I have a very large collection of objects in node.js (millions) that I need to keep in memory for caching purposes (they are maintained in several global hash objects). Each hash collections stores about 750k keys.
In order to keep GC to a minimum, I want to find out the best way to store these items. Would it be better to split these items into 100s of thousands of hashes? Should I maybe not use hashes at all? Is there any way to keep them off the heap completely so they never are examined by GC (and if so how would I do so)?
There is no public API to control garbage collection from JavaScript.
But GC has gone a long way over the years. Modern GC implementations will notice that some objects live long and put them into a special "area" which will be collected only rarely.
How this exactly works is completely implementation dependent; every browser does it's own thing and usually, this also often changes when a new browser version is released.
EDIT The layout on memory and the organization is completely irrelevant. Modern GCs are really hard to understand in detail without investing a couple of weeks reading the actual code. So what I explain now is a really simplified picture; the real code will work different (and some GCs will use completely different tricks to achieve the same goal).
Imagine that the GC has a counter for each object in which it counts how often it has seen it in the past. Plus it has several lists where it keeps objects of different age, that is with objects whose counters have passed certain thresholds. So when the counter reaches some limit, the object is moved to the next list.
The first list is visited every time GC runs. The second list is only considered for every Nth GC run.
An alternate implementation might add new objects to the top of the "GC list" and for every GC run, it will only check N elements. So long living objects will move down the list and after a while, they don't be checked every time.
What this means for you is that you don't have to do anything; the GC will figure out that your huge map lives a long time (and the same is true for all objects in the map) and after a while, it will start to ignore this data structure.