@sematext/gc-stats/README.md

Exposes node v8 garbage collection stats

# Note!
This is a fork of [dainis/node-gcstats](https://github.com/dainis/node-gcstats). I did not write the code nor claim I have. I'm only maintaining the dependencies that are required for my own open-source projects. Repos I maintain that need gc-stats to work:

- https://github.com/sematext/sematext-agent-express
- https://github.com/sematext/spm-agent-nodejs
- https://github.com/sematext/spm-agent

To install this package run:
```bash
npm install @sematext/gc-stats
```

# Usage

Create a new instance of the module and subscribe to `stats`-events from that:

    var gc = (require('@sematext/gc-stats'))();

    gc.on('stats', function (stats) {
        console.log('GC happened', stats);
    });

This will print blobs like this whenever a GC happened:

    GC happened {
      startTime: 9426055813976,
      endTime: 9426057735390,
      pause: 1921414,
      pauseMS: 1,
      gctype: 1,
      before: {
         totalHeapSize: 11354112,
         totalHeapExecutableSize: 3670016,
         usedHeapSize: 7457184,
         heapSizeLimit: 1501560832,
         totalPhysicalSize: 9725880,
         totalAvailableSize: 1488434544,
         mallocedMemory: 8192,
         peakMallocedMemory: 1186040
      },
      after: {
         totalHeapSize: 12402688,
         totalHeapExecutableSize: 3670016,
         usedHeapSize: 6485792,
         heapSizeLimit: 1501560832,
         totalPhysicalSize: 10166144,
         totalAvailableSize: 1489388528,
         mallocedMemory: 8192,
         peakMallocedMemory: 1186040
      },
      diff: {
         totalHeapSize: 1048576,
         totalHeapExecutableSize: 0,
         usedHeapSize: -971392,
         heapSizeLimit: 0,
         totalPhysicalSize: 440264,
         totalAvailableSize: 953984,
         mallocedMemory: 0,
         peakMallocedMemory: 0
      }
    }

## Property insights
* totalHeapSize: Number of bytes V8 has allocated for the heap. This can grow if usedHeap needs more.
* usedHeapSize: Number of bytes in use by application data
* total HeapExecutableSize: Number of bytes for compiled bytecode and JITed code
* heapSizeLimit: The absolute limit the heap cannot exceed
* totalPhysicalSize: Committed size (node 0.11+)
* totalAvailableSize: Available heap size(node 4+)
* startTime: Nanoseconds for start, using hrtime()
* endTime: Nanoseconds for end, using hrtime()
* pause: Nanoseconds from start to end of GC using hrtime()
* pauseMS: pause expressed in milliseconds
* mallocedMemory
* peakMallocedMemory
* numberOfNativeContexts
* numberOfDetachedContexts
* gctype can have the following values([v8 source](https://github.com/nodejs/node/blob/554fa24916c5c6d052b51c5cee9556b76489b3f7/deps/v8/include/v8.h#L6137-L6144)):
  * 1: Scavenge (minor GC)
  * 2: Mark/Sweep/Compact (major GC)
  * 4: Incremental marking
  * 8: Weak/Phantom callback processing
  * 15: All

# Installation

    npm install gc-stats

# Node version support
node-gcstats depends on C++ extensions which are compiled when the *gc-stats* module is installed. Prebuilt binaries are available on github.