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[Bug 9823] New: Add "maxExecutionContexts" property with number of hardware execution contexts

From: <bugzilla@jessica.w3.org>
Date: Fri, 28 May 2010 23:09:03 +0000
To: public-webapps@w3.org
Message-ID: <bug-9823-2927@http.www.w3.org/Bugs/Public/>
http://www.w3.org/Bugs/Public/show_bug.cgi?id=9823

           Summary: Add "maxExecutionContexts" property with number of
                    hardware execution contexts
           Product: WebAppsWG
           Version: unspecified
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Web Workers (editor: Ian Hickson)
        AssignedTo: ian@hixie.ch
        ReportedBy: robert.ennals@intel.com
         QAContact: member-webapi-cvs@w3.org
                CC: mike@w3.org, public-webapps@w3.org


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

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Received on Friday, 28 May 2010 23:09:05 GMT

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