Execution Stream (ES)

Introduction

Execution Stream (ES) is the execution of a sequential instruction stream. Each instruction does not necessarily need to be SISD, but can also be vector or SIMD instructions. When bound to a hardware processing element (PE), ES can be thought as its software equivalent. Argobots considers one ES per PE. Oversubscription of ESs is possible but is not recommended.

ES is conceptually a container of work units, which are User-level Threads (ULTs) or tasklets. Work units execute to completion, but their execution may be interleaved inside an ES. Since ES is a sequential instruction stream, there is no parallel execution of work units in a single ES. Only one work unit runs in an ES at a certain point. However, work units in different ESs can be executed in parallel. ES has implicitly-managed progress semantics handled by kernel or hardware, and thus one blocked ES cannot block other ESs.

Each ES executes all associated work units according to the scheduling policy. The scheduler associated with the ES is in charge of scheduling. ES has its own scheduler and different ESs can adopt different schedulers. The Argobots runtime provides some predefined schedulers, but users can also create their own schedulers.

ES Types

Argobots categorizes ESs into two types: primary and secondary. A major difference between them is that users can only create secondary ESs but not the primary ES.

• Primary ES (initial ES). This is the ES that automatically starts when the Argobots library is initialized. It cannot be created explicitly, and an application has only one primary ES. When the primary ES starts, it implicitly contains one ULT, which has started the main function. In other words, when the Argobots is initialized, there exists one primary ES containing one ULT. Primary ES establishes parent/child relationship with secondary ESs. When it terminates, all secondary ESs are terminated.
• Secondary ES. Secondary ESs are ESs other than the primary ES. They are explicitly created at run time, and any work unit can create them. There is no parent/child relationship between secondary ESs. Termination of a secondary ES does not affect execution of other ESs including both primary ES and secondary ESs.