On 3/4/2012 8:12 PM, Brian Troutwine wrote:
(First of all, sorry for the jumbled first paragraph. I was
trying to do two things at once).
An interesting thought. If you enable graph output in your puppet.conf
http://docs.puppetlabs.com/references/stable/configuration.html#graph
you can identify those sub-graphs that could run in parallel pretty
easily, just by eyeballing it. Parallel execution is wonderful for an
optimizing compiler because you're strongly CPU limited. I'd be
willing to bet that a parallel puppet agent would be of less use: once
the catalog is compiled on the master, puppet's execution time is
going to be limited by network and disk IO. Probably. Running puppet
agent through DTrace/SystemTap would be instructive.
My intuition, albeit as a novice, makes me question this.
Just look at how well parallel make improves things.
There would probably be a point of diminishing returns,
or even negative returns, but that's true for "make -j" also.
Just "puppet -j2" could result in enough of a benefit to
make this approach worthwhile.
Also, some resources are implicitly exclusive: two Packages with 'apt'
providers cannot be run in parallel because Debian/Ubuntu keeps a
global lock on... hmm, I've forgotten what, exactly. It's there,
though. Ensuring that implicitly parallel resources block one another
would increase the implementation complexity of Puppet, of end-user
modules or, as is likely, both.
I see what you mean. The same is true for yum, which is what
I'm most familiar with. Maybe puppet could do something so
that these kind of inherent sequential processes with yum/apt (and
anything else) could be reflected in the graph.
Aside from all other considerations, multi-core parallelism in MRI is
not so great. JRuby's better, being hosted on the JVM, but, well,
MRI's RAM consumption is already bad enough.
I'm talking about process-level parallelism so I don't think
the virtual machine implementation matters here.
Jon
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