Subj : Re: Performance Advice on Maintaining Many Contexts
To   : netscape.public.mozilla.jseng
From : peter.kyme
Date : Thu Mar 03 2005 03:52 am

Brendan Eich <brendan@meer.net> wrote in message news:<422666B8.9090900@meer.net>...
> Peter Kyme wrote:
> > My problem involves a system containing a large number (> 100) of
> > objects, each of which has an associated Spidermonkey context.
> > 
> > The objects execute a small javascript repeatedly, iterating until
> > termination. This javascript is user defined, and so may contain
> > arbitrary local variables,
> 
> 
> Do you really mean local as in function-local, or just top-level vars 
> private to the script?  If the latter, you need a global object per 
> script to hold those global vars, but not a context per script unless 
> you are executing scripts concurrently.
>

I just meant top-level vars private to the script. I now see that I
only need a global object per script, not a seperate context as well.

My design involves having a static class that contains C++ functions
that are callable from javascript. I was using an STL map to associate
JS contexts with the target C++ object. From what I now understand I
should use JS_GetGlobalObject to get the current global object of the
context, and use that as the key for my map.

However, I also have the situation where one of the C++ callback
functions can create a new C++ object and associated JS global object,
which is then executed once before the callback returns control to the
original JS code. eg:

JS_SetGlobalObject(cx, gobj1)
globalObject1 -> running JS code, which calls 'fork' C++ function
globalObject1 -> C++ 'fork' function, creates new globalObject2
JS_SetGlobalObject(cx, gobj2)
globalObject2 -> runs JS code, completes
JS_SetGlobalObject(cx, gobj1)
globalObject1 -> returns control to original JS code

Is this sort of behaviour acceptable? Or would I need seperate
contexts, even though there aren't strictly seperate execution
threads.

> 
> > which was my motivation for creating a
> > context per object (no multithreading is involved).
> 
> 
> A JSContext is the state you need to execute concurrently, so if you 
> have only one thread, you need no more than one JSContext.
> 
> Why did you make a context per object?
> 
> 
> > This appears to work well, but I've encountered a nasty performance
> > issue at cleanup time. Running the program through once, without doing
> > any memory cleanup takes ~1.2s. If I then add code to destroy the JS
> > contexts before exits, the program take ~17s to execute.
> 
> 
> What version of the engine are you using?
> 
> /be

I'm running 1.5-rc6a, compiled on Linux. The above numbers were for
the debug version of libjs, the optimised version comes down to 9s.

Thanks for your help.

.