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High response, low processor usage

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Hi All,
We have a single servlet that we are running through Weblogic 5.1 (for various reasons). The servlet has been running fine in production for 5-6 months now. Recently the usage of the system has gone up by 40-50%. Suddenly the response time for some of the requests has gone up from 10-20 milliseconds to 20000 milliseconds. Most of the requests return quickly enough. The processor usage on the server is pretty low (7-8%).
On restarting the application, the response times fell back to being reasonable. But we are noticing them rising again and they are reaching 2000 milliseconds (also the load right now is much lower than the peak load can get).
We would have assumed that the servlet does not scale if:
a) All requests are slowed down (currently every 30th or so request is slowed down)
b) The CPU usage is peaking (it is only 7-8 %)
c) Restarting does not help (restarting reduced the times from 20000 ms to 100 ms)
Any and all ideas on what could cause this kind of behavior or hints on troubleshooting are welcome. Pointers to documentation that might help are highly appreciated.
Thanks in advance,
Sharad Agarwal
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I meant the subject to be: "Bad Response Time, Low Processor/Memory Usage".
To add more information, the Java memory parameters are set as follows: -Xms64m -Xmx256m In the weblogic console, the Maximum Java Heap is reported to be 64 MB (65280K). Also GC kicks off when memory reaches 64MB. Why would that be?
These machines have 4GB RAM. We are deliberating to move the memory parameter to -Xmx1024m We are using jdk1.3.1 Does Garbage Collection thread cause all other threads to go on hold? We don't want to move the memory too high as the GC thread may cause a high perceived slow down every time it kicks off.
Thanks in advance for all ideas,
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