Modes of Operation

Calling Semantics

Asynchronous (Polled)

Hardware “request/response” interface is inherently asynchronous (non-blocking).

  • Calling function returns once request submitted.

  • Callback invoked when response available (polled).

../_images/polled_async.png

Asynchronous (Interrupts)

Hardware “request/response” interface is inherently asynchronous (non-blocking).

  • Calling function returns once request submitted.

  • Callback invoked when response available (interrupt-driven).

../_images/async_interrupts.png

Synchronous

Software interface is traditionally synchronous (blocking).

  • Calling function blocks until response available.

  • Can be implemented “on top of” asynchronous hardware semantics.

../_images/synch.png

Pros And Cons

Pros and cons of modes of operation

Asynchronous

Synchronous
CPU Utilization
Software thread can do other things while hardware processes job, without need for expensive context switch.
Software thread blocked or idle awaiting response. Can use multi-threading, but context switching can be expensive.
Acceleration Utilization
A single software thread can have multiple requests outstanding, keeping multiple accelerator engines.
Hardware has at most one request outstanding per CPU/software thread, remaining threads are idle.
Ease of Use
Can be difficult if application is designed to use synchronous APIs.
Easier to integrate if application is designed to use synchronous APIs.

Note

Asynchronous API tends to be optimal for performance, but harder to integrate.