It is likely that people will want to use the Web Workers API for creating multiple threads to perform some kind of CPU-bound computation more efficiently than they could with a single thread. In particular, Section 1.2.6 (Delegation) talks about splitting a task across multiple workers in order to gain performance.  In this particular example, the number of workers is fixed at 10, but this is likely to be the wrong number in most cases. 

Right now, the spec gives no guidance to developers about how many workers they should use for compute-bound jobs. In the absence of such information, it seems likely that developers will do something ugly like choose a fixed number that seemed to work well on the device they tested on, attempt to identify which of a finite number of known devices the app is running on using user-agent sniffing, or just create far more workers than needed in the hope that the user agent will deal with the problem. 

I suggest we just add a simple maxExecutionContexts property with descriptive text like:
 This value is the maximum number of hardware execution contexts that may be available to applications running in the User Agent.  Other activity in the User Agent or on the system may be using these resources at any time (including during or after the request for information is made).  It is not the number of free, unused resources. User Agents may exclude dedicated processors that they know are not available for applications or may choose to set thread priorities low for applications that overuse system resources by starting too many WebWorkers on a busy system.

maxExecutionContexts is not an optimal or recommended number of workers to create. If another app is using some of the cores, then the optimal number of cores may be lower. If your workers are often IO bound, then the optimal number of cores may be higher. Similarly if worker-communication costs are significant, it may not be useful to use all available cores.

maxExecutionContexts is however a number that can be useful for an app that wants to choose an appropriate number of workers to create. At the simplest level, the fact that maxExecutionContexts is greater than 1 tells an app that it may be able to gain some performance from some level of parallelism, and the fact that maxExecutionContexts is a large number may indicate that it is wise for the app to split its work into finer-grain chunks than if it was smaller.

It is up to an individual developer to determine how the number of workers they create corresponds to "maxExecutionContexts"; however it is likely that the availability of this number will help them make better decisions than they would  if this information was not available.


-Rob