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Using All These Cores: Transactional Memory in PyPy

Summary

PyPy, the Python implementation written in Python, experimentally supports Transactional Memory (TM). The strength of TM is to enable a novel use of multithreading, inheritently safe, and not limited to special use cases like other approaches. This talk will focus on how it works under the hood.

Description

PyPy is a fast alternative Python implementation. Software Transactional Memory (STM) is a current academic research topic. Put the two together --brew for a couple of years-- and we get a version of PyPy that runs on multiple cores, without the infamous Global Interpreter Lock (GIL).

The current research is based on a recent new insight that promises to give really good performance. The speed of STM is generally measured by two factors: the ability to scale with the number of CPUs, and the amount of overhead when compared with other approaches in a single CPU (in this case, with the regular PyPy with the GIL). Scaling is not really a problem here, but single-CPU performance is --or used to be. This new approach gives a single-threaded overhead that should be very low, maybe 20%, which would definitely be news for STM systems. Right now (February 2014) we are still implementing it, so we cannot give final numbers yet, but early results on a small interpreter for a custom language are around 15%. This looks like a deal-changer for STM.

In the talk, I will describe our progress, hopefully along with real numbers and demos. I will then dive under the hood of PyPy to give an idea about how it works. I will conclude with a picture of how the future of multi-threaded programming might looks like, for high-level languages like Python. I will also mention CPython: how hard (or not) it would be to change the CPython source code to use the same approach.

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