Arrays are probably your best bet (but don't ever `delete` elements, or change the size of your array!).
I don't understand what you mean by "placeholder". But FWIW, array keys (i.e., small integers) are usually unboxed, i.e., not heap-allocated. A general advise, though: don't be over-concerned with GC. There are a lot of misconceptions about its performance. It is quite efficient in most practical cases -- much more so than most attempts to work around it and manage life-times manually. On 9 May 2016 at 20:06, /#!/JoePea <[email protected]> wrote: > Thanks Andreas, that's helpful to know. In general, is there some way to > keep a list (adding removing things over time) while avoiding GC? > > For example, I thought I could place and remove items into a list (f.e. > Set or Map, while having my own references to all the items and the list > and outside of the list as well as to the list itself), but as you pointed > out that still can have need for GC. > > I know you said > > > In general, there is very little you can do in JavaScript that does not > potentially cause allocation and thus GC > > But, I would at least want to know that I minimized GC as much as > possible. My first hunch (for my case) would be to make a linked list out > of the items I need to have a "list" of, where each item's link (reference) > to the next item is also one of the references that I already hold outside > of the list? > > It seems that with Arrays (and Sets and Maps) that deleting an item from > the list also deletes the placeholder that was used for that item (in the > case of Array, the placeholders would be the numerical keys that are > deleted when an item is popped for example), and I'm guessing (with my > still expanding VM knowledge) that those placeholders are GCed separately > from the thing that was referenced to by the placeholder (so even if the > placeholder contained `null` there would still be GC). Is that a good guess? > > I'll be experimenting... > > On Mon, May 9, 2016 at 2:28 AM, Andreas Rossberg <[email protected]> > wrote: > >> The `undefined` value is represented in exactly the same way as `true`, >> `false`, and `null` in V8. They're so called "oddballs" internally, global >> values that are neither allocated nor freed. >> >> Either way, the key/values of a (regular) map do not keep anything alive >> about the map. Adding or removing entries from a set or map can of course >> cause allocation/deallocation, regardless of the keys/values themselves. >> (Similarly if you are using regular objects as maps, btw.) >> >> There are also lots of other things that cause allocation/deallocation >> under the hood of a JavaScript engine, e.g. compilation, optimisation, >> deoptimisation, etc. Some of that predominantly happens at start-up. If you >> want to reduce random noise from your experiments, try to warm up the code >> first. But even then, don't read too much into micro-benchmarks -- JS VMs >> are far too complicated, dynamic, and heuristics-based to draw useful >> conclusions from tiny tests (that's e.g. why JSPerf tests are often >> bollocks). >> >> In general, there is very little you can do in JavaScript that does not >> potentially cause allocation and thus GC. Certainly nothing involving >> dynamic data structures. >> >> /Andreas >> >> >> On 5 May 2016 at 07:48, /#!/JoePea <[email protected]> wrote: >> >>> > The only v8 shell I have lying around is too old (3.14.5.10) to have >>> Set, so I can't tell you what it would do. >>> >>> On my first attempt, I noticed 8 Major GCs: >>> https://cloud.githubusercontent.com/assets/297678/15036715/f41ea4d4-1247-11e6-8823-f153c3c1b7bb.png >>> >>> On second attempt, no Major GCs: >>> https://cloud.githubusercontent.com/assets/297678/15036788/c066483a-1248-11e6-970b-3f9d20710bbc.png >>> >>> I wonder why. I'm in Chrome 50. >>> >>> So, the second attempt looks good. I'm not sure why the first is so >>> different. I tried it a few times, but I only got that Major GC zig-zag the >>> first time. >>> >>> Thanks for pointing out Set! >>> >>> On Wed, May 4, 2016 at 7:30 PM, Boris Zbarsky <[email protected]> wrote: >>> >>>> On 5/4/16 5:03 PM, Steve Fink wrote: >>>> >>>>> The only v8 shell I have lying around is too old (3.14.5.10) to have >>>>> Set, so I can't tell you what it would do. >>>>> >>>> >>>> I have v8 "4.8.0 (candidate)" (meaning whatever rev I checked out), and >>>> it does 1163 minor ("Scavenge") GCs on your testcase. It also does 1163 >>>> minor GCs if I take out the add() calls. It does none if I remove the >>>> clear() calls, no matter whether the add() calls are there or not. >>>> >>>> -Boris >>>> _______________________________________________ >>>> es-discuss mailing list >>>> [email protected] >>>> https://mail.mozilla.org/listinfo/es-discuss >>>> >>> >>> >>> _______________________________________________ >>> es-discuss mailing list >>> [email protected] >>> https://mail.mozilla.org/listinfo/es-discuss >>> >>> >> >
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