# Parallel Object Model¶

## Introduction¶

Object-oriented programming provides high level abstractions for software engineering. In addition, the nature of objects makes them ideal structures to distribute data and executable codes over heterogeneous distributed hardware and to make them interact between each other. Nevertheless, two questions remain:

• Question 1: which objects should run remotely?
• Question 2: where does each remote object live?

The answers, of course, depend on what these objects do and how they interact with each other and with the outside world. In other words, we need to know the communication and the computation requirements of objects. The parallel object model presented in this chapter provides an object-oriented approach for requirement-driven high performance applications in a distributed heterogeneous environment.

## Parallel Object Model¶

POP stands for Parallel Object Programming, and POP parallel objects are generalizations of traditional sequential objects. POP-Java is an extension of Java that implements the POP model. POP-Java instantiates parallel objects transparently and dynamically, assigning suitable resources to objects. POP-Java also offers various mechanisms to specify different ways to do method invocations. Parallel objects have all the properties of traditional objects plus the following ones:

• Parallel objects are shareable. References to parallel objects can be passed to any other parallel object. This property is described in Shareable Parallel Objects.
• Syntactically, invocations on parallel objects are identical to invocations on traditional sequential objects. However, parallel objects support various method invocation semantics: synchronous or asynchronous, and sequential, mutex or concurrent. These semantics are explained in section Invocations semantics.
• Parallel objects can be located on remote resources in separate address spaces. Parallel objects allocations are transparent to the programmer. The object allocation is presented in section Parallel Object Allocation.
• Each parallel object has the ability to dynamically describe its resource requirement during its lifetime. This feature is discussed in detail in section Requirement-driven parallel objects.

As for traditional objects, parallel objects are active only when they execute a method (non active object semantic). Therefore, communication between parallel objects are realized thank to remote methods invocation.

## Shareable Parallel Objects¶

Parallel objects are shareable. This means that the reference of a parallel object can be shared by several other parallel objects. Sharing references of parallel objects are useful in many cases. For example, Figure 1 illustrates a scenario of using shared parallel objects: input and output parallel objects are shareable among worker objects. A worker gets work units from input which is located on the data server, performs the computation and stores the results in the output located at the user workstation. The results from different worker objects can be automatically synthesized and visualized inside output.

To share the reference of a parallel object, POP-Java allows parallel objects to be arbitrarily passed from one place to another as arguments of method invocations.

## Invocations semantics¶

Syntactically, method invocations on parallel objects are identical to those on traditional sequential objects. However, to each method of a parallel object, one can associate different invocation semantics. Invocation semantics are specified by programmers when declaring methods of parallel objects. These semantics define different behaviors for the execution of the method as described below (example of syntax in Developing POP-Java applications):

• Interface semantics, the semantics that affect the caller of the method:

• Synchronous invocation: the caller waits until the execution of the called method on the remote object is terminated. This corresponds to the traditional method invocation.

• Asynchronous invocation: the invocation returns immediately after sending the request to the remote object. Asynchronous invocation is important to exploit the parallelism. However, as the caller does not wait the end of the execution of the called method, no computing result is available. This excludes asynchronous invocations from producing results. Results can be actively returned to the caller object using a callback to the caller. To do so the called object must have a reference to the caller object. This reference can be passed as an argument to the called method (see Figure 2).

• Object-side semantics, the semantics that affect the order of the execution of methods in the called parallel object:

• A mutex call is executed after completion of all calls previously arrived.
• A sequential call is executed after completion of all sequential and mutex calls previously arrived.
• A concurrent call can be executed concurrently (time sharing) with other concurrent or sequential calls, except if mutex calls are pending or executing. In the latter case the call is executed after completion of all mutex calls previously arrived.

In a nutshell, different object-side invocation semantics can be expressed in terms of atomicity and execution order. The mutex invocation semantics guarantees the global order and the atomicity of all method calls. The sequential invocation semantics guarantees only the execution order of sequential methods. Concurrent invocation semantics guarantees neither the order nor the atomicity.

Figure 3 illustrates different method invocation semantics. Sequential invocation Seq1() is served immediately, running concurrently with Conc1(). Although the sequential invocation Seq2() arrives before the concurrent invocation Conc2(), it is delayed due to the current execution of Seq1() (no order between concurrent and sequential invocations). When the mutex invocation Mutex1() arrives, it has to wait for other running methods to finish. During this waiting, it also blocks other invocation requests arriving afterward (Conc3()) until the mutex invocation request completes its execution (atomicity and barrier).

## Parallel Object Allocation¶

The first step to allocate a new object is the selection of an adequate placeholder. The second step is the object creation itself. Similarly, when an object is no longer in use, it must be destroyed in order to release the resources it is occupying in its placeholder. The POP-C++ runtime system provides automatic placeholder selection, object allocation, and object destruction. Those automatic features result in a dynamic usage of computational resources and gives to the applications the ability to adapt to the changes in both the environment and the user behavior.

The creation of POP-Java parallel objects is driven by high-level requirements on the resources where the object should lie (see section Requirement-driven parallel objects). If the programmer specifies these requirements they are taken into consideration by the runtime system for the transparent object allocation. The allocation process consists of three phases: first, the system finds a suitable resource, where the object will lie; then the object code is transmitted and executed on that resource; and finally, the corresponding interface is created and connected to the object.

## Requirement-driven parallel objects¶

Parallel processing is increasingly being done using distributed systems, with a strong tendency towards web and global computing. Efficiently extracting high performance from highly heterogeneous and dynamic distributed environments is a challenge today. POP-C++ and POP-Java were conceived under the belief that for such environments, high performance can only be obtained if the two following conditions are satisfied:

• The application should be able to adapt to the environment;
• The programming environment should somehow enable objects to describe their resource requirements.

The application adaptation to the environment can be fulfilled by multilevel parallelism, dynamic utilization of resources or adaptive task size partitioning. One solution is to dynamically create parallel objects on demand.

Resource requirements can be expressed by the quality of service that objects require from the environment. Most of the systems offering quality of service focus on low-level aspects, such as network bandwidth reservation or real-time scheduling. Both POP-C++ and POP-Java integrate the programmer requirements into parallel objects in the form of high-level resource descriptions. Each parallel object is associated with an object description that depicts the characteristics of the resources needed to execute the object. The resource requirements in object descriptions are expressed in terms of:

• Resource (host) name (low level description, mainly used to develop system services).
• The maximum computing power that the object needs (expressed in MFlops).
• The maximum amount of memory that the parallel object consumes.
• The expected communication bandwidth and latency.
• The preferred communication protocol.
• The preferred encoding protocol.

An object description can contain several items. Each item corresponds to a type of characteristics of the desired resource. The item is classified into two types: strict item and non-strict item. A strict item means that the designated requirement must be fully satisfied. If no satisfying resource is available, the allocation of parallel object fails. Non-strict items, on the other hand, give the system more freedom in selecting a resource. Resource that partially match the requirements are acceptable although a full qualification resource is preferable. For example, a certain object has a preferred performance 150MFlops although 100MFlops is acceptable (non-strict item), but it needs memory storage of at least 128MB (strict item).

The construction of object descriptions occurs during the parallel object creation. The programmer can provide an object description to each object constructor. The object descriptions can be parametrized by the arguments of the constructor. Object descriptions are used by the runtime system to select an appropriate resource for the object. Some examples of the syntax of object descriptions can be found in the section Object description.

It can occur that, due to some changes on the object data or some increase of the computation demand, an object description needs to be re-adjusted during the life time of the parallel object. If the new requirement exceeds some threshold, the adjustment could cause the object migration. The current implementations of POP-C++ and POP-Java do not support object migration yet.