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
