doRedis on ec2:no new jobs being assigned to workers after first job completes

I have been trying to use the R/Quantstrat applyParameters() function to
analyze the performance for 1200 different Moving Average parameter
combinations in the "Luxor" trading strategy. As I want to test with 6
years of GBP/USD pricing info at a 30 minute interval, each individual
tests requires 15 minutes or more, so this is where doRedis comes into
play.

The platform that I am using is Amazon EC2. I configure one Redis server
as a master, and from this master I also run the R program. Then I start
up a bunch of slaves, containing 8 CPUs and 7GB of memory each
(c1.xlarge in Amazon EC2 parlor), which automatically connect to the
redis server, so I end up with a range of workers, which is a multiple of 8.

Now if I test with only 10 days of data (instead of 6 years), or even 3
months, everything is great, whether I use 1 slave or 40 slaves (= 320
workers). But whenever I run with more than 1 year of data, I start
getting problems, where workers will no longer be assigned a new job
after the first one completes, and the worker count decreases in the
redis-server window. Whenever this occurs, then if I restart the workers
manually, they will log on to the redis server again and get assigned a
new job; however once again no new job will be assigned to them after
completion.

It seems that this has something to do with the amount of time that it
takes to complete the task. When renting multiple EC2 machines of the
same instance type, they often appear to be slightly different
nevertheless. And I found that, when running with 15 months of price
data, one particular type (CPU freq = 2.13GHz / cache size=4096MB) gets
assigned one job for each of its 8 workers, but then none to any worker
anymore, where another much faster type (CPU freq=2.33GHz / cache
size=6144MB) runs continuously without a flaw. FYI: all slaves share the
same identical disk image.

Anyone any ideas how to proceed and fix this? Or test I can do to find out
what goes wrong?

--
Jan Humme - OpenTrades

WWW:     http://www.opentrades.nl
Email:   jan at opentrades.nl
Twitter: @opentrades