On 03/08/2017 06:21 AM, Ehsan Akhgari wrote:
On 2017-03-07 2:49 PM, Eric Rahm wrote:
I often wonder if unified builds are making things slower for folks who use
ccache (I assume one file changing would mean a rebuild for the entire
unified chunk), I'm not sure if there's a solution to that but it would be
interesting to see if compiling w/o ccache is actually faster at this point.
Unified builds are the only way we build so this is only of theoretical
interest. But at any rate, if your use case is building the tree once
every couple of days, you should definitely disable ccache with or
without unified builds. ccache is only helpful for folks who end up
compiling the same code over and over again (for example, if you use
interactive rebase a lot, or if you switch between branches, or use
other VCS commands that touch the file modified times without changing
the dates and times. Basically if you don't switch between branches a
lot and don't write a lot of C++ code, ccache probably hurts you more
than it helps you.
We only build unified, but it's not a binary thing -- within the JS
engine, I found that unified builds substantially speed up full
compiles, but also substantially slow down incremental compiles. And
that wasn't a great tradeoff when it is pretty common to iterate on a
single C++ file, since you'd end up rebuilding the concatenation of
dozens of files, which took a long time to get the compile errors out
of. I found setting FILES_PER_UNIFIED_FILE to 6 to be a pretty good
compromise [1], but it depends on the average size of your .cpp files
and the common usage patterns of people using that portion of the tree.
Oh, and the number of files in a directory -- if you don't have that
many files and you have a lot of cores, then you want enough chunks to
keep your cores busy.
Also note that dropping the unified sizes did *not* speed up the time to
get a new JS shell binary after a single-cpp change, oddly enough. The
compile time was much faster, but it made up for it seemingly by moving
the time into a slower link. I guess unified compilation is sort of
doing some of the linking up-front? Still, when iterations are bound by
the time it takes to see the latest round of compile errors, dropping
the unified chunk sizes can be a pretty big win. You might want to
consider it for your component.
1. http://searchfox.org/mozilla-central/source/js/src/moz.build#14
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