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From "Robin D. Wilson" <rwils...@gmail.com>
Subject Strange 'pause' activity during testing
Date Wed, 14 Dec 2011 21:35:54 GMT
I have a marginally complicated test case that performs a 'registration' on
my site. It gets the home page, POSTS a form, gets the response, POSTS
another form, gets that response, and then completes. The test runs fine
with 100 threads, and 30000 iterations - IF I don't "Retrieve All Embedded
Resources from HTML Files". In this mode, I am really testing the throughput
of my 'tomcat' application, not the other elements of my system. (I'm
assuming that the other elements are being retrieved from our Content Data
Network instead of our main system in this case.)

If I enable the "Retrieve All Embedded Resources from HTML Files" flag, and
tune the test down to 10 threads with 3000 total iterations, I notice a very
strange behavior. The test runs along at a pretty good clip for the first
~600 iterations (about 1 min, 25 seconds into the run), and then it just
stops making requests for about 35 seconds. Then, it picks back up again and
runs for another 1 m 25s, and then stops again for 35 seconds... (NOTE: with
the 10 threads, 3000 total iterations - but with "Retrieve All Embedded..."
disabled - I don't see the 'stop' behavior either - so it isn't caused by
tuning it down...)

I recently added the "Perfmon Metrics Collector" to the test script, so I
could see if one of the servers was maxed out - but it looks like all the
servers are idle during the 'stop' period. Likewise, I added the Perfmon for
the "localhost" (running the JMeter test) to see if it was swamped - but it
too is idle during the 'stop'. I swapped out our network switch (the test
environment is on an isolated network switch) with a _much_ higher capacity
switch - in case there was a network issue, still no change.

I'm running out of ideas for things to check - so I thought I'd ask you guys
if you have any suggestions of things I should look at.

My system consists of:

	WinXP - running JMeter 2.4.1 - driving the test script in GUI mode
	Server 1 - running Red Hat Linux, with "Apache (2.2.21)" as the web
server - using AJP Proxy to Server 2
	Server 2 - running Red Hat Linux, with Tomcat 7.0.21 as the App
Server - connecting through Hibernate to Server 3
	Server 3 - running Red Hat Linux with MySQL 5.x as the DB Server

All 4 machines are running on a private switched network (32Gbs backplane).

The requests are downloading about 3MB total (per thread per iteration) over
4 main URL requests, and 30+ 'Retrieve All Embedded' requests.

At first I thought it was the network - but the new switch seemed to deny
that thought (the old switch had a much slower backplane). Also, I'm having
no trouble collecting the PerfMon data during the 'stop' period - so the
network is still functioning just fine...
Then I thought it might be garbage collection on the tomcat - but I watched
the gc.log - and it doesn't do any GCs during the 'stop' period.
Then I thought it might be the garbage collection on the JMeter side, so I
started the JMeter.bat from a 'cmd' prompt with gc logging enabled - it
doesn't do any GCs during the 'stop' period either.

The apache, tomcat, and DB are all 'idle' (no CPU to speak of, no network
I/O, no disk I/O, etc.) during the 'stop' period.

The JMeter box (WinXP) is doing very little during that time too (I
attribute the little bit of activity to displaying the PerfMon graphs, and
Remote Desktop display to my desktop computer)...

--
Robin D. Wilson
Sr. Director of Web Development
KingsIsle Entertainment, Inc.
VOICE: 512-777-1861
www.KingsIsle.com




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