idle-gc

IDLE-GC

"The Devil's hands are idle playthings." -- Futurama, S04E18

RATIONALE

This module runs the garbage collector at times when node.js is otherwise idle.

It is a replacement for the built-in functionality that is scheduled for removal in node.js v0.10.

The reasons for removing it from node.js core are twofold:

1. The implementation has severe deficiencies. Many people have reported issues where their otherwise idle node.js server uses 100% CPU, trying to collect (often non-existent) garbage.

2. The garbage collector has much improved. In the old days, forcing the garbage collector to run at opportune times could significantly increase throughput. The current incremental collector however is much better, so much so that in most cases the idle GC scheme is superfluous at best and possibly a deoptimization.

This module attempts to fix the deficiencies while preserving the functionality for people that need it. Reasons for using it include:

1. Low latency. If you have an application that needs low latency, this module can help amortize the overhead of the garbage collector - but only if your application is sufficiently idle.

2. Reducing memory usage. The garbage collector trades space for time; given the chance, it will allocate more heap memory if that means it won't have to scan the existing heap as often.

   That's almost always a worthwhile trade-off unless your application runs in a restricted environment (say a budget VPS with only 256 or 512 MB of RAM.) where the larger memory footprint is actually detrimental.

USAGE

The module exports two functions, start() and stop().

var g = require('idle-gc');
g.start();      // Run at 5 second intervals.
g.start(7500);  // Run at 7.5 second intervals. Stops the old timer first.
g.stop();       // Stop the timer.

The default interval is 5 seconds. That means the first GC run starts 5 seconds after the last activity.

'Activity' in this context means 'any operation that somehow causes the event loop to move forward', be that file or network I/O, a JavaScript timer, etc.

If there is no more garbage to collect, the timer disables itself. It is automatically re-enabled when new activity happens (unless explicitly stopped, of course.)

CAVEATS

As mentioned in the USAGE section, timers are considered activity. It therefore follows that in this example the idle GC never runs because the JavaScript timer pre-empts it every time.

var g = require('idle-gc');
g.start(2500);         // Run at 2.5 second intervals.
setInterval(f, 2000);  // Run at 2.0 second intervals.
function f() {}