cpu-profile-analysis

if (sizeMB < 400) {
    const data = JSON.parse(fs.readFileSync(profilePath, 'utf8'));
    fs.writeFileSync(profilePath, JSON.stringify(data, null, 2));
}

// Node lookup
const nodeMap = new Map();       // id -> node
const parentMap = new Map();     // id -> parent id
 
// Absolute timestamps from deltas
const timestamps = [data.startTime];
for (let i = 0; i < data.timeDeltas.length; i++) {
    timestamps.push(timestamps[i] + data.timeDeltas[i]);
}
 
// Stack walker (leaf to root)
function getStack(sampleNodeId) {
    const stack = [];
    let id = sampleNodeId;
    while (id !== undefined) {
        const node = nodeMap.get(id);
        if (node) stack.push(node.callFrame.functionName);
        id = parentMap.get(id);
    }
    return stack; // [leaf, ..., root]
}

{
  "pid": 3406,           // Process ID
  "tid": 7534980,        // Thread ID
  "ts": 200420830729,    // Timestamp in microseconds
  "ph": "X",             // Phase (event type)
  "cat": "devtools.timeline",  // Category
  "name": "EventDispatch",     // Event name
  "dur": 9,              // Duration in microseconds (for complete events)
  "tdur": 8,             // Thread duration (excludes time thread was suspended)
  "args": { ... },       // Event-specific arguments
  "tts": 7078808         // Thread timestamp
}

const procNames = events.filter(e => e.name === 'process_name');
// => [{args: {name: 'Renderer'}, pid: 3406}, {args: {name: 'Browser'}, pid: 3348}, ...]
 
const threadNames = events.filter(e => e.name === 'thread_name');
// => [{args: {name: 'CrRendererMain'}, pid: 3406, tid: 7534980}, ...]

import { readFileSync, statSync } from 'fs';
 
const stat = statSync(tracePath);
const sizeMB = stat.size / (1024 * 1024);
console.log(`File size: ${sizeMB.toFixed(0)}MB`);
 
let data;
if (sizeMB < 400) {
    data = JSON.parse(readFileSync(tracePath, 'utf8'));
} else {
    // Too large -- use Buffer-based extraction (see "Handling Huge Files" section)
    data = parseTraceFromBuffer(readFileSync(tracePath));
}
const events = data.traceEvents;

if (sizeMB < 400) {
    fs.writeFileSync(tracePath, JSON.stringify(data, null, 2));
}

const data = JSON.parse(fs.readFileSync(tracePath, 'utf8'));
const events = data.traceEvents;
 
// Identify Renderer main thread
const rendererPid = events.find(e => e.name === 'process_name' && e.args?.name === 'Renderer')?.pid;
const mainTid = events.find(e => e.name === 'thread_name' && e.pid === rendererPid && e.args?.name === 'CrRendererMain')?.tid;
 
// Filter to main thread events for most analysis
const mainEvents = events.filter(e => e.pid === rendererPid && e.tid === mainTid);

const userTimings = events.filter(e => e.cat?.includes('blink.user_timing') && !e.cat.includes('rail'));
// Each has: name (e.g. 'code/didResolveTextFileEditorModel'), ts (microseconds), args.data.startTime (ms from navigation)

const longTasks = mainEvents
    .filter(e => e.name === 'RunTask' && e.ph === 'X' && e.dur > 50000) // > 50ms
    .sort((a, b) => b.dur - a.dur);

const funcCalls = mainEvents
    .filter(e => e.name === 'FunctionCall' && e.dur > 10000) // > 10ms
    .sort((a, b) => b.dur - a.dur);
// args.data contains: functionName, url, lineNumber, columnNumber, scriptId

const layoutEvents = mainEvents.filter(e =>
    e.name === 'UpdateLayoutTree' || e.name === 'Layout' ||
    e.name === 'PrePaint' || e.name === 'Paint'
);
// UpdateLayoutTree.args.elementCount tells you how many elements were restyled

const profileEvent = events.find(e => e.name === 'Profile' && e.pid === rendererPid);
const chunks = events.filter(e => e.name === 'ProfileChunk' && e.pid === rendererPid && e.id === profileEvent.id);
 
// Each chunk's args.data.cpuProfile contains: {nodes: [...], samples: [...]}
// Each chunk's args.data.timeDeltas contains sample timing
// Merge all chunks to reconstruct a full cpuprofile-like structure
const allNodes = [];
const allSamples = [];
const allDeltas = [];
for (const chunk of chunks) {
    const cp = chunk.args.data.cpuProfile;
    if (cp.nodes) allNodes.push(...cp.nodes);
    if (cp.samples) allSamples.push(...cp.samples);
    if (chunk.args.data.timeDeltas) allDeltas.push(...chunk.args.data.timeDeltas);
}
// Now analyze allNodes/allSamples/allDeltas using the same approach as .cpuprofile

const gcEvents = mainEvents.filter(e => e.name === 'MinorGC' || e.name === 'MajorGC');
const totalGcTime = gcEvents.reduce((sum, e) => sum + (e.dur || 0), 0);
// Also check cppgc events for Blink GC
const cppgcEvents = events.filter(e => e.cat?.includes('cppgc'));

const dispatches = mainEvents.filter(e => e.name === 'EventDispatch');
// args.data.type tells you the event type: 'click', 'keydown', 'mousedown', etc.
// dur tells you how long the handler took
const longHandlers = dispatches.filter(e => e.dur > 50000).sort((a, b) => b.dur - a.dur);

import { readFileSync, statSync } from 'fs';
 
function parseProfileFromBuffer(buf) {
    // Helper: find the array value for a given key, return parsed array
    function extractArray(key) {
        const keyBuf = Buffer.from(`"${key}":[`);
        const pos = buf.indexOf(keyBuf);
        if (pos === -1) throw new Error(`${key} not found`);
        const arrayStart = pos + keyBuf.length;
        // Find matching ']' -- arrays of numbers have no nested brackets
        const arrayEnd = buf.indexOf(0x5D, arrayStart); // 0x5D = ']'
        return JSON.parse('[' + buf.subarray(arrayStart, arrayEnd).toString('utf8') + ']');
    }
 
    // Helper: find a scalar value for a given key
    function extractScalar(key) {
        const keyBuf = Buffer.from(`"${key}":`);
        const pos = buf.indexOf(keyBuf);
        if (pos === -1) throw new Error(`${key} not found`);
        const valueStart = pos + keyBuf.length;
        // Scan to next comma or closing brace
        let end = valueStart;
        while (end < buf.length && buf[end] !== 0x2C && buf[end] !== 0x7D) end++;
        return JSON.parse(buf.subarray(valueStart, end).toString('utf8'));
    }
 
    // Extract the nodes array -- contains objects, so we need bracket matching
    function extractNodesArray() {
        const keyBuf = Buffer.from('"nodes":[');
        const pos = buf.indexOf(keyBuf);
        if (pos === -1) throw new Error('nodes not found');
        const start = pos + keyBuf.length - 1; // include '['
        let depth = 0, end = -1;
        for (let i = start; i < buf.length; i++) {
            if (buf[i] === 0x5B) depth++;
            else if (buf[i] === 0x5D) { depth--; if (depth === 0) { end = i + 1; break; } }
            // Skip strings to avoid counting brackets inside them
            if (buf[i] === 0x22) { i++; while (i < buf.length) { if (buf[i] === 0x5C) i++; else if (buf[i] === 0x22) break; i++; } }
        }
        if (end === -1) throw new Error('nodes array end not found');
        return JSON.parse(buf.subarray(start, end).toString('utf8'));
    }
 
    const nodes = extractNodesArray();
    const samples = extractArray('samples');
    const timeDeltas = extractArray('timeDeltas');
    const startTime = extractScalar('startTime');
    const endTime = extractScalar('endTime');
 
    return { nodes, samples, timeDeltas, startTime, endTime };
}

import { readFileSync } from 'fs';
 
function parseTraceFromBuffer(buf) {
    // 1. Extract metadata (small, near the top of the file)
    let metadata = {};
    const metaKeyBuf = Buffer.from('"metadata":{');
    const metaPos = buf.indexOf(metaKeyBuf);
    if (metaPos !== -1) {
        const metaStart = metaPos + metaKeyBuf.length - 1; // include '{'
        let depth = 0, metaEnd = -1;
        for (let i = metaStart; i < buf.length; i++) {
            if (buf[i] === 0x7B) depth++;
            else if (buf[i] === 0x7D) { depth--; if (depth === 0) { metaEnd = i + 1; break; } }
            if (buf[i] === 0x22) { i++; while (i < buf.length) { if (buf[i] === 0x5C) i++; else if (buf[i] === 0x22) break; i++; } }
        }
        if (metaEnd !== -1) {
            metadata = JSON.parse(buf.subarray(metaStart, metaEnd).toString('utf8'));
        }
    }
 
    // 2. Extract traceEvents by parsing in chunks
    //    Each event is a JSON object {...}. Scan for object boundaries.
    const eventsKeyBuf = Buffer.from('"traceEvents":[');
    const eventsPos = buf.indexOf(eventsKeyBuf);
    if (eventsPos === -1) throw new Error('traceEvents not found');
    const arrayStart = eventsPos + eventsKeyBuf.length;
 
    const traceEvents = [];
    let i = arrayStart;
    while (i < buf.length) {
        // Skip whitespace and commas
        while (i < buf.length && (buf[i] === 0x20 || buf[i] === 0x0A || buf[i] === 0x0D || buf[i] === 0x09 || buf[i] === 0x2C)) i++;
        if (i >= buf.length || buf[i] === 0x5D) break; // end of array
 
        if (buf[i] !== 0x7B) { i++; continue; } // expect '{'
 
        // Find matching '}'
        let depth = 0, objEnd = -1;
        for (let j = i; j < buf.length; j++) {
            if (buf[j] === 0x7B) depth++;
            else if (buf[j] === 0x7D) { depth--; if (depth === 0) { objEnd = j + 1; break; } }
            if (buf[j] === 0x22) { j++; while (j < buf.length) { if (buf[j] === 0x5C) j++; else if (buf[j] === 0x22) break; j++; } }
        }
        if (objEnd === -1) break;
 
        // Parse this individual event object -- each is small enough for JSON.parse
        traceEvents.push(JSON.parse(buf.subarray(i, objEnd).toString('utf8')));
        i = objEnd;
    }
 
    return { metadata, traceEvents };
}