Hello Andrew, reading your email, Alan's email and the Mladen email piqued
my interest because I am currently working on a gig to improve the
performance and monitoring of two Tomcat instances supporting 3 web
applications and one web service. I am inclined to agree with Alan. And, did
you read the ML replies to the Xmx and Xms Subject line emails? I must agree
with the ML contributors that answered the email named in the previous
sentence: baseline test, apply Eden parameters to the JVM then monitor the
results of the load testing. Once you have all your monitoring results
including logs the next step is to create a new metric by comparing the
newly acquired data to your initial baseline test. There are very good
points made on both sides but I have to believe that Tomcat tuning a priori
is like trying to predict the weather. In my gig there are too many unknowns
to resolve:
(1) this is a legacy system which means differing verions of JDKs, Tomcat
instances, web apps or web services built with framework versions no longer
supported e.g. AXIS 1.3.
(2) Commercial vendors that have taken FOSS and re-packaged it as
proprietary software and as a result there is no direct support from the
vendors for: SLA source code or updated binaries that were written in this
century.
(3) I know my client wants everything upgraded and migrated if possible
when in reality I will have to improve the monitoring and performance issues
with the current servlet containers, web services and network topology as it
stands now.
I know the rest of the world is moving away from clustering (horizontal
scaling) and more toward virtualization (vertical scaling). In my case I
will have to settle for horizontal scaling and the Tomcat software load
balancing. I welcome anyone wanting to expound on Tomcat load balancing:
say, a comparison between Tomcat JK connector load balancing and using an
appliance like Big IP.
Like you Andrew I would cheer a <calculated> solution if it existed: just
dump in the number of nodes, instances, network(s), applications, web
services, bandwidth and client users and viola! out comes the network
diagram with annotations. Discussion, suggestions, advice, solutions, rants
and raves welcomed.
Andrew Hole wrote ..
Hello
I read an interesting document from Mladen Turk (with whom I want to
speak
directly, but I don't know direct contact) that there is a formula to
calculate the number of concurrent request:
http://people.apache.org/~mturk/docs/article/ftwai.html
Calculating Load
When determining the number of Tomcat servers that you will need to
satisfy
the client load, the first and major task is determining the Average
Application Response Time (hereafter AART). As said before, to satisfy
the
user experience the application has to respond within half of second.
The
content received by the client browser usually triggers couple of
physical
requests to the Web server (e.g. images). The web page usually consists
of
html and image data, so client issues a series of requests, and the time
that all this gets processed and delivered is called AART. To get most
out
of Tomcat you should limit the number of concurrent requests to 200 per
CPU.
So we can come with the simple formula to calculate the maximum number
of
concurrent connections a physical box can handle:
500
Concurrent requests = ( ---------- max 200 ) * Number of CPU's
AART (ms)
The other thing that you must care is the Network throughput between the
Web
server and Tomcat instances. This introduces a new variable called
Average
Application Response Size (hereafter AARS), that is the number of bytes
of
all context on a web page presented to the user. On a standard 100Mbps
network card with 8 Bits per Byte, the maximum theoretical throughput is
12.5 MBytes.
12500
Concurrent requests = ---------------
AARS (KBytes)
For a 20KB AARS this will give a theoretical maximum of 625 concurrent
requests. You can add more cards or use faster 1Gbps hardware if need to
handle more load.
The formulas above will give you rudimentary estimation of the number of
Tomcat boxes and CPU's that you will need to handle the desired number
of
concurrent client requests. If you have to deploy the configuration
without
having actual hardware, the closest you can get is to measure the AART
on a
test platform and then compare the hardware vendor Specmarks.
I would like to launch a discussion on the validity of this formula and,
in
case of inappropriate, to try to get a more accurate formula.
Thanks a lot
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