clinic heapprofiler to discover the functions which allocates more memory at heap – run clinic heapprofiler -hJavaScript is Garbage Collected language. Rather than manually freeing objects, they are simply "cleaned away" at some point after all references to an object have been removed.
At a basic level, the Garbage Collector traverses the JavaScript objects at various intervals to find any "orphaned" objects (objects which no longer have any references). If there are too many objects, and/or too many orphaned objects this can cause performance issues – because the Garbage Collector uses the same thread as the JavaScript event loop. In other words, JavaScript execution pauses while the Garbage Collector clears away de-referenced objects.
At a more detailed level, GC collection is triggered by memory activity, rather than time and objects are classified by the GC into young and old. "Young" objects are traversed (scavenged) more frequently, while "old" objects will stay in memory for longer. So there are actually two GC types, a frequent scavenge of new space (short lived objects) and a less regular traversal of old space (objects that survived enough new space scavenges).
Several heuristics may trigger detection of a GC issue, but they all center around high memory usage.
One possible cause of a detected GC issue is a memory leak, where objects are being accidentally allocated. However there are other (more common) cases where the is no leak but the memory strategy needs to be adapted.
One such common case is when large objects (such as may be generated for big JSON payloads), are created during periods of high activity (e.g. under request load). This can cause the objects to be moved into old space – if they survive two (by default) GC scavenges – where they will live for longer due to the less frequent scavenges. Objects can then build up in "old space" and cause intermittent process stalling during Garbage Collection.
Depending on the use case this may be solved in different ways. For instance if the goal is to write out serialized objects, then the output could be written to the response as strings (or buffers) directly instead of creating the intermediate objects (or a combined strategy where part of the object is written out from available state). It may just be a case that a functional approach (which is usually recommended) is leading to the repeated creation of very similar objects, in which case the logical flow between functions in a hot path could be adapted to reuse objects instead of create new objects.
Another possibility is that a very high amount of short lived objects are created, filling up the "young" space and triggering frequent GC sweeps – if this case isn't an unintended memory leak, then then an object pooling strategy may be necessary.
To solve Garbage Collection issues we have to analyse the state of our process in order to track down the root cause behind the high memory consumption.
clinic heapprofiler to create a flamegraph of memory allocations in the application.clinic heapprofiler --help for how to generate the memory profile.http: overload-protectionclinic flame to discover CPU intensive function calls – run clinic flame -h--trace-sync-io to track synchronous I/O operationsJavaScript is a single-threaded event-driven non-blocking language.
In Node.js I/O tasks are delegated to the Operating System, JavaScript functions (callbacks) are invoked once a related I/O operation is complete. At a rudimentary level, the process of queueing events and later handling results in-thread is conceptually achieved with the "Event Loop" abstraction.
At a (very) basic level the following pseudo-code demonstrates the Event Loop:
while (event) handle(event)
The Event Loop paradigm leads to an ergonomic development experience for high concurrency programming (relative to the multi-threaded paradigm).
However, since the Event Loop operates on a single thread this is essentially a shared execution environment for every potentially concurrent action. This means that if the execution time of any line of code exceeds an acceptable threshold it interferes with processing of future events (for instance, an incoming HTTP request); new events cannot be processed because the same thread that would be processing the event is currently blocked by a long-running synchronous operation.
Asynchronous operations are those which queue an event for later handling, they tend to be identified by an API that requires a callback, or uses promises (or async/await).
Whereas synchronous operations simply return a value. Long running synchronous operations are either
functions that perform blocking I/O (such as fs.readFileSync) or potentially resource intensive
algorithms (such as JSON.stringify or react.renderToString). The --trace-sync-io can help
by printing a stack trace whenever synchronous I/O is detected after the first run of the event
loop.
To solve the Event Loop issue, we need to find out where the synchronous bottleneck is. This may (commonly) be identified as a single long-running synchronous function, or the bottleneck may be distributed which would take rather more detective work.
clinic flame to generate a flamegraphclinic flame --help to get startedhttp: overload-protectionclinic bubbleprof to explore asynchronous delays – run clinic bubbleprof -h to get started.Node.js provides a platform for non-blocking I/O. Unlike languages that typically block for IO (e.g. Java, PHP), Node.js passes I/O operations to an accompanying C++ library (libuv) which delegates these operations to the Operating System.
Once an operation is complete, the notification bubbles up from the OS, through libuv, which can then trigger any registered JavaScript functions (callbacks) for that operation. This is the typical flow for any asynchronous I/O (where as Sync API's will block, but should never be used in a server/service request handling context).
The profiled process has been observed is unusually idle under load, typically this means it's waiting for external I/O because there's nothing else to do until the I/O completes.
To solve I/O issues we have to track down the asynchronous call(s) which are taking an abnormally long time to complete.
I/O root cause analysis is mostly a reasoning exercise. Clinic.js Bubbleprof is a tool developed specifically to inform and ease this kind of reasoning.
clinic bubbleprof to create a diagram of the application's asynchronous flow.clinic bubbleprof --help for how to generate the profileconsole.timeconsole.timeEnd--on-port flag to immediately begin load testing, this is known to reduce the chances of interference.clinic bubbleprof and/or clinic flame commands.--inspect flag with the Chrome Devtools Memory tab.An unknown issue occurs when Clinic.js' analysis algorithms are unable to categorize the sampling results but nevertheless an issue of some kind has been detected.
This outcome can be attributed to one of two scenarios:
clinic doctor doesn't recognize it.In the case of ambient noise, there may still be a specific, categorizable performance issue.
We can make eliminate the possibility of ambient noise and make it easier for Clinic.js to definitively recognize the issue by:
--on-port flag. This can reduce the chances of unknown issues because there is no time gap nor additional system activity between the server starting and the load test beginning.By way of example, instead of running clinic -- node app.js in one terminal and autocannon localhost:3000 in another, it is preferable and recommended to trigger both in one command using the following command:
clinic doctor --on-port="autocannon localhost:3000" -- node app.js
An even simpler form of this is to use the --autocannon flag,
clinic doctor --autocannon / -- node app.js
If after taking these steps an unknown categorization continues to occur then we can instead attempt to infer the nature of the performance issue using specialist diagnostic tooling, such
as clinic flame, clinic bubble or Node Inspector.
--on-port flag is being used to trigger load testing instead of initiating load testing independentlyclinic bubbleprof to create a diagram of the application's asynchronous flow (see clinic bubbleprof --help)clinic bubbleprof will visualize promise activity, make a point of looking out for it in the diagram.clinic flame to generate a flamegraphclinic flame --help to get started--inspect flag with the Chrome Devtools Memory tab.node --inspect <FILENAME>inspect link for that target – this will connect Chrome Devtools to the Node processes remote debug interface