task_proxy: cache prereq status for speed

[benfitzpatrick]

This adds some caching to speed up task proxy ready_to_run.

(By the way, all these speed up PRs are coming from a single branch I've been working on for a few weeks. I've decided to split it up into related pieces.)

If a task doesn't have conditional prerequisites, then we can simply cache the 'am I satisfied' result instead of rechecking it every loop. We only normally need to recheck it once one or more relevant outputs have been found in satisfy_me.

I've moved the conditional expression checking into its own method for neatness and ease of future profiling - I wonder how much it costs if we have lots of 'OR' dependent tasks!

This reduces the cost of calling ready_to_run (another dominant cost in a modified busy suite with lots of waiting tasks) by around two thirds.

@hjoliver, @kaday please review.

[hjoliver]

@benfitzpatrick - why can't we cache conditional results too? If a conditional prerequisite is unsatisfied, it only needs checking if any of the involved outputs get completed, and once satisfied it doesn't need rechecking short of a manual state reset. But maybe I'm missing something? The test battery passes OK after a quick stab at implementing this on top of your branch: hjoliver@bc815b8 (in a simple test suite this halved the number of calls to eval() the conditional expression).

[benfitzpatrick]

I think you're right, looking at it. I've incorporated your commit.

[hjoliver]

Thanks. Review 1 - good, test battery passes here. Over to @kaday

[kaday]

Looks okay to me. Passes test battery in my environment.