tl;dr You may experience a change in behavior in build performance
In bug 884587, we just changed how file purging occurs during builds.
Due to historical reasons (notably bad build dependencies and to prevent
old files from leaking into the distribution package), we wipe out large
parts of objdir/dist at the beginning of a top-level build [1].
Until recently, we simply did a |rm -r| because there was no way for the
build system to know which files should or shouldn't be retained.
The moz.build transition has given us a "whole world view" that now
allows us to identify which files should or should not belong. Instead
of performing |rm -r|, we now read in manifest files, scan directories
being purged, and selectively delete files that are unaccounted for.
An unfortunate side-effect of this change is that we need to walk parts
of objdir at the top of the build, which may be slow, especially if this
directory isn't in the filesystem/stat cache. This may result in a
multi-second "jank" at the beginning of the build. Hopefully, this jank
is offset by the build performing less work (deleting fewer files at the
top of the build means fewer files need to be restored during the build).
If you notice "unacceptable" jank during purging and you aren't building
on an SSD, please buy an SSD (if you can). I'll say it again: *please
build Firefox on an SSD if you can*. If you have an SSD, I will be very
interested in investigating further. File a new bug that depends on bug
884587 and we'll look into things.
[1]
https://hg.mozilla.org/integration/mozilla-inbound/rev/8d90527c22c6#l1.12
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