[MAPSERVER-USERS] missing lock in mapfile.c:msFreeLabelCacheSlot() ?

[rich.fromm]

Tamas Szekeres wrote:
> 
> You can probably control the caching effect by setting LABELCACHE OFF for
> all of layers. However I'm a bit hesitant to think that this is due to a
> missing lock around the labelcache itself since it doesn't contain static
> variables.
> 

I think you might be right, and that this crash might be another red
herring.

What I'm doing here is stressing the limits of the system to see what it can
handle.  There's various interactions between mapserver, the JVM, tomcat,
and
postgres, and when things go badly, it's not entirely clear what's the cause
and what's the effect.

I tried a few related tests:

1) I commented out all LAYER's that had any LABEL declarations.  In this
   instance, tomcat indeed did NOT crash.  However, under stress there were
   problems reported by tomcat and by postgres, but that's (sort of) okay.

2) I kept all of the LAYER's, but for those with LABEL declarations I set
   LABELCACHE OFF.  The previous crash at mapfile.c:msFreeLabelCacheSlot()
did
   NOT occur.  However, intead, there was a new crash:

(msClipPolygonRect) mapprimitive.c:605
(msDrawShape) mapdraw.c:1609
(msDrawVectorLayer) mapdraw.c:887
(msDrawLayer) mapdraw.c:718
(msDrawMap) mapdraw.c:432
(mapObj_draw) mapscript/java/mapscript_wrap.c:1714
(Java_edu_umn_gis_mapscript_mapscriptJNI_mapObj_1draw)
mapscript/java/mapscript_wrap.c:20612

   But again this coincided with other things going badly under stress, so
I'm
   not really sure if mapserver is to blame.  Ideally, tomcat and the JVM
   should never crash.  If things go badly, the system should become less
   responsive, and possibly refuse additional requests, or cancel existing
   ones.  Crashing is clearly a wrong thing to do with respect to
correctness.
   But given the nature of what I'm seeing, it's not clear to me that
   mapserver is the piece at fault.

I've spent some time trying to tune tomcat and postgres and some OS
parameters
a bit.  While I'm still having a few issues, some of which might (or might
not) be genuine mapserver issues, now that the system is tuned better, I'm
not
seeing any more JVM crashes, even under heavy load.  (Although it might just
be that the definition of "heavy" has shifted higher, and I'm no longer
stressing things that much relative to that.)

Anyway, I will deal with the issues that I am having in separate mailing
list
threads.  But for the time being I think that maybe my original concern here
is a non-issue.  I'll reply back with more details if I see any more
observations that cause me to think differently.

- Rich

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