Using Chef's Cookbook recipes to dynamically override an attribute - ruby

Question

I'm trying to setup a new cookbook for Cassandra, and on cassandra.yaml file which has the follow comments about optimal settings:
# For workloads with more data than can fit in memory, Cassandra's
# bottleneck will be reads that need to fetch data from
# disk. "concurrent_reads" should be set to (16 * number_of_drives) in
# order to allow the operations to enqueue low enough in the stack
# that the OS and drives can reorder them.
#
# On the other hand, since writes are almost never IO bound, the ideal
# number of "concurrent_writes" is dependent on the number of cores in
# your system; (8 * number_of_cores) is a good rule of thumb.
However, there's no way to determine the numbers of cores or numbers of disk drives predefined in the attributes because the deployed servers could have different hardware settings.
Is it possible to dynamically override the attributes with the deployed hardware settings? I read the Opscode doc and I don't think it has a way to capture the output from
cat /proc/cpuinfo | grep processor | wc -l
I was thinking about something like this:
cookbook-cassandra/recipes/default.rb
cores = command "cat /proc/cpuinfo | grep processor | wc -l"
node.default["cassandra"]["concurrent_reads"] = cores*8
node.default["cassandra"]["concurrent_writes"] = cores*8
cookbook-cassandra/attributes/default.rb
default[:cassandra] = {
...
# determined by 8 * number of cores
:concurrent_reads => 16,
:concurrent_writes => 16,
..
}

Answer 1

You can capture stdout in Chef with mixlib-shellout (documentation here: https://github.com/opscode/mixlib-shellout).
In your example, you could do something like:
cc = Mixlib::ShellOut.new("cat /proc/cpuinfo | grep processor | wc -l")
cores = cc.run_command.stdout.to_i # runs it, gets stdout, converts to integer

Answer 2

I have found a way to do this in recipes, but I haven't deployed it yet to any box to verify it yet.
num_cores = Integer(`cat /proc/cpuinfo | grep processor | wc -l`)
if ( num_cores > 8 && num_cores != 0 ) # sanity check
node.default["cassandra"]["concurrent_reads"] = (8 * num_cores)
node.default["cassandra"]["concurrent_writes"] = (8 * num_cores)
end

Answer 3

I am using chef 11 so this may not be available on previous versions, but there's a node['cpu'] attribute with info about the cpus, cores and etc.
chef > x = nodes.show 'nodename.domain'; true
=> true
chef > x['cpu']['total']
=> 16
And you can use it on your recipes. That's how the Nginx cookbook does it.