node-memwatch
is here to help you detect and find memory leaks in
Node.JS code. It provides:
-
A
stats
event, emitted on full MarkSweepCompact GCs giving you data describing your heap usage and trends over time. -
A
HeapDiff
class that lets you compare the state of your heap between two points in time, telling you what has been allocated, and what has been released.
npm install @raghb1/node-memwatch
There are a growing number of tools for debugging and profiling memory usage in Node.JS applications, but there is still a need for a platform-independent native module that requires no special instrumentation. This module attempts to satisfy that need.
To get started, import node-memwatch
like so:
var memwatch = require('@raghb1/node-memwatch');
Currently unsupported while we explore heuristics
The best way to evaluate your memory footprint is to look at heap
usage right after V8 performs garbage collection. memwatch
does
exactly this - it checks heap usage only after GC to give you a stable
baseline of your actual memory usage.
When V8 performs a garbage collection (technically, we're talking
about a full GC with heap compaction), memwatch
will emit a stats
event.
memwatch.on('stats', function(stats) { ... });
The stats
data will look something like this:
{
gcScavengeCount: 1,
gcScavengeTime: 1100880, // ns
gcMarkSweepCompactCount: 2,
gcMarkSweepCompactTime: 21157231, // ns
gcIncrementalMarkingCount: 0,
gcIncrementalMarkingTime: 0, //ns
gcProcessWeakCallbacksCount: 0,
gcProcessWeakCallbacksTime: 0, // ns
total_heap_size: 16097280, // bytes
total_heap_size_executable: 3670016, // bytes
total_physical_size: 10741880, // bytes
total_available_size: 1487689928, // bytes
used_heap_size: 5691584, // bytes
heap_size_limit: 1501560832, // bytes
malloced_memory: 8192,
peak_malloced_memory: 1185464,
gc_time: 4587251 // ns
}
V8 has its own idea of when it's best to perform a GC, and under a
heavy load, it may defer this action for some time. To aid in
speedier debugging, memwatch
provides a gc()
method to force V8 to
do a full GC and heap compaction.
For leak isolation, it provides a HeapDiff
class that takes two snapshots and
computes a diff between them. For example:
// Take first snapshot
var hd = new memwatch.HeapDiff();
// do some things ...
// Take the second snapshot and compute the diff
var diff = hd.end();
The contents of diff
will look something like:
{
"before": { "nodes": 11625, "size_bytes": 1869904, "size": "1.78 mb" },
"after": { "nodes": 21435, "size_bytes": 2119136, "size": "2.02 mb" },
"change": { "size_bytes": 249232, "size": "243.39 kb", "freed_nodes": 197,
"allocated_nodes": 10007,
"details": [
{ "what": "String",
"size_bytes": -2120, "size": "-2.07 kb", "+": 3, "-": 62
},
{ "what": "Array",
"size_bytes": 66687, "size": "65.13 kb", "+": 4, "-": 78
},
{ "what": "LeakingClass",
"size_bytes": 239952, "size": "234.33 kb", "+": 9998, "-": 0
}
]
}
}
The diff shows that during the sample period, the total number of
allocated String
and Array
classes decreased, but Leaking Class
grew by 9998 allocations. Hmmm.
You can use HeapDiff
in your on('stats')
callback; even though it
takes a memory snapshot, which triggers a V8 GC, it will not trigger
the stats
event itself. Because that would be silly.
Please see the Issues to share suggestions and contribute!