Sabbatical Log: November 27th

This blog series is running about a week behind my actual work, giving me time to clean things up.  The date the actual work was done is in the title, horizontal lines indicate when I stopped writing and started coding.

I’ve got falling down a pit mostly working, but I want to add a little drop shadow to show where the player is falling. This will be the first exit transition that creates an entity, so that’ll be interesting.

Whelp, that’s wrapped up

Again, I’m not gonna win any animation awards – but it works.

I think my final task for this sabbatical month is going to be to implement an actual enemy to fight, but before I do that there’s one more piece of infrastructure I want to build: a “job system”.

Pretty much everything that matters has multiple cores now, and it’s kind of silly not to take advantage of them – but multi-threading has some complications in the context of a game. Whereas a web app can assign threads to different logical requests, games have a single logical process. Unlike web apps where all threads are more or less equal, games typically have a privileged thread (the UI thread). And since this whole endeavor is a learning exercise, I think I should play around within these constraints.

I’ve worked up a very simple job system, composed of 3 parts:

  • JobRunner
  • Job (which implements IJob)
  • JobsCompletionToken

JobRunner is pre-sized for a set of threads, and a maximum number of concurrent jobs. Other code will use a single JobRunner to pre-allocate Job’s based on a delegate, and whatever additional state they need (the current GameState will always be passed, as it should always be needed). JobsCompletionToken is returned by the JobRunner.StartJobs method, wraps around whatever IJobs are passed, and has a WaitForCompletionMethod().

The basic idea is that JobRunner pre-allocates threads and JobCompletionTokens, various bits of code create their Job’s when they’re spun up, those Job’s (as IJobs) are re-used to run the code but with an updated GameState. I built it using the standard Thread, Monitor, and Interlocked classes. Threads do the actual work (the main thread is never stolen), the Interlocked class is used to add and remove IJobs and JobsCompletionTokens from queues, and Monitor is used to pause and resume threads. Since everything is pre-allocated, once startup is done this runs with no allocations.

Multithreaded code is difficult, and I’m not convinced I got this correct (though I did write tests). It’s also difficult to describe, so I’m just going to link to it: JobRunner, Job, and JobsCompletionToken.

Now to actually use it.

It’s the end of the day and I’ve converted two systems to use the JobRunner: the CollisionDetectionSystem, and the UpdatePositionSystem. Both of these were doing four passes of the same logical step, one for each level of a room.

(single threaded is on the left, multi-threaded on the right)

If you squint you can see some small improvements in DEBUG, although they’re mostly washed out in RELEASE builds. The multi-threading for the collision system is probably too coarse, honestly, since most everything is on one level. Regardless, I feel this exercise was a good one.

Tomorrow I’ll start on a proper enemy.

Continue onto November 28th