Firefox's Optimized Zip Format: Reading Zip Files Really Quickly
Some googling showed me that over past 11 years people have noticed that Firefox uses optimized zip files. This inspired me to document thinking behind the optimized zip format I implemented in Firefox in the pre-pandemic 2010. I had a lot of fun writing this code, was surprised that I failed to blog about it.
Zip files seem to be designed for cheap appends. For every file inside a zip, there is a Local Header + optionally compressed file # contents.
Unfortunately reading files started from ending precludes readahead. It is also suboptimal to read files from zip in random order.
Since we already do PGO2 for Firefox builds, I added a ZipArchiveLogger for logging zip-entries being accessed to the Firefox profiling stage.
So for all cases where zip file access pattern matches one recorded during profiling phase Firefox can read the relevant resources in a single IO!
https://github.com/humphd/mozilla-central-old/blob/9d4d9f265e24e6358c067ae1e300c1ce3227a91d/config/optimizejars.py. This code later got subsumed into mozpack ↩︎
