Wash dishes. Don't collect garbage.

Jul 6, 2014   #Garbage collection 

In computer science literature, “garbage collection” refers to the process by which unused computer memory is reclaimed for use by a program. Such memory is usually referred to as “garbage” and the “garbage collector” periodically runs to do this job. Though I understand the process to some extent, I’ve never been happy with the metaphor since it doesn’t help at all with suggesting possible techniques for doing the task and is just used to label this part of a programming system with automatic memory management. In this post, I explore “dish washing” as a metaphor for the same process and argue why it is a better one to adopt for teaching purposes.

What’s the fuss about metaphors?

If you grok the concept, why bother coming up with different metaphors to explain it? Why not just teach the known algorithms and leave innovation to the imagination of the student/programmer?

Computer science and programming are abstract activities. Much of the history of development tools is about making these abstract activities accessible to our senses, including recently popular talks and writing by Bret Victor. I think metaphors contribute to the same by bringing the abstract activities into the fold of the physically familiar and hence accessible to mental simulation. Even “garbage collection” triggers the imagination of finding a garbage can full, taking it out and dumping it so that the “collector” who pops by once in a while can take it away to be dumped into a landfill or recycled as the case may be.

What’s wrong with “garbage collection” as a metaphor?

Wait a minute. Do I still have to explain this after I wrote about the image that garbage collection invokes in the previous paragraph? If you do know about GC in computer science, does that imagery bear any resemblance to the process that scans memory for previously used memory that is no longer usable, and makes it available to future code that needs it?

If you think that’s a clear enough analogy for you, please stop here and move on. … But wait, before you actually move on, tell me a few possible techniques for doing that which you can come up with by working with the metaphor alone. Feel free to post in the comments section. I’m genuinely interested to know.

Washing dishes and memory management

I make something to eat in my kitchen. I use vessels to cook food and vessels to put the cooked food in. At some point, I run out of free vessels. I can’t use the cooking vessels because they’re not clean, and I can’t use some of the storage vessels either for the same reason. Some of the storage vessels still contain food and I cannot use them.

I need to wash me some dishes.

I run a program to do some useful work. It uses memory to compute information I consume in some form. At some point, it runs out of free memory. It can’t use the memory it already touched because it doesn’t know off hand whether the information in them will be used or not. Some of the memory does contain information that will be used.

My program needs to find it some memory.

Cooking up metaphors

Let’s imagine some scenarios that call for kitchen work.

  1. When cooking dinner for two in a large well stocked kitchen, you can afford to just keep using vessels as you need without ever needing to wash anything. A program that doesn’t need much memory, running on a system with gobs of RAM, can afford to not deallocate memory at all.

  2. When cooking for a party with a small kitchen, you’ll need to wash dishes frequently so that you have vessels available to do the cooking. If you don’t have enough vessels to serve the food, you don’t have a party. A program that needs a lot of memory to run, when run on a system with constrained memory, will need to reclaim memory frequently to get the job done, and if there isn’t enough memory to hold the program’s output, the job crashes.

  3. When kitchen size roughly matches the cooking task, different strategies may work depending on the kitchen structure. If it takes time to get new vessels, it may be faster to wash dishes frequently. If washing takes time, you may wish to schedule naps at long intervals. A program may likewise find it faster to reclaim unused memory frequently if allocation takes time, and would need to allow for fewer long pauses to minimize total time spent reclaiming memory.

  4. An economical chef can maximize his kitchen’s availability if vessels are cleaned up as soon as he’s done with using them, at the cost of working slightly slower due to repeated cleaning of the same vessels. This strategy would work for small kitchens. A program may likewise reclaim memory as soon as it knows when such memory becomes unused. It would need some way of tracking when memory becomes unused in order to do this, which would correspond to reference counting strategies, for ex. This strategy would work for embedded devices with constrained resources.

  5. A chef may categorize vessels into “need this for most work” and “stores stuff for a long time”. He’d make sure the first category gets washed frequently while not bothering to wash the second category as often. A program may likewise find that some kinds of allocations store stuff for a short term only and some other kinds store stuff for longer periods. It can then choose to focus on reclaiming the memory with high turn over with less frequent attempts to reclaim memory that’s known to store information for a longer time.

  6. Well, you’re the Chef. You can hire a hand to cleanup your vessels for you, so that the vessels that you need are available to you without delay. Of course, you’re both working in the same space and would need to coordinate a bit so you don’t bump into each other too often. A program can dedicate a processor to continuously reclaim memory so the main processing task isn’t starved. Of course both programs are running on the same computer and they’d need to coordinate memory accesses and memory reclamation for consistency.

Heard of “generational garbage collection”? Now explain that with the original metaphor for me please.

Wrapping up

I believe I’ve shown dish washing to be a better way to explain various strategies for what’s traditionally called “garbage collection”. Here’s a rough legend for this analogy -

  1. Kitchen = Computer
  2. Chef = Processor
  3. Cooking = Processing
  4. Vessels = Memory
  5. Food = Information

If the next time you’re hanging around in your kitchen and come up with some interesting way to manage your cooking, think what would happen if you applied that to memory management in a computer system and have an aha moment …

… implement it, benchmark it against known approaches, write a paper about it, publish some open source code that shows the technique … and be sure to write me a thank you note here for making that connection for you :)

Happy cooking!