Why servlet halt a moment in concurrent request

I use Axis for webservice service.

when more than 8 concurrent , there are some request halt randomly for about 30 seconds.

I debug by log in every line and found from my code:

public class foo{

    void bar(){
             a();
             log.debug('exit from a');
    }

    void a开发者_JS百科(){

             log.debug('exit a');
    }

the time between "exit a" and "exit from a" are delay about 30 second. This is no reason.

I try to increase web container thread pool , heap but didn't help

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Enviroment websphere 6.1

Spring 2.5 , Hibernate 3 ,Axis 1.6.5

Min heap to 768

Max heap to 1024

Thread pool defult max to 300

Thread pool web container max to 500

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A shot in the dark, but it happened to me once: Is there a chance that you perform massive logging, so the delays derive from the logger having to rotate or flush massive files?