3 ms·

Some of these performance tips are just not believable without some kind of justification. I'm to believe that garbage collection can't work if you use delete?

Some of these performance tips are just not believable without some kind of justification. I'm to believe that garbage collection can't work if you use delete? That seems pretty silly.

mintplant 10y ago
This one actually seems fair to me. They're not saying that GC won't kick in on the value the deleted property used to point to, but rather that using `delete` on a property can be more expensive than you need if you're just trying to let the GC know something can be cleaned up. This is because optimized JIT code and caches guard on the structure of objects that pass through them, and changing that structure (eg. deleting a property) will lead to deoptimizations / tossing away some of that code. On SpiderMonkey, deleting a property that isn't the last one that was defined may incur a "dictionary mode" conversion [0]. This means the information describing each object property, which is usually immutable and shared between similar objects, will be copied one-by-one into a unique chain owned by the particular object instance. This can only happen once per object instance, however, and subsequent usages of `delete` on its properties will be less expensive -- but still more expensive than simply setting property values to `null`. I believe something similar happens with V8's "hidden classes" mechanism. [0] https://dxr.mozilla.org/mozilla-central/source/js/src/vm/Shape.cpp#961-970 https://dxr.mozilla.org/mozilla-central/source/js/src/vm/Sha...
- recursive 10y ago
  I'm not saying it's not true. I don't know enough deep javascript magic to make that claim. It just didn't sound true, given that they've provided no supporting information or even a plausible-sounding explanation of why one way is better than another way. If the entire site were rewritten using this level of detail plus benchmarks, it would be 10x more useful.

Some of these performance tips are just not believable without some kind of justification. I'm to believe that garbage collection can't work if you use delete? That seems pretty silly.

mintplant 10y ago

This one actually seems fair to me. They're not saying that GC won't kick in on the value the deleted property used to point to, but rather that using `delete` on a property can be more expensive than you need if you're just trying to let the GC know something can be cleaned up. This is because optimized JIT code and caches guard on the structure of objects that pass through them, and changing that structure (eg. deleting a property) will lead to deoptimizations / tossing away some of that code. On SpiderMonkey, deleting a property that isn't the last one that was defined may incur a "dictionary mode" conversion [0]. This means the information describing each object property, which is usually immutable and shared between similar objects, will be copied one-by-one into a unique chain owned by the particular object instance. This can only happen once per object instance, however, and subsequent usages of `delete` on its properties will be less expensive -- but still more expensive than simply setting property values to `null`. I believe something similar happens with V8's "hidden classes" mechanism. [0] https://dxr.mozilla.org/mozilla-central/source/js/src/vm/Shape.cpp#961-970 https://dxr.mozilla.org/mozilla-central/source/js/src/vm/Sha...

recursive 10y ago
I'm not saying it's not true. I don't know enough deep javascript magic to make that claim. It just didn't sound true, given that they've provided no supporting information or even a plausible-sounding explanation of why one way is better than another way. If the entire site were rewritten using this level of detail plus benchmarks, it would be 10x more useful.