The JavaScript Start-up Bytecode Cache⁰ is a project which aims at reducing
the page load time by recording the bytecode generated during the last
visits and by-pass the JavaScript parser.
This project changes the way we process JavaScript script tags which are
fetched from the network, and cached. After multiple visits¹, the bytecode
would be encoded incrementally², as soon as the bytecode emitter generates
it. Once we reached some idle time³, we save the content encoded
incrementally as an alternate data on the cache⁴. The cache contains a
compressed version of the source, the bytecode of functions which got
executed during the start-up of the page, and all non-executed functions
encoded as source indexes⁵.
On follow-up visits the script loader would load the alternate data instead⁶
of the source, and decode the bytecode either off-thread⁷ or on the
current-thread. This is expected to replace the syntax checker and the
bytecode emitter for all recorded functions.
This feature is currently pref-ed off and can be enabled by setting the
following preferences in about:config⁸:
- dom.script_loader.bytecode_cache.enabled = true
- dom.script_loader.bytecode_cache.strategy = 0
For any issue caused by this optimization, filed it as a blocker of Bug 900784.
In the upcoming days, I will add telemetry probes to better tune the
heuristics¹ for the web and monitor the known sources of fallback and
failures. In addition, I will request for a pref-experiment, such that we
can get more data from nightly users. At the moment, I expect to enable this
feature around mid-July.
⁰ https://bugzilla.mozilla.org/show_bug.cgi?id=900784
¹ These are heuristics which would be customized by running a
pref-experiment. (see https://bugzilla.mozilla.org/show_bug.cgi?id=1362114)
² We cannot do it off-thread, nor after the execution (see
https://bugzilla.mozilla.org/show_bug.cgi?id=1316081)
³ Currently set to the next cycle after the processing of the OnLoad event.
(see https://bugzilla.mozilla.org/show_bug.cgi?id=1372207)
⁴ Thanks to Valentin Gosu for his work and support on the alternate data
interface as part of necko. (see
https://bugzilla.mozilla.org/show_bug.cgi?id=1231565)
⁵ https://bugzilla.mozilla.org/show_bug.cgi?id=917996
⁶ This forces us to store the compressed source as part of the encoded
bytecode, but prevent additional round-trip between the parent and child
processes.
⁷ https://bugzilla.mozilla.org/show_bug.cgi?id=1316078
⁸
http://searchfox.org/mozilla-central/rev/d840ebd5858a61dbc1622487c1fab74ecf235e03/modules/libpref/init/all.js#212-233
--
Nicolas B. Pierron
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