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From Martin Schulz <sch...@videotron.ca>
Subject Re: Any synchronization issues with SMP?
Date Sat, 26 Jun 2004 01:24:08 GMT
it appears that the JVM slows everything down to a crawl,
including the code path which should lead to another accept being
called., for up to 8 minutes!!!

Furthermore, the mpstat has the nice porperty that CPU usage adds
up to exactly 100%, i.e. a single CPU is used... no more, no less.
This corresponds to 12% or 13% CPU utilization shown in prstat
based on 8 CPUs.  My interpretation is that the JVM is effectively
preventing parallel execution (which otherwise appears to work fine).

Nearly all threads either wait, read from a Socket, or zip/unzip data.

I'm not sure what all that means, but Tomcat appears to be a victim
of it.  I'll experiment some more.  Main difference with the systems
Rainer mentioned is the JVM (1.4.2_04) and the CPU (Sparc III 1.2GHz).

If any of this rings a bell, drop me a note.  I'll be happy to share data
as appropriate.

I'll repost to the list only if I learn anything which impacts Tomcat 
directly
(other than that the code path to hand of the socket accept responsibility
is not suitable for _very_ high hit rates, which does not worry me too
much at this point).

Cheers!
    Martin

Martin Schulz wrote:

> Rainer,
>
> Thanks for the tips.  I am about to take timing stats
> internally in the ThreadPool and the Tcp workers.
> Also, the described symptoms do not disappear, but seem to be of much
> shorter duration when only 4 CPUs are used for the application.
> I'll summarize what I find.
>
> Martin
>
> Rainer Jung wrote:
>
>> Hi,
>>
>> we know one application running on 9 systems with 4 US II CPUs each
>> under Solaris 9. Peak request rates at 20 requests/second per system.
>> Tomcat is 4.1.29, Java is 1.3.1_09. No symptoms like yours!
>>
>> You should send a signal "QUIT" to the jvm process during the
>> unresponsiveness time. This is a general JVM mechanism (at least for sun
>> JVMs). The signal writes a stack trace for each thread on STDOUT (so you
>> should also start tomcat with redirection of STDOUT the output to some
>> file). Beware: older JVM in rare cases stopped working after getting
>> this signal (not expected with 1.3.1_09).
>>
>> In this stack dump you should be able to figure out, in which methods
>> most of your threads stay and what the status is.
>>
>> Is there native code included (via JNI)? Any synchronization done in the
>> application itself? Are you using Tomcat clustering? Which JVM?
>>
>> Sincerely
>>
>> Rainer Jung
>
>


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