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os: optimise cgo clearenv #70672
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For programs with very large environments, calling unsetenv(3) for each environment variable can be fairly expensive because of CGo overhead, but clearenv(3) is much faster. The only thing we have to track is whether GODEBUG is being unset by the operation, which can be done very quickly without resorting to doing unsetenv(3) for every variable. This change makes os.Clearenv() ~30% faster when run an environment with 1000 environment variable set (and gets better with more environment variables): cpu: AMD Ryzen 7 7840U w/ Radeon 780M Graphics │ old.txt │ new.txt │ │ sec/op │ sec/op vs base │ Env-16 213.6μ ± 1% 185.1μ ± 3% -13.32% (p=0.000 n=10) (The time taken to set up 1000 variables in this benchmark is 117.2μs, so while the naive timing is 13% the speedup for os.Clearenv() itself is from 96.4μs to 67.9μs which is ~30%.) Change-Id: I589794845854e1721e6fc5badc671474c2bbd037
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For programs with very large environments, calling unsetenv(3) for each
environment variable can be fairly expensive because of CGo overhead,
but clearenv(3) is much faster. The only thing we have to track is
whether GODEBUG is being unset by the operation, which can be done very
quickly without resorting to doing unsetenv(3) for every variable.
This change makes os.Clearenv() ~30% faster when run an environment with
1000 environment variable set (and gets better with more environment
variables):
(The time taken to set up 1000 variables in this benchmark is 117.2μs,
so while the naive timing is 13% the speedup for os.Clearenv() itself is
from 96.4μs to 67.9μs which is ~30%.)
Note: Unfortunately, not all POSIX platforms support clearenv(3).
glibc/Linux has had it for a long time, but Darwin doesn't have it and
FreeBSD only got it in FreeBSD 14. So for now, we will only implement
this for Linux, but it should be trivial to add this for other platforms
that have clearenv(3).