Monitoring the Garbage Collection of a node application

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Monitoring a running nodejs application in production is a critical activity which allows detecting applications performance degradation automatically without manually testing and looking at it.

Besides monitoring only the total memory utilisation, the monitoring of Garbage Collection performance over time can expose more information about a potential problem in the application which can lead to a memory leak.

Running a node app with the --trace_gc flag will write GC information to the logs at every GC event. Following is the output after running an example app, e.g. node --trace_gc app.js

From the GC logs you can observe that there are two types of entries:

How to read the GC cycle entries?

Both types of entries contain data separated by single or multiple spaces.

The Scavenge cycle

[30420:0x103e45000] 99 ms: Scavenge 3.8 (5.7) -> 3.3 (8.2) MB, 0.6 / 0.0 ms (average mu = 1.000, current mu = 1.000) allocation failure

The Scavenge entry contains:

The Mark-sweep cycle

[30420:0x103e45000] 8168 ms: Mark-sweep 6.4 (9.7) -> 4.5 (7.1) MB, 2.6 / 0.0 ms (+ 0.2 ms in 3 steps since start of marking, biggest step 0.1 ms, walltime since start of marking 13 ms) (average mu = 1.000, current mu = 1.000) finalize incremental marking via task GC in old space requested

The Mark-sweep entry contains the same data as Scavenge with some extra information regarding the marking phase:

Now that you have enabled monitoring for Garbage Collection cycles of a node app what are some of the indications one should watch after.


Tracking down memory leaks is a challenging activity which cannot be done without data and internal insight about what exactly the application is doing to manage its memory.

Node has a built-in mechanism to output information about V8 engine’s Garbage Collection cycles. Which you can use to monitor how often the GC cycles happen, how long it takes and how much memory it frees up by comparing the total heap used before and after the GC cycle.

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