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Yeah, of the four things listed, I think it's the one that would trip me up the most. I think the safety net here would be dead code elimination: `unused()` sho

Yeah, of the four things listed, I think it's the one that would trip me up the most. I think the safety net here would be dead code elimination: `unused()` should be detected as never called and removed during compilation/transpilation so that this wouldn't be an issue. I am sort of surprised that `unused` is not picked up by the garbage collector in the first place though. Since JS functions are objects, shouldn't it detect an object that's not referenced during the mark and sweep? In general, I really hate having to debug memory leaks in JS or Python. The interpreter for both will randomly allocate additional memory as it runs, so using tools like Valgrind is next to impossible. The only reliable method I've found is to pepper my code with logging statements that show what the current memory usage is, run the code like 1000-10,000 times, and see the points between which the memory usage goes up without coming down on a consistent basis. Python's built in `gc` module seems nearly useless for determining what's actually stuck in memory, and having a billion libraries that can have their own memory leaks is also not fun. These are the times I miss C: when you leak memory in C you know it because it becomes painful fast and it's usually easy to find, if your code is sane.

favorited 9y ago
I haven't done JS in a while, but it sounds to me like it is referenced, just not in source code. `someMethod` references it implicitly. Is my understanding correct?
- gsnedders 9y ago
  Yes. The function's frame contains a reference to the object storing local variables of the parent frame. To do better you need to store a list of variables referenced (which you can only do if there's neither direct-eval nor a with statement).
Ajedi32 9y ago
> In general, I really hate having to debug memory leaks in JS or Python. The interpreter for both will randomly allocate additional memory as it runs, so using tools like Valgrind is next to impossible. I don't know about Python, but for JavaScript can't you use the built-in devtools? Maybe try grabbing a heap snapshot, or recording a record allocation profile and go from there?
wahern 9y ago
The fix is to implement the closures properly, by only closing over individual variable slots. It looks like the engines are implementing closures by closing over entire windows of slots--that is, if two functions have the same scope, they inherit the _union_ of the variables they reference as a single window/block of variable slots. The original article has a much simpler explanation and solution: https://blog.meteor.com/an-interesting-kind-of-javascript-memory-leak-8b47d2e7f156 https://blog.meteor.com/an-interesting-kind-of-javascript-me... To learn more about closures than you ever thought possible, try reading this paper describing how closures are implemented in Lua: http://www.cs.tufts.edu/~nr/cs257/archive/roberto-ierusalimschy/closures-draft.pdf http://www.cs.tufts.edu/~nr/cs257/archive/roberto-ierusalims...

favorited 9y ago

I haven't done JS in a while, but it sounds to me like it is referenced, just not in source code. `someMethod` references it implicitly. Is my understanding correct?

gsnedders 9y ago
Yes. The function's frame contains a reference to the object storing local variables of the parent frame. To do better you need to store a list of variables referenced (which you can only do if there's neither direct-eval nor a with statement).

Ajedi32 9y ago

> In general, I really hate having to debug memory leaks in JS or Python. The interpreter for both will randomly allocate additional memory as it runs, so using tools like Valgrind is next to impossible. I don't know about Python, but for JavaScript can't you use the built-in devtools? Maybe try grabbing a heap snapshot, or recording a record allocation profile and go from there?

wahern 9y ago

The fix is to implement the closures properly, by only closing over individual variable slots. It looks like the engines are implementing closures by closing over entire windows of slots--that is, if two functions have the same scope, they inherit the _union_ of the variables they reference as a single window/block of variable slots. The original article has a much simpler explanation and solution: https://blog.meteor.com/an-interesting-kind-of-javascript-memory-leak-8b47d2e7f156 https://blog.meteor.com/an-interesting-kind-of-javascript-me... To learn more about closures than you ever thought possible, try reading this paper describing how closures are implemented in Lua: http://www.cs.tufts.edu/~nr/cs257/archive/roberto-ierusalimschy/closures-draft.pdf http://www.cs.tufts.edu/~nr/cs257/archive/roberto-ierusalims...