Compression of the JavaScript Startup Bytecode Cache (JSBC)

[goodusername123]

Compression of the JavaScript Startup Bytecode Cache (JSBC)

[yokoffing]

the Alternative Data size column shows/holds a version of the file that was processed by Firefox in some way to reduce the amount of work needed to use it again

Ahh. Okay. Thank you for the clarification.

For my quick test, based on the Alternative Data size column:

box2d.js 757,914 bytes no compression
box2d.js 234,034 bytes level 2
box2d.js 228,689 bytes level 3
box2d.js 202,330 bytes level 7

Pretty cool.

Alternative Data size column has a version of the .js file that was parsed and converted to bytecode which is why the sizes are different, And browser.cache.jsbc_compression_level only affects the Alternative Data size.

I see now.

[yokoffing]

Thank you for the thorough explanation! This is valuable.

about:cache?storage=disk provides odd Alternative Data size when doing this. With browser.cache.jsbc_compression_level at 3, some values in the right column Alternative Data size is bigger than the left column Data size. Is that correct?

[yokoffing]

Looks like there's also the prefs to alter when bytecode is saved, in case you'd like to test page load times with dom.script_loader.bytecode_cache.strategy at -1. You could also switch to -1 temporarily to do faster testing.

// PREF: strategy to use for when the bytecode should be encoded and saved
// [1] https://searchfox.org/mozilla-release/source/modules/libpref/init/StaticPrefList.yaml#3461-3488
// -1 = saved as soon as the script is seen for the first time, independently of the size or last access time
// 0 = saved in order to minimize the page-load time (default)
//user_pref("dom.script_loader.bytecode_cache.enabled", true); // DEFAULT
//user_pref("dom.script_loader.bytecode_cache.strategy", 0); // DEFAULT

[goodusername123]

Thank you for the thorough explanation! This is valuable.

about:cache?storage=disk provides odd Alternative Data size when doing this. With browser.cache.jsbc_compression_level at 3, some values in the right column Alternative Data size is bigger than the left column Data size. Is that correct?

The Data size column shows/holds the unprocessed file as it was cached/downloaded from the URL/server while the Alternative Data size column shows/holds a version of the file that was processed by Firefox in some way to reduce the amount of work needed to use it again so in the case of .js files the Alternative Data size column has a version of the .js file that was parsed and converted to bytecode which is why the sizes are different, And browser.cache.jsbc_compression_level only affects the Alternative Data size.

More information about both the JSBC system which caches compiled JavaScript bytecode and the Alternative Data system/column are available at this link: https://blog.mozilla.org/javascript/2017/12/12/javascript-startup-bytecode-cache/