# 3.1. Introduction¶

This manual describes Adaptive MPI (AMPI), which is an implementation of the MPI standard on top of Charm++. AMPI acts as a regular MPI implementation (akin to MPICH, OpenMPI, MVAPICH, etc.) with several built-in extensions that allow MPI developers to take advantage of Charm++’s dynamic runtime system, which provides support for process virtualization, overlap of communication and computation, load balancing, and fault tolerance with zero to minimal changes to existing MPI codes.

In this manual, we first describe the philosophy behind Adaptive MPI, then give a brief introduction to Charm++ and rationale for AMPI. We then describe AMPI in detail. Finally we summarize the changes required for existing MPI codes to run with AMPI. Appendices contain the details of installing AMPI, and building and running AMPI programs.

Note

Currently, AMPI supports the MPI-2.2 standard, and the MPI-3.1 standard is under active development, though we already support non-blocking and neighborhood collectives among other MPI-3.1 features.

## 3.1.1. Overview¶

Developing parallel Computational Science and Engineering (CSE) applications is a complex task. One has to implement the right physics, develop or choose and code appropriate numerical methods, decide and implement the proper input and output data formats, perform visualizations, and be concerned with correctness and efficiency of the programs. It becomes even more complex for multi-physics coupled simulations, many of which are dynamic and adaptively refined so that load imbalance becomes a major challenge. In addition to imbalance caused by dynamic program behavior, hardware factors such as latencies, variability, and failures must be tolerated by applications. Our philosophy is to lessen the burden of application developers by providing advanced programming paradigms and versatile runtime systems that can handle many common programming and performance concerns automatically and let application programmers focus on the actual application content.

Many of these concerns can be addressed using the processor virtualization and over-decomposition philosophy of Charm++. Thus, the developer only sees virtual processors and lets the runtime system deal with underlying physical processors. This is implemented in AMPI by mapping MPI ranks to Charm++ user-level threads as illustrated in Figure 3. As an immediate and simple benefit, the programmer can use as many virtual processors (“MPI ranks”) as the problem can be easily decomposed to. For example, suppose the problem domain has $$n*2^n$$ parts that can be easily distributed but programming for general number of MPI processes is burdensome, then the developer can have $$n*2^n$$ virtual processors on any number of physical ones using AMPI.

AMPI’s execution model consists of multiple user-level threads per Processing Element (PE). The Charm++ scheduler coordinates execution of these user-level threads (also called Virtual Processors or VPs) and controls execution. These VPs can also migrate between PEs for the purpose of load balancing or other reasons. The number of VPs per PE specifies the virtualization ratio (degree of over-decomposition). For example, in Figure 3 the virtualization ratio is $$3.5$$ (there are four VPs on PE 0 and three VPs on PE 1). Figure 4 shows how the problem domain can be over-decomposed in AMPI’s VPs as opposed to other MPI implementations.

Another benefit of virtualization is communication and computation overlap, which is automatically realized in AMPI without programming effort. Techniques such as software pipelining require significant programming effort to achieve this goal and improve performance. However, one can use AMPI to have more virtual processors than physical processors to overlap communication and computation. Each time a VP is blocked for communication, the Charm++ scheduler picks the next VP among those that are ready to execute. In this manner, while some of the VPs of a physical processor are waiting for a message to arrive, others can continue their execution. Thus, performance improves without any changes to the application source code.

Another potential benefit is that of better cache utilization. With over-decomposition, a smaller subdomain is accessed by a VP repeatedly in different function calls before getting blocked by communication and switching to another VP. That smaller subdomain may fit into cache if over-decomposition is enough. This concept is illustrated in Figure 3 where each AMPI rank’s subdomain is smaller than the corresponding MPI subdomain and so may fit into cache memory. Thus, there is a potential performance improvement without changing the source code.

One important concern is that of load imbalance. New generation parallel applications are dynamically varying, meaning that processors’ load is shifting during execution. In a dynamic simulation application such as rocket simulation, burning solid fuel, sub-scaling for a certain part of the mesh, crack propagation, particle flows all contribute to load imbalance. A centralized load balancing strategy built into an application is impractical since each individual module is developed mostly independently by various developers. In addition, embedding a load balancing strategy in the code complicates it greatly, and programming effort increases significantly. The runtime system is uniquely positioned to deal with load imbalance. Figure 5 shows the runtime system migrating a VP after detecting load imbalance. This domain may correspond to a weather forecast model where there is a storm cell in the top-left quadrant, which requires more computation to simulate. AMPI will then migrate VP 1 to balance the division of work across processors and improve performance. Note that incorporating this sort of load balancing inside the application code may take a lot of effort and complicate the code.

There are many different load balancing strategies built into Charm++ that can be selected by an AMPI application developer. Among those, some may fit better for a particular application depending on its characteristics. Moreover, one can write a new load balancer, best suited for an application, by the simple API provided inside Charm++ infrastructure. Our approach is based on actual measurement of load information at runtime, and on migrating computations from heavily loaded to lightly loaded processors.

For this approach to be effective, we need the computation to be split into pieces many more in number than available processors. This allows us to flexibly map and re-map these computational pieces to available processors. This approach is usually called “multi-domain decomposition”.

Charm++, which we use as a runtime system layer for the work described here, simplifies our approach. It embeds an elaborate performance tracing mechanism, a suite of plug-in load balancing strategies, infrastructure for defining and migrating computational load, and is interoperable with other programming paradigms.

## 3.1.2. Charm++¶

Charm++ is an object-oriented parallel programming library for C. It differs from traditional message passing programming libraries (such as MPI) in that Charm++ is “message-driven”. Message-driven parallel programs do not block the processor waiting for a message to be received. Instead, each message carries with itself a computation that the processor performs on arrival of that message. The underlying runtime system of Charm++ is called Converse, which implements a “scheduler” that chooses which message to schedule next (message-scheduling in Charm++ involves locating the object for which the message is intended, and executing the computation specified in the incoming message on that object). A parallel object in Charm++ is a C object on which a certain computations can be asked to be performed from remote processors.

Charm++ programs exhibit latency tolerance since the scheduler always picks up the next available message rather than waiting for a particular message to arrive. They also tend to be modular, because of their object-based nature. Most importantly, Charm++ programs can be dynamically load balanced, because the messages are directed at objects and not at processors; thus allowing the runtime system to migrate the objects from heavily loaded processors to lightly loaded processors.

Since many CSE applications are originally written using MPI, one would have to rewrite existing code if they were to be converted to Charm++ to take advantage of dynamic load balancing and other Charm++ features. This is indeed impractical. However, Converse - the runtime system of Charm++ - supports interoperability between different parallel programming paradigms such as parallel objects and threads. Using this feature, we developed AMPI, which is described in more detail in the next section.

## 3.1.3. AMPI¶

AMPI utilizes the dynamic load balancing and other capabilities of Charm++ by associating a “user-level” thread with each Charm++ migratable object. User’s code runs inside this thread, so that it can issue blocking receive calls similar to MPI, and still present the underlying scheduler an opportunity to schedule other computations on the same processor. The runtime system keeps track of the computational loads of each thread as well as the communication graph between AMPI threads, and can migrate these threads in order to balance the overall load while simultaneously minimizing communication overhead.

### 3.1.3.1. MPI Standards Compliance¶

Currently AMPI supports the MPI-2.2 standard, with preliminary support for most MPI-3.1 features and a collection of extensions explained in detail in this manual. One-sided communication calls in MPI-2 and MPI-3 are implemented, but they do not yet take advantage of RMA features. Non-blocking collectives have been defined in AMPI since before MPI-3.0’s adoption of them. ROMIO (http://www-unix.mcs.anl.gov/romio/) has been integrated into AMPI to support parallel I/O features.