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Tracing V8

Peiyong Lin edited this page Oct 31, 2016 · 4 revisions
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Introduction

Currently, V8 provides support for tracing. It works automatically when V8 is embedded in Chrome through the Chrome tracing system. But you can also enable it in any standalone V8 or within an embedder that uses the Default Platform. More details about the trace-viewer can be found here.

Tracing in d8

To start tracing, use the --enable-tracing option. V8 generates a v8_trace.json that you can open in Chrome. To open it in Chrome, go to "chrome://tracing" then click on Load and then load the v8-trace.json file.

Each trace event is a associated with a set of categories, you can enable/disable the recording of trace events based on their categories. With the above flag only, we only enable the default categories (a set of categories that has a low overhead). To enable more categories and have a more fine control of the different parameters you will need to pass a config file. An example of a config file traceconfig.json:

{
 "record_mode": "record-continuously",
 "included_categories": ["v8", "disabled-by-default-v8.runtime_stats"]
}

An example of calling d8 with tracing and a traceconfig file:

d8 --enable-tracing --trace-config=traceconfig.json

The trace config format is compatible with the one of Chrome Tracing, however, we don't support regular expression in included categories list, and V8 doesn't need excluded categories list, thus the trace config file for V8 can be reused in Chrome tracing, but you can't reuse Chrome trace config file in V8 tracing if the trace config file contains regular expression, besides, V8 will ignore excluded categories list.

Enabling Runtime Call Statistics in tracing

To get Runtime Call Statistics, please record the trace with the following 2 categories enabled: v8 and disabled-by-default-v8.runtime_stats. Each top level V8 trace event will contain the runtime statistics for the period of that event. By selecting any of those events in trace-viewer, the runtime stats table will be displayed in the lower panel, selecting multiple will create a merged view.

Enabling GC Object Statistics in tracing

To get the GC Object Statistics in tracing, you need to collect a trace with disabled-by-default.v8.gc_stats category enabled also you need to use the following js-flags:

--track_gc_object_stats --noincremental-marking

Once you load the trace in trace-viewer, search for slices named: V8.GC_Object_Stats, in the lower panel you will find the statistics. Selecting multiple slices will create a merged view.