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From "anorakgirl" <tom...@anorakgirl.co.uk>
Subject concurrent user threading problem
Date Thu, 13 Nov 2003 09:59:23 GMT
hi all,
orry for the long mail; i'm at my witts end here, and just wanted to check
that i'm not losing my marbles!

we have been running a commercial app on tomcat for the last two years.
we've had intermittent out of memory problems processing large pages
meaning we've had to reboot occasionally (the app produces xml, transforms
it using xslt to output html), but in general the performance has been
good and the app has been stable. as the business has been growing, we
recently upgraded to a new funky dell server, multiprocessor with
hyperthreading, but this seems to have exacerbated the problem
dramatically:
we now have a situation where if a single user requests a page, it is
lightening fast, but once there are two requests at the same time, both
are about 100 times slower; i would expect twice slower at most. once
returning to only one request at a time, the speed recovers immediately.
in top you can see that java has 100% of the CPU time, and one of the 4
effective CPU's is maxed out.

so i'm desperately trying to track down the cause of this problem; i'm
guessing it must be some kind of race condition. we have reviewed all our
code, ensured that methods accessing shared objects are synchronized, been
through all the config, removed most of the logging etc etc, and upgraded
to tomcat 4.1.29 and all to no avail; we don't seem to be able to narrow
down where the problem could be.

what i wanted to check was: the app has one servlet only, through which
all requests are processed. all requests go through the doGet method. to
try and check if our code was to blame, we synchronised this method. my
understanding is that this would mean if two requests were received at
approximately the same time, one would have to wait until after the first
had been processed, meaning the two requests could not access any of our
code at the same time, and therefore the second would be say twice as slow
as the first. however, this is not the case, still they are both about 100
times slower. this makes me think that it cannot be an error in our code?
but then tomcat seems to serve concurrent users on other people's apps
with no problem, so i can't believe tomcat is to blame.

is my assumption about synchronizing the doGet method correct?  is there
anything special i should know about using tomcat on a multiprocessor
server? any suggestions at all greatfully received!

thanks,
tamsin

ps:
tomcat: 4.1.29
java: 1.4.2
postgresql: 7.3.4
saxon
xerces




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