select_executor is slow

[giordano]

% julia --project=/tmp --startup-file=no -q                              
julia> using Sandbox; @time @eval Sandbox.select_executor(false)
  4.016078 seconds (10.87 M allocations: 574.060 MiB, 5.55% gc time, 99.88% compilation time)
UnprivilegedUserNamespacesExecutor

For comparison:

% julia --startup-file=no -q
julia> using BinaryBuilderBase; @time @eval BinaryBuilderBase.preferred_runner()
  0.009291 seconds (1.20 k allocations: 72.375 KiB, 97.16% compilation time)
UserNSRunner

Note that by forcing the Sandbox.jl runner with

% FORCE_SANDBOX_MODE=privileged julia --project=/tmp --startup-file=no -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false)
  2.859711 seconds (10.62 M allocations: 561.097 MiB, 7.55% gc time, 100.03% compilation time)
PrivilegedUserNamespacesExecutor

the time is cut from 4 to 3 seconds, still 300x slower than BinaryBuilderBase.preferred_runner. They're both type-unstable (of course), but Sandbox.select_executor seems to be particularly bad, even when forcing the runner.

[giordano]

As an extra datapoint:

% julia --compile=min --project=/tmp --startup-file=no -q
julia> using Sandbox; @time @eval Sandbox.select_executor(false)
  0.366757 seconds (838.45 k allocations: 42.643 MiB, 3.60% gc time, 81.86% compilation time)
UnprivilegedUserNamespacesExecutor

[giordano]

I believe most of the time is spent compiling

Sandbox.jl/src/Docker.jl

Lines 31 to 33 in ed87f07

	return with_executor(DockerExecutor) do exe
	return probe_executor(exe; test_read_only_map=true, test_read_write_map=true, verbose)
	end

[giordano]

For the record, with JuliaLang/julia#47184 and Sandbox.select_executor(false) precompiled:

% ./julia --startup-file=no -q --project=/tmp
julia> @timed @eval using Sandbox
(value = nothing, time = 0.180352858, bytes = -8781611, gctime = 0.0, gcstats = Base.GC_Diff(-8781611, 7, 0, 378317, 327, 0, 0, 0, 0))

julia> @time @eval Sandbox.select_executor(false)
  0.006543 seconds (8.59 k allocations: 539.896 KiB)
PrivilegedUserNamespacesExecutor

🚀