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According to google, V8 uses an efficient garbage collection by employing a "stop-the-world, generational, accurate, garbage collector". Part of the claim is that the V8 stops program execution when performing a garbage collection cycle.

An obvious question is how can you have an efficient GC when you pause program execution?

I was trying to find more about this topic as I would be interested to know how does the GC impacts the response time when you have possibly tens of thounsands requests per second firing your node.js server.

Any expert help, personal experience or links would be greatly appreciated

Thank you

1 Answers1

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"Efficient" can mean several things. Here it probably refers to high throughput. When looking at response time, you're more interested in latency, which could indeed be worse than with alternative GC strategies.

The main alternatives to stop-the-world GCs are

  • incremental GCs, which need not finish a collection cycle before handing back control to the mutator1 temporarily, and
  • concurrent GCs which (virtually) operate at the same time as the mutator, interrupting it only very briefly (e.g. to scan the stack).

Both need to perform extra work to be correct in the face of concurrent modification of the heap (e.g. if a new object is created and attached to an already-scanned object, this new reference must be noticed). This impacts total throughput, i.e., it takes longer to actually clean the entire heap. The upside is that they do not (usually) interrupt the program for very long, if at all, so latency is low(er).

Although the V8 documentation still mentions a stop-the-world collector, it seems that an the V8 GC is incremental since 2011. So while it does stop program execution once in a while, it does not 2 stop the program for however long it takes to scan the entire heap. Instead it can scan for, say, a couple milliseconds, and let the program resume.

1 "Mutator" is GC terminology for the program whose heap is garbage collected.

2 At least in principle, this is probably configurable.