Overlapping Computation and Communication in HPC Applications
Improving MPI Performance by Hiding Latency
Scaling MPI applications on large HPC systems requires efficient communication patterns. Whenever possible, the application needs to overlap communication and computation to hide the communication latency. In previous articles, we’ve discussed implementing the non-blocking collective communications provided by Intel® MPI Library for speeding up non-blocking communication and I/O collective operations (see Hiding Communication Latency Using MPI-3 Non-Blocking Collectives and MPI-3 Non-Blocking I/O Collectives in Intel® MPI Library). These improvements are based on the asynchronous progress threads support that allows the Intel MPI Library to perform communication and computation in parallel. This article will focus on non-blocking point-to-point communication, mainly the MPI_Isend and MPI_Irecv functions, and how an efficient ghost cell exchange mechanism for a domain decomposition code can effectively hide communication latency in MPI applications.
Benchmarking the Point-to-Point Overlap
To test the ability of Intel MPI Library to overlap communication and computation, we’ve implemented a simple ping-pong benchmark, where computation is simulated by the sleep function and the communication uses the non-blocking send (Isend) and receive (Irecv). This is similar to the approach used by Wittmann et al in Asynchronous MPI for the Masses.
We’ll set up the benchmark so that the first task initializes the communication with MPI_Isend, sleeps for time Tcpu to simulation computation, then posts an MPI_Wait while the second task posts an MPI_Irecv. This is performed several times to collect enough statistical data (in our case, we run it for 100 iterations). Here’s the code snippet:
We measure the total time Tt of the benchmark and the time of the compute part Tcpu and, in case of no-asynchronous progress, the total time Tt = Tcpu + Tc where Tc is the communication time. When asynchronous communication is enabled, the total time of the benchmark results in Tt = max(Tcpu, Tc), showing an overlap between computation and communication.
Using Intel MPI Library release_mt version, the following environment variable must be set to enable the asynchronous progress thread feature:
(Find more details in the Intel® MPI Library Developer Guide for Linux* OS and Intel® MPI Library Developer Reference for Linux* OS.)
By default, one progress thread is generated per MPI task.
Figure 1 shows the results of our ping-pong benchmark. Different colors correspond to different message sizes for the Isend/Irecv communication, starting from 16MB (blue lines) to 64MB (green lines). Smaller messages might be sent as non-blocking, even for blocking communication, due to the eager protocol for the MPI communication. The two lines with the same color correspond to the cases with and without asynchronous progress enabled (triangle and circle markers, respectively). The effectiveness of overlapping communication and computation is clearly visible in the behavior of the 16MB async, 32MB async, and 64MPI async lines. The horizontal portions show that the total time is dominated by communication overhead. Indeed, the compute time is completely invisible in the timings. On the other hand, for larger compute times (represented by the x-axis), the total time is dominated by the computation. The communication is completely hidden and Tt = Tcpu, resulting in computation to communication overlap of 100%.
Ghost Cell Exchange in Domain Decomposition
To better investigate the performance benefit of the asynchronous progress threads, let’s investigate the behavior of a typical domain decomposition application that uses a ghost cell exchange mechanism. Figure 2 shows an example of two-dimensional domain partitioning in four blocks, each assigned to a different MPI process (denoted by different colors). The white cells with colored dots correspond to the cells that each process computes independently. The colored cells at the borders of each sub-domain correspond to the ghost cells that must be exchanged with neighboring MPI processes.
The benchmark application solves the diffusive heat equation in three dimensions, using 2D domain decomposition with a stencil of one element in each direction. The general update scheme can be summarized as follows:
- Copy data of the ghost cells to send buffers
- Halo exchange with Isend/Irecv calls
- Compute (part 1): Update the inner field of the domain
- Copy data from receive buffers to the ghost cells
- Compute (part 2): Update the halo cells
- Repeat N times
The computation exhibits strong scaling with the number of MPI processes used to run the simulation. The copy step is needed to fill the buffers used for the data exchange. In terms of simulation time, the overhead of copying is negligible. The halo exchange is used for instantiating the communication of the ghost cells, which are not used during computation of the inner field. Therefore, updating the inner field can be executed asynchronously with respect to the halo exchange. The outer boundary of the domain is ignored for simplicity.
Strong Scaling Benchmark
On a multi-node system, we measured how the number of iterations per second changes with increasing resources. Each node is a two-socket, Intel® Xeon® Gold 6148 processor system. Figure 3 shows the results of strong scaling, where the problem size is fixed to a 64 x 64 x 64 grid and the number of nodes increases. The difference between the blue and orange lines shows the performance improvement if we overlap communication and computation using the asynchronous progress thread implementation of Intel MPI Library. The improvement is significant, up to 1.9x for the eight-node case. This is confirmed by plotting the computation and communication times (Figure 4). The blue lines correspond to the standard MPI, while the orange correspond to the asynchronous progress thread implementation. The compute and the communication time are represented in dashed and solid lines, respectively. As the number of nodes increases, the benefit isn’t as high because the computation per MPI process becomes smaller and the application is dominated by the communication overhead.
Increasing the Number of MPI Processes per Node
At higher numbers of MPI processes per node, we increase the grid size to 128 x 128 x 128 in order to have enough computation to be overlapped with the communication of the ghost cells. Figures 5 and 6 show the number of iterations per second versus the number of nodes for the 10 and 20 MPI processes per node cases, respectively. For larger numbers of MPI processes, we need to balance the available resources. Intel MPI Library lets us pin the MPI processes and the asynchronous progress thread to specific CPU cores using these environment variables:
Using these environment variables and pinning the asynchronous thread close to the corresponding MPI process gives best performance. We observed high speedups in both cases, up to 2.2x for the 16-node case and 10 MPI processes per node.
Weak Scaling Benchmark
For the last performance test, we performed a weak scaling investigation. In Figure 7, we report the performance speedup when overlapping communication with computation, where we increase the grid size by two along with the number of nodes, up to 128. The computation size and the number of MPI processes per node is kept constant at four processes. The overall benefit is significant, up to a 32% performance improvement for the largest test.
Improving Speed and Scaling
We’ve investigated and analyzed the possibility of overlapping communication and computation for the non-blocking point-to-point MPI functions using the asynchronous progress thread control provided by Intel MPI Library. We showed the performance benefit of using this implementation for a typical domain decomposition code using the ghost cells exchange mechanism. We believe applications with similar communication patterns using the non-blocking Isend/Irecv scheme could observe similar speedups and scaling behavior.
Configuration: Testing by Intel as of Feb. 14, 2020. Node configuration: 2x Intel® Xeon® Gold 6148 Processor CPU @ 2.40GHz, 20 cores per CPU, 96 GB per node, nodes connected by the Intel® Omni-Path Fabric (Intel® OP Fabric), Intel® MPI Library 2019 update 4, release_mt, asynchronous progress control enabled via export I_MPI_ASYNC_PROGRESS=1, export I_MPI_ASYNC_PROGRESS_THREADS=1. Intel’s compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice. Notice revision #20110804.