https://bugs.llvm.org/show_bug.cgi?id=38610
Bug ID: 38610
Summary: ThinLTO doesn't use all virtual cores as advertised
Product: new-bugs
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: mh+l...@glandium.org
CC: llvm-bugs@lists.llvm.org
The ThinLTO documentation (https://clang.llvm.org/docs/ThinLTO.html) says:
By default, the ThinLTO link step will launch up to
std::thread::hardware_concurrency number of threads in parallel. For machines
with hyper-threading, this is the total number of virtual cores.
That appears not to be true. On a machine with 8 cores and 16 hyper-threads,
std::thread::hardware_concurrency returns 16, but the linkage (with lld) only
uses 8 threads. I have to manually pass --thinlto-jobs=16 for 16 threads to be
used. This does make a difference, where the linkage with 16 threads takes 75%
of the time the one with 8 threads takes.
Tangentially, GCC has (opt-in) Make jobserver support, which is even better.
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