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      Node.js trace_events 跟踪事件

      Node.js trace_events 跟踪事件

      Trace Event provides a mechanism to centralize tracing information generated by V8, Node.js core, and userspace code.

      Tracing can be enabled with the –trace-event-categories command-line flag or by using the trace_events module. The –trace-event-categories flag accepts a list of comma-separated category names.

      The available categories are:

      • node - An empty placeholder.
      • node.async_hooks - Enables capture of detailed async_hooks trace data. The async_hooks events have a unique asyncId and a special triggerId triggerAsyncId property.
      • node.bootstrap - Enables capture of Node.js bootstrap milestones.
      • node.fs.sync - Enables capture of trace data for file system sync methods.
      • node.perf - Enables capture of Performance API measurements.
        • node.perf.usertiming - Enables capture of only Performance API User Timing measures and marks.
        • node.perf.timerify - Enables capture of only Performance API timerify measurements.
      • node.promises.rejections - Enables capture of trace data tracking the number of unhandled Promise rejections and handled-after-rejections.
      • node.vm.script - Enables capture of trace data for the vm module’s runInNewContext(), runInContext(), and runInThisContext() methods.
      • v8 - The V8 events are GC, compiling, and execution related.

      By default the node, node.async_hooks, and v8 categories are enabled.

      node --trace-event-categories v8,node,node.async_hooks server.js
      

      Prior versions of Node.js required the use of the –trace-events-enabled flag to enable trace events. This requirement has been removed. However, the –trace-events-enabled flag may still be used and will enable the node, node.async_hooks, and v8 trace event categories by default.

      node --trace-events-enabled
      
      // is equivalent to
      
      node --trace-event-categories v8,node,node.async_hooks
      

      Alternatively, trace events may be enabled using the trace_events module:

      
      const trace_events = require('trace_events');
      const tracing = trace_events.createTracing({ categories: ['node.perf'] });
      tracing.enable();  // Enable trace event capture for the 'node.perf' category
      
      // do work
      
      tracing.disable();  // Disable trace event capture for the 'node.perf' category
      

      Running Node.js with tracing enabled will produce log files that can be opened in the chrome://tracing tab of Chrome.

      The logging file is by default called node_trace.${rotation}.log, where ${rotation} is an incrementing log-rotation id. The filepath pattern can be specified with –trace-event-file-pattern that accepts a template string that supports ${rotation} and ${pid}:

      node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js
      

      Starting with Node.js 10.0.0, the tracing system uses the same time source as the one used by process.hrtime() however the trace-event timestamps are expressed in microseconds, unlike process.hrtime() which returns nanoseconds.

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