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Comment by softfalcon | Hacker News Reader

Interesting, thanks for the info, I'll do some reading on what you're saying. I agree, you're right about JS having issues with hiccups in the UI due to scheduling on a single process thread.

Makes a lot of sense, cool that the garbage collector can run independently of the call stack and function scheduler.

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OP doesn’t know what he’s talking about. Creating an object per byte is insane to do if you care about performance. It’ll be fine if you do 1000 objects once or this isn’t particularly performance sensitive. That’s fine. But the GC running concurrently doesn’t change anything about that, not to mention that he’s wrong and the scavenger phase for the young generation (which is typically where you find byte arrays being processed like this) is stop the world. Certain phases of the old generation collection are concurrent but notably finalization (deleting all the objects) is also stop the world as is compaction (rearranging where the objects live).

This whole approach is going to be orders of magnitude of overhead and the GC can’t do anything because you’d still be allocating the object, setting it up, etc. Your only hope would be the JIT seeing through this kind of insanity and rewriting to elide those objects but that’s not something I’m aware AOT optimizer can do let alone a JIT engine that needs to balance generating code over fully optimal behavior.

Don’t take my word for it - write a simple benchmark to illustrate the problem. You can also look throughout the comment thread that OP is just completely combative with people who clearly know something and point out problems with his reasoning.

Even if you stop the world while you sweep the infant generation, the whole point of the infant generation is that it's tiny. Most of the memory in use is going to be in the other generations and isn't going to be swept at all: the churn will be limited to the infant generation. That's why in real usage the GC overhead is I would say around 15% (and why the collections are spaced regularly and quick enough to not be noticeable).

I've been long on JS but never heard things like this, could you please prove it by any means or at least give a valid proof to the _around 15%_ statement? Also by saying _quick enough to not be noticeable_, what's the situation you are referring too? I thought the GC overhead will stack until it eventually affects the UI responsiveness when handling continues IO or rendering loads, as recently I have done some perf stuff for such cases and optimizing count of objects did make things better and the console definitely showed some GC improvements, you make me nerve to go back and check again.

Yeah I mean don't take my word, play around with it! Here's a simple JSFiddle that makes an iterator of 10,000,000 items, each with a step object that cannot be optimized except through efficient minor GC. Try using your browser's profiler to look at the costs of running it! My profiler says 40% of the time is spent inside `next()` and only 1% of the time is spent on minor GCs. (I used the Firefox profiler. Chrome was being weird and not showing me any data from inside the fiddle iframe).

To me this is just "fake test". As I have said really world cases involves consistent IO loads and/or rendering loops, for example in my case I need to load tons of pixel data and decode them in works, then at the same time use canvas to render the decoded image and huge chunk of array data, they are real world high loads, there are tons of objects created during the process and way less counts than the "fake test", yet still optimizing the object counts made huge difference to the final performance.

Let's say talk about this in another more general case: virtual windowing. If anyone has tried to implement stuff and hit performance bottle neck and then find virtual windowing could help, it definitely involves two problems to solve, first is the UI responsiveness when more and more stuff got created and rendered, the object count usually should be way less than "10,000,000", yet still you could hit the wall.

I think I might be too negative about it, but I just want to share the real cases here.

JSFiddle link missing.

sry https://jsfiddle.net/L10dw6gn/3/

Thanks for this. I was feeling similarly reading the original post.

I was trying to keep an open mind, it's easy to be wrong with all that's going on in the industry right now.

Thanks for clarifying some of the details back to what I was originally thinking.

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