Merge branch 'develop' of https://github.com/baidu/Paddle into conv_op
hedaoyuan
8 years ago
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# Design Doc: Operation Graph Based Parameter Server
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## Abstract
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We propose an approach to implement the parameter server. In this
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approach, there is no fundamental difference between the trainer and
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the parameter server: they both run subgraphs, but subgraphs of
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different purposes.
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## Background
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The previous implementations of the parameter server does not run a
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subgraph. parameter initialization, optimizer computation, network
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communication and checkpointing are implemented twice on both the
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trainer and the parameter server.
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It would be great if we can write code once and use them on both the
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trainer and the parameter server: reduces code duplication and
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improves extensibility. Given that after the current refactor, we are
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representing everything as a computing graph on the
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trainer. Representing everything as a computing graph on the parameter
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server becomes a natural extension.
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## Design
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### Graph Converter
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The *graph converter* converts the user-defined operation (OP) graph
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into subgraphs to be scheduled on different nodes with the following
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steps:
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1. OP placement: the OPs will be placed on different nodes according
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to heuristic that minimizes estimated total computation
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time. Currently we will use a simple heuristic that puts parameter
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varable on parameter server workers and everything else on trainer
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workers.
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1. Add communication OPs to enable the communication between nodes.
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We will need these OPs: *Send*, *Recv*, *Enqueue*, *Dequeue*.
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Below is an example of converting the user defined graph to the
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subgraphs for the trainer and the parameter server:
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<img src="src/local-graph.png" width="300"/>
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After converting:
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<img src="src/dist-graph.png" width="700"/>
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1. The parameter variable W and it's optimizer subgraph are placed on the parameter server.
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1. Operators are added to the subgraphs.
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- *Send* sends data to the connected *Recv* operator. The
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scheduler on the receive node will only schedule *Recv* operator
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to run when the *Send* operator has ran (the *Send* OP will mark
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the *Recv* OP runnable automatically).
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- *Enueue* enqueues the input variable, it can block until space
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become available in the queue.
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- *Dequeue* outputs configurable numbers of tensors from the
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queue. It will block until the queue have the required number of
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tensors.
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### Benefits
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- Model parallelism become easier to implement: it's an extension to
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the trainer - parameter server approach. we already have the
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communication OPs, but need to extend the graph converter's
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placement functionality.
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- User-defined optimizer is easier to add - user can now express it as
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a subgraph.
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- No more duplication logic inside the trainer and the parameter
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server mentioned in the background section.
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### Challenges
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- It might be hard for the graph converter to cut a general graph
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(without any hint for which subgraph is the optimizer). We may need
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to label which subgraph inside the OP graph is the optimizer.
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- It's important to balance the parameter shards of on multiple
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parameter server. If a single parameter is very big (some
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word-embedding, fully connected, softmax layer), we need to
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automatically partition the single parameter onto different
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parameter servers when possible (only element-wise optimizer depends
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on the parameter variable).
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### Discussion
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- In the "Aync SGD" figure, the "W" variable on the parameter server
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could be read and wrote concurrently, what is our locking strategy?
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E.g., each variable have a lock cpp method to be invoked by every
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OP, or, have a lock OP.
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- Can the Enqueue OP be implemented under our current tensor design
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(puts the input tensor into the queue tensor)?
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- *Dequeue* OP will have variable numbers of output (depends on the
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`min_count` attribute), does our current design support it? (similar
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question for the *Add* OP)
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### References:
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[1] [TensorFlow: Large-Scale Machine Learning on Heterogeneous Distributed Systems](https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/45166.pdf)
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/*
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Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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#include <cuda.h>
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#include <cuda_runtime.h>
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#include "paddle/framework/lod_tensor.h"
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#include "paddle/platform/assert.h"
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#include <gtest/gtest.h>
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__global__ void test(size_t* a, int size) {
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for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < size;
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i += blockDim.x * gridDim.x) {
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a[i] *= 2;
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}
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}
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TEST(LoDTensor, LoDInGPU) {
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paddle::framework::Tensor tensor;
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paddle::framework::LoDTensor lod_tensor;
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paddle::platform::GPUPlace place(0);
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paddle::framework::LoD src_lod;
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src_lod.push_back(std::vector<size_t>{0, 2, 4, 6, 8, 10, 12, 14});
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tensor.Resize({14, 16});
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tensor.mutable_data<float>(place);
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lod_tensor.set_lod(src_lod);
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lod_tensor.set_tensor(&tensor);
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CHECK_EQ(lod_tensor.lod_element(0, 2), 4);
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CHECK_EQ(lod_tensor.lod_element(0, 4), 8);
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auto lod = lod_tensor.lod();
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test<<<1, 8>>>(lod[0].data(), lod[0].size());
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cudaDeviceSynchronize();
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for (size_t i = 0; i < src_lod[0].size(); ++i) {
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CHECK_EQ(lod[0].data()[i], src_lod[0].data()[i] * 2);
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}
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}
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