README.md

thetool

thetool is a CLI tool to capture different cpu, memory and other profiles for your node app in Chrome DevTools friendly format.

Quick start

npx thetool -o . -t memorysampling npm run test
# .. open DevTools frontend and do three clicks to get:

Getting Started

thetool works only with Node >= 10.x.

thetool interface is simple as 1-2-3.

1. Specify output folder using -o flag, e.g. -o . to put output in current folder.
2. Specify tool using -t, available tools: cpu, memorysampling, memoryallocation, coverage, type, heapsnapshot.
3. Specify any command to start node, e.g. node index.js or npx thetool or npm run test.

When report is ready, thetool will dump thetool> Report captured in ... message in terminal with a hint how to analyze it.

Why not to use Chrome DevTools directly?

• it can be used in environments where it is not possible to run Chrome DevTools, e.g., on the server, run thetool <yourapp> there, send report back and analyze it locally,
• it supports child processes and workers,
• it supports some other tools, e.g., node tracing and type profiler.

Tool selector

Problem	Tool	Insight	DevTools tab
my app is slow	cpu	where in code does app spend most time?	Performance
my app requires too much memory	memorysampling	where in code does app allocate most memory?	Memory
my app requires too much memory	memoryallocation	most precise version of memorysampling with much bigger overhead	Memory
my app requires too much memory	heapsnapshot	what is inside the heap right now?	Memory
my app package is too big	coverage	what code was executed and how many times?
my app needs type annotations	type	what are the types of function arguments and returns?

On-demand tooling

You can use --ondemand flag to profile only part of your app:

1. Add --ondemand flag to the list of thetool arguments.
2. Call startTheTool/stopTheTool from your Node scripts (thetool will add these methods to Node context).

startTheTool/stopTheTool methods are asynchronous, so you should await them or chain them using promise.then

Couple examples:

async function main() {
  await startTheTool();
  // code of your app
  await stopTheTool();
}
// .. or using promises..
function main() {
  startTheTool().then(() => {
    // code of your app
  }).then(() => stopTheTool());
}

CPU: Profiler

thetool -o . -t cpu npm run test

To analyze: open Chrome DevTools, to to Performance tab, click load button, select file with data.

Memory: Sampling Profiler

thetool -o . -t memorysampling npm run test

To analyze: open Chrome DevTools, go to Memory tab, click load button, select file with data.

--samplingInterval option is available: average sample interval in bytes, poisson distribution is used for the intervals. The default value is 32768 bytes

Memory: Allocation Profiler

thetool -o . -t memoryallocation npm run test

To analyze: open Chrome DevTools, go to Memory tab, click load button, select file with data.

Memory: Heap Snapshot

thetool -o . -t heapsnapshot node -e "captureTheTool.then(captureTheTool).then(captureTheTool)"

Given command will capture three heap snapshots. To analyze: open Chrome DevTools, go to Memory tab, click load button, select file with data. You can load multiple snapshots and compare them from DevTools UI.

Tracing

thetool -o . -t tracing --recordMode recordAsMuchAsPossible --includedCategories node,v8 npm run test

To analyze: open Chrome DevTools, go to Performance tab, click load button, select file with data.

--recordMode controls how the trace buffer stores data (recordUntilFull, recordContinuously, recordAsMuchAsPossible) --includedCategories please take a look on different available categories on https://nodejs.org/api/tracing.html

E.g. you can capture V8 sampling profiler using following command:

thetool -o . -t tracing --recordMode recordAsMuchAsPossible --includedCategories v8.execute,v8.cpu_profiler,v8.cpu_profiler.hires npm run test

Coverage Profiler

thetool -o . -t coverage npm run test

To analyze: in current folder create ./coverage/tmp folder and move files with data to this folder, run c8: npx c8 report. Please take a look at c8 README.md to see what output formats are supported.

Type Profiler

thetool -o . -t type npm run test

To analyze: no tool yet.