Bug 339 fixed some thread-safety issues but left some potential performance
bottlenecks. This is enhancement bug is to discuss and address them. Here are
the relevant comments from bug 339:
------- Additional Comment -- 10 From Sean Gillies 2004-10-10 11:04 -------
Recent progress on thread safety:
I've fixed trouble at three spots
- parsing expression strings outside of msLoadMap
- evaluating mapserver logical expressions
- loading symbol set files outside of msLoadMap
while leaving Frank's mapthread code unchanged. It's simple and works, no
need to change it.
Issues of thread safety can now be divided into two separate application
phases: a "loading" phase and an "execution" phase, which is everything else.
During the loading phase, we acquire a parser lock immediately and release
it only after the entire mapfile has been loaded. Within this phase we
will use parsing functions with no locking of their own, particularly
loadExpressionString, and loadSymbolSet.
Outside of the loading phase, we must use locking versions of these functions.
msLoadExpressionString and msLoadSymbolSet are wrappers for the previous
functions. I've also added parser locking to msEvalExpression and msEvalContext,
and have changed all execution-phase code to use these parser locking
functions.
My tests pass, the code is working fine in my multithreaded Zope app, so
I'm going to call it "FIXED".
We are now safer, but still have a performance bottleneck. The one
parser is shared between threads in mapfile loading and map rendering (or
"execution") phases. In the first case a lock is held relatively long,
while in the second case it's acquired and released (inside msEvalExpression)
quickly with every call to msLayerNextShape. I believe the tendency would be
for map rendering to be upstaged by mapfile loading when activity is high.
I see at least three solutions that we may want to consider as a new
tracker issue for 4.4:
1) implement a second global parser for use in msEvalExpression
and/or
2) pull the parser locking out of msEvalExpression, and instead lock around
the loops which call msLayerNextShape.
or
3) rewrite msEvalExpression so that it doesn't need a parser for logical
expressions.
------- Additional Comment -- 11 From Frank Warmerdam 2004-10-12 08:47 -------
Sean Gillies wrote:
> 1) implement a second global parser for use in msEvalExpression
Sean,
My understanding (and Steve obviously knows more about this) is that the
mapfile parsing uses only the lexer while msEvalExpression() uses the
yacc/bison parser which in turn uses the same low level lexer as the
mapfile code uses. It might be possible to build a seperate lexer instance
for the expressions.
> and/or
>
> 2) pull the parser locking out of msEvalExpression, and instead lock around
> the loops which call msLayerNextShape.
I don't think this would be all that wise. My take would be that
mapfile parsing is pretty quick while in some cases processing shapes
can take a long time, much of which is not spent in the parser at all.
> or
>
> 3) rewrite msEvalExpression so that it doesn't need a parser for logical
> expressions.
>
> I'd love to get some feedback from folks who are interested in using
> mapserver in a multithreaded environment.
Well, my longer term suggestion has always been to look into whether we
can get a thread safe lexer in place, either by researching flex options
in greater depth or if necessary by rewriting the lexer by hand. Lexers
aren't really that hard.
