GD Threshold Issue

Question

We are getting this message:
<Sep 23, 2003 10:21:05 AM PDT> <Warning> <WebLogicServer> <000335> <Percent
of s
tarting server free memory is now at "4%". This is less than the configured
thre
shold of "5%". We will try to garbage collect, but you may consider
rebooting.>
WLS 7.X has a GC threshold page. The benefit of a forced GC is that you are
distributing the time required for a single GC across several pre-emptive
GC calls. I have seen no indication or information that would indicate that
threshold drive GCs are anything but a bad idea.
Consider what is being done here. When there is memory available there is no
need to run the GC. When there is memory available your request for memory
will be fulfilled. When a request cannot be fulfilled the GC will run and
release memory. If and only if all memory is held by non-collectible objects
will you run out of memory.
Setting a threshold means that you are lowering the trigger point so the GC
runs when there is no point in doing so. Consider what happens when you get
to the threshold of, say, 10% and there legitimately there is only 10% free
memory: everything else is reachable and by definition it cannot be
collected. The GC will start to run continuously! You have effectively taken
10% of your memory out of service. It seems as though setting artificial
memory thresholds is counterproductive.
I added a memory watchdog for diagnostic purposes a year ago. We determined
that we there was no legitimate out of memory error as the GC would always
return to about the same baseline. Running GC at regular intervals fixed did
not solve the crashing problem and out of memory problem we were trying to
solve. Apparently there is a known problem with WLS and hotspot but WLS
support has precisely zero information on that a year ago.
Question: what is the purpose of the GC threshold and can anybody point me
to a credible information source that actually recommends this. This is not
the same as running the GC at regular intervals which has the benefit of
distributing the effort over time. Can anybody explain what the threshold is
all about and whether I am missing some important information that has led
me to the wrong conclusion about this?
TIA

Answer 1

I've had the same question on my mind....
"dealmein" <[email protected]> wrote in message
news:[email protected]...
We are getting this message:
<Sep 23, 2003 10:21:05 AM PDT> <Warning> <WebLogicServer> <000335><Percent
of s
tarting server free memory is now at "4%". This is less than theconfigured
thre
shold of "5%". We will try to garbage collect, but you may consider
rebooting.>
WLS 7.X has a GC threshold page. The benefit of a forced GC is that youare
distributing the time required for a single GC across several pre-emptive
GC calls. I have seen no indication or information that would indicatethat
threshold drive GCs are anything but a bad idea.
Consider what is being done here. When there is memory available there isno
need to run the GC. When there is memory available your request for memory
will be fulfilled. When a request cannot be fulfilled the GC will run and
release memory. If and only if all memory is held by non-collectibleobjects
will you run out of memory.
Setting a threshold means that you are lowering the trigger point so theGC
runs when there is no point in doing so. Consider what happens when youget
to the threshold of, say, 10% and there legitimately there is only 10%free
memory: everything else is reachable and by definition it cannot be
collected. The GC will start to run continuously! You have effectivelytaken
10% of your memory out of service. It seems as though setting artificial
memory thresholds is counterproductive.
I added a memory watchdog for diagnostic purposes a year ago. Wedetermined
that we there was no legitimate out of memory error as the GC would always
return to about the same baseline. Running GC at regular intervals fixeddid
not solve the crashing problem and out of memory problem we were trying to
solve. Apparently there is a known problem with WLS and hotspot but WLS
support has precisely zero information on that a year ago.
Question: what is the purpose of the GC threshold and can anybody point me
to a credible information source that actually recommends this. This isnot
the same as running the GC at regular intervals which has the benefit of
distributing the effort over time. Can anybody explain what the thresholdis
all about and whether I am missing some important information that has led
me to the wrong conclusion about this?
TIA