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Section: New Software and Platforms
Pipedream
Keywords: Performance analysis - CPU - Reverse engineering
Scientific Description: Pipedream reverse engineers the following performance characteristics: - Instruction latency – The number of cycles an instruction requires to execute. - Peak micro-op retirement rate – How many fused micro-ops the CPU can retire per cycle. - Micro-fusion – The number of fused micro-ops an instruction decomposes into. - Micro-op decomposition and micro-op port usage – The list of unfused micro-ops every instruction decomposes into and the list of execution ports every one of these micro-ops can execute on.
The first step of the reverse engineering process consists of generating a number of microbenchmarks. Pipedream then runs these benchmark, measuring their performance using hardware counters. The latency, throughput, and micro-fusion of different instructions can then be read directly from these measurements.
The process of finding port mappings, i.e. micro-op decompositions and micro-op port usage, however, is more involved. For this purpose, we have defined a variation of the maximum flow problem which we call the "instruction flow problem". We have developed a linear program (LP) formulation of the instruction flow problem which can be used to calculate the peak IPC and micro-operations per cycle (MPC) a benchmark kernel can theoretically achieve with a given port mapping. The actual port mapping of the underlying hardware is then determined by finding the mapping for which the throughput predicted by instruction flow best matches the actual measured IPC and MPC.
Functional Description: Pipedream is a tool for measuring specific performance characteristics of CPUs It is used to build the performance model of another tool called Gus (https://gitlab.inria.fr/nderumig/gus). Pipedream finds measured performance characteristics such as the throughput and latency of instructions by running a large set of automatically generated microbenchmarks. The tool can also find port mappings, a model of part of the CPU instruction scheduler, by analysing performance measurements of specially crafted microkernels using a LP solver. We have used it to produce a port mapping for the Intel Skylake CPU architecture. Pipedream is able to find the port mappings for some instructions for which existing approaches fall back to manual analysis.