On 04/25/2018 08:38 PM, Bobby Holley wrote:
On Wed, Apr 25, 2018 at 10:21 AM, Ted Mielczarek <t...@mielczarek.org> wrote:
On Wed, Apr 25, 2018, at 12:32 PM, Jeff Muizelaar wrote:
At minimum we should make --enable-profiling build with rust-opt.
This sounds reasonable, although the quirk is that we default
--enable-profiling on for nightly[1], so anyone building m-c will have it
enabled. We could make the build system only do this for "explicitly
enabled --enable-profiling", it just might be slightly confusing. (For
reference, the rustc opt level is controlled here[2].)
We could also change this default. In general, Nightly is usually fine for
profiling, so I think the set of people doing local builds for profiling is
small enough that we could reasonably expect them to set the flag in their
mozconfig.
Somewhat off-topic, but we tried hard last year to increase the number of
people doing profiling,
and Quantum Flow is still ongoing and performance as important as ever.
Profiling should be part of daily development work.
Because of that it would be great to have local builds profile-able by default.
But still, I kind of like the idea of build profiles. Something which screams
in the terminal what kind of build one is about
to get. At the beginning of build process and I guess end too there would be a
message hinting whether the build is good for profiling or so.
IIRC the slowdown from using opt-level=2 vs. 1 is primarily in LLVM? It
would probably be useful if we instrumented the build to record compilation
times for all of the crates we build at various optimization levels and see
where the biggest issues are. Perhaps we could get someone on the Rust team
to make some improvements if we know what the worst offenders are.
Well, there's the opt level thing, but there's also the much larger bit
that we don't do Rust LTO without --enable-release, which IIUC has a
substantial performance impact.
-Ted
1. https://dxr.mozilla.org/mozilla-central/rev/
26e53729a10976f52e75efa44e17b5e054969fec/js/moz.configure#243
2. https://dxr.mozilla.org/mozilla-central/rev/
26e53729a10976f52e75efa44e17b5e054969fec/build/moz.
configure/toolchain.configure#1397
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