Jason Kim's Blog

Fixing Memory Leak on Production Node.js Application

The last few days at work were rough. My team was intensely focused on preparing the production environment to be stable. We've been having some serious issues on the production environment. One of the most serious production issues was a nasty memory leak.

The graph above displays the memory usage of 12 node applications on our production environment. The y-axis shows the memory usage in percentage and the x-axis shows the time span of 7 days. This narrow sawtooth pattern on memory usage is extremely bad. As you can see, all 12 servers are accumulating usage in memory rapidly. As a remedy for the memory leak, we had to periodically restart our servers. This was less than ideal, but because we had so many other really high priority items last few weeks, we just had to bite our tongue and suck it up. However, the frequency at which we had to restart our server started to increase. We had to restart our servers every few days, then every day, then it came to a point where we were restarting our servers every 4 to 5 hours. Thank god we have a globally distributed team (Croatia, and Argentina), it could've been a lot tougher without having team members in other timezones. I can't emphasize this point enough, and I will take another opportunity to praise having globally distributed software development team in another post in the future.

After some pitfalls, we fixed the memory leak. As you can see below, the overall memory usage stays flat after 13:30 PM after our memory leak fix was applied.

We came up with several different strategies to fix the memory issue, but the working solution came down to comparing two memory dumps at different times and comparing their content. I used gcore on a production server to gather a memory dump soon after the server restart when the memory usage is around 30%. After around 3 hours when the memory usage for the server starts to hover around 60%, I took another memory dump. I used mdb on a local VM running Solaris 11 to analyze the two memory dumps.

Here is the memory dump of the production server at around 30% memory usage.

{% highlight text %} 73ce2207ea9 4643 2 Arguments: 0, length 101b5ab4f71 5519 4 Object: albumId, albumNames, ... 9719767799 6070 2 Object: localeCode, value 971977c371 6966 1 Object: entry 1102bfc1dc31 8775 2 Object: id, genreIds 971977b021 9086 2 Object: ids, href 971973e051 9430 3 Array 3015e01 16203 0 Object 971974c9c9 19854 1 Object: nr_exclusive_duration_millis 9719710199 19917 13 TraceSegment: name, transaction, ... 971971cb11 20186 7 Timer: state, touched, duration, ... 342d8d9 49769 0 Array 971971cbb9 59489 1 Array 971970f1c9 71743 2 Array OBJECT #OBJECTS #PROPS CONSTRUCTOR: PROPS {% endhighlight %}

And here is the memory dump of the production server at around 80% around 4 hours after the server restart.

{% highlight text %} 1102bfc1dc31 8775 2 Object: id, genreIds


110879a2c9 14099 6 ExponentialBackoffStrategy: ... 1108779561 15593 3 Array 45357fdf31 17059 2 Object: ids, href 3015db1 34680 0 Object 110875b221 35287 4 Object: albumId, albumNames, ... 110875b2c9 38401 1 Object: entry 110875aff1 38481 2 Object: localeCode, value 1108708d51 198161 0 Array 1108706ae1 202984 1 Array 1108706a81 206039 1 Object: nr_exclusive_duration_millis 11087046d9 209896 13 TraceSegment: name, transaction, ... 1108704801 223481 7 Timer: state, touched, duration, ... 1108706aa1 507224 2 Array OBJECT #OBJECTS #PROPS CONSTRUCTOR: PROPS {% endhighlight %}

The most notable cause of the memory increase is TraceSegment: name, transaction, .... You can see that the number of TraceSegment object count increased by 10 folds going from 19917 to 209896. You can also see some other object such as Timer and Array increasing. This is because those are properties of the TraceSegment object. Contrast this number with the genre-tags object that stayed exactly the same 2 Object: id, genreIds at 8775. That object is a json file parsed into memory at startup of the app and it does not increase. This suggests to me that the memory dump is indeed accurate, and Node retains memory steady on correctly coded objects.

It turns out TraceSegment objects are created by NewRelic. The fact that I wasn't able to recreate the severity of memory leak convincingly on the development environment from previous attempts also tells me that the memory leak was caused by something happening on the production environment.

We get some benefits of having NewRelic monitoring, but disabling NewRelic on production was no-brainer at this point. We had to do it. We all acknowledged that NewRelic is not the initial cause of the memory leak. There is still something in our code that is behaving naughtily with NewRelic, but we got to put our the damn fire on production first. So we did it. We disabled NewRelic on production and the memory usage stopped climbing and it remains very steady for now.

I want to thank Yunong Xiao for sharing Netflix's experience on debugging Node.js in production. It informed me about useful tools in the wild and taught me how I can use those tools to find the cause of the memory leak. I also want to thank Bryce Neal for writing a detailed tutorial on setting up Solaris to use MDB.