENCV
#include <opencv2/opencv.hpp>
#endif // MXNET_USE_OPENCV
Expand Down Expand Up @@ -53,11 +54,14 @@ class LibraryInitializer {

// disable openmp for multithreaded workers
#ifndef _WIN32
using op::custom::CustomOperator;
pthread_atfork(
[]() {
CustomOperator::Get()->Stop();
Engine::Get()->Stop();
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can we call Engine stop before customop stop. This will ensure that all ops pushed to engine have executed before stopping engine.

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I take that back. Maybe that won't work since Stopping engine before custom op means there may be ops that havent been pushed to engine by custom op but it has stopped. Another option is to add waitall after the worker joined in CustomOp stop to make sure all pushed ops have completed.

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@arcadiaphy arcadiaphy Mar 17, 2019 •
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The second option doesn't work either. When it goes into Stop function, the custom worker thread has already frozen on python function, leading to the blocking of worker join. I cannot think of a simple and graceful way to complete or ignore the unfinished custom operation other than manually add waiting command in python code.

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do you know why it is frozen on worker join ? The worker should just push the operator to engine and return.

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It's very easy to understand the first situation. The destruction of static variable on engine exit happens when python interpreter has been shutdown, so something bad happens when you call CFUNCTYPE function at that time.
I push a new commit to move custom Stop function to MXNotifyShutdown, forcing custom function called before python shutdown, so the first situation is all right.

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The second situation of forking is so mysterious, I don't have a clue now. The custom Stop function is registered in prepare fork handler, everything should be OK at that time, but the worker thread just freezes.

},
[]() {
CustomOperator::Get()->Start();
Engine::Get()->Start();
},
[]() {
Expand All @@ -71,6 +75,7 @@ class LibraryInitializer {
#endif // MXNET_USE_OPENCV
engine::OpenMP::Get()->set_enabled(false);
Engine::Get()->Start();
CustomOperator::Get()->Start();
});
#endif
}
Expand Down
29 changes: 21 additions & 8 deletions src/operator/custom/custom-inl.h
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Expand Up @@ -137,6 +137,24 @@ class CustomOperator {
}

~CustomOperator() {
this->Stop();
}

static CustomOperator* Get() {
static CustomOperator inst;
return &inst;
}

void Start() {
num_free_threads = 0;
destructing_ = false;
naive_engine_ = true;
if (std::string("NaiveEngine") != dmlc::GetEnv("MXNET_ENGINE_TYPE", std::string())) {
naive_engine_ = false;
}
}

void Stop() {
if (naive_engine_) return;
{
std::unique_lock<std::mutex> lock(mutex_);
Expand All @@ -145,17 +163,12 @@ class CustomOperator {
}
for (auto &worker : workers_)
worker.join();
workers_.clear();
}

static CustomOperator* Get();

private:
CustomOperator() : num_free_threads(0) {
destructing_ = false;
naive_engine_ = true;
if (std::string("NaiveEngine") != dmlc::GetEnv("MXNET_ENGINE_TYPE", std::string())) {
naive_engine_ = false;
}
CustomOperator() {
this->Start();
}
void ThreadTarget() {
std::unique_lock<std::mutex> lock(mutex_);
Expand Down
5 changes: 0 additions & 5 deletions src/operator/custom/custom.cc
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Expand Up @@ -34,11 +34,6 @@ namespace mxnet {
namespace op {
namespace custom {

CustomOperator* CustomOperator::Get() {
static CustomOperator inst;
return &inst;
}

struct CustomParam {
std::string op_type;
size_t num_args, num_outs, num_auxs;
Expand Down