1 15. Research Plan in Japan

2 a. Present research related to research plan

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4 I am currently working on a PhD in the domain of runtimes for parallel programming models. The object of my research is to improve the VMS runtime framework. VMS addresses the productivity gap between sequential and parallel programming. The difficulty of parallel programming is reduced when the programming model offers high-level concepts that closely match the application's concepts. However, the more specialized a programming model is, the less users it has, and so less effort can be spent on developing the model due to diminishing returns (it becomes useless when the time to write the runtime is larger than the extra time it takes to write the applications in a less suitable model).

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6 VMS brings the simplicity of sequential programming to the development of parallel runtimes by providing a base layer that takes charge of the most difficult aspect, synchronization. Building on this, the runtime writer only needs to provide specific methods in the form of a plugin to support a certain programming model. These plugin methods are easy to write because they need to be neither thread-safe nor reentrant.

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8 I have been evaluating the performance of the current VMS implementation. It was designed for cache-coherent shared memory machines not exceeding a dozen cores and performs well on these machines, offering comparable performance to the default runtime libraries for models such as OmpSs. However, some of the assumptions in the VMS abstractions are unsuited to larger and/or distributed systems. Preparing for the research to take place during the exchange period, additional benchmarks using different combinations of features will be developed to evaluate the runtime's performance, and establish a baseline to compare improvements to. Next, a distributed version of VMS will be developed. The goal is to have a first working implementation ready that will be improved using the combined knowledge of distributed systems at TokyoTech and parallel runtimes at TU Berlin.

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10 b. Purpose of proposed research

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12 Often, a significant part of the runtime overhead is spent on ensuring compatibility with a wide range of features of the programming model, or even with other programming models, even though most of these features will not be used in this particular application. If the runtime could know if these features were needed or not, it would be possible to reduce overhead by eliding the unnecessary functionalities.

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14 The proposed research will enable building modularized runtimes that can load a reduced feature set based on the developer's indication of which features they wish to use. A further aim is to allow development of hardware support that can be flexibly used by replacing selected runtime models with ones taking advantage of special-purpose hardware when it is available.

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16 c. Proposed plan

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18 Main tasks during the research stay:

19 - Define a set of basic abstractions suited to distributed systems that enable plugins for many different programming models. These abstractions should support several goals:

20 1. Generality: allow implementation of different types of programming models (thread-based, task-based, dataflow, ...)

21 2. Simplicity: present only very few basic abstractions that can be quickly understood

22 3. Modularity: encourage separation of the different runtime functionalities. Modularity enables two approaches to boost performance. One is to selectively load only the necessary functionalities for a specific application, eschewing overhead due to preparing features that are never actually used. The other is to make it easier to integrate specialized hardware that can support specific runtime functions.

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24 Possibly offer simplified abstractions for common models (thread- and task-based programming models in particular).

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26 - Develop a modularized plugin for the OmpSs programming model using the new abstractions. This programming model will serve as case study to evaluate the suitability of the abstractions both from a productivity and from a performance perspective.

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28 d. Expected results and impacts

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30 The expected immediate benefit is to have a runtime whose overhead is proportional to the featureset actually used by the application. This allows the usage of powerful programming models for a larger group of applications.

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32 This work will also enable important future research both at TokyoTech and TU Berlin. Future plans are to use the improved VMS platform to develop runtimes that can automatically adjust task size to different systems' capabilities (number of cores, computation vs. communication speed, etc). It will also serve to develop a set of hardware accelerators for schedulers.

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34 The embedded systems of tomorrow are increasingly resembling the high-performance systems of today, and both domains share the drive to extract maximum performance from the systems. Combining the experience and techniques from both domains is profitable for both sides.