Henri Gomez wrote:
The TomcatoMips indicator was just something to tell that it's not the
raw CPU power which is important, but the estimated LOAD capacity of
an instance.
But its still apache working out TomcatoMips. I think that approach is
still flawed.
I'm saying only the server end of the AJP knows the true situation. The
current setup presumes that the running apache instance has all the
facts necessary to determine balancing. When all it knows about is the
work it has given the backend and the work rate its betting it back.
I'm thinking both ends apache and tomcat should make load calculations
based on that they know at hand. As far as I know there is no provision
in the AJP to announce "Willingness to serve". Both ends should feed
their available information and configuration biases it into their
respective algorithm and come out with a result that can be compared
against each other. The worker would then announce as necessary (there
maybe a minimum % change to damper information flap) down the connector
the info to apache. There probably need to be a random magic number and
wrapping sequence number in the packet to help the apache end spot
obvious problems.
This would allow kernel load avg/io load (and anything else) to be
periodically taken into account at the tomcat end. It would be expected
that each member of the backend tomcat cluster is using the same
algorithm to announce willingness. Otherwise you get disparity when
apache comes to make a decision.
So I suppose its just the framework to allow an LB worker to announce
its willingness to serve I am calling for here. Not any specific
algorithm, that issue can be toyed with until the end of time.
An initial implementation would need to experiment and work out:
* How that willingess value impacts/biases the existing apache LB
calculations.
* Guidelines on how to configure algorithm at each end up based on
known factors (like CPUs, average background workload, relative IO
performance).
I'm thinking with that you can hit the widest audience to make a usable
default without giving much thought to configuration. The type of
approach kernels make these days, you only have to tweak and think about
configuration in extreme scenarios but for the most it works well out of
the box.
Darryl
