[Issue 14770] New: std.process should use lightweight forks where available

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=14770

          Issue ID: 14770
           Summary: std.process should use lightweight forks where
                    available
           Product: D
           Version: D2
          Hardware: x86_64
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: rsw0x at rsw0x.me

fork(even with CoW pages) can be extremely slow with modern memory usages due
to the antiquated default page size of OSes because it requires copying the
page table.

For example, a program using 400MiB of memory with 4KiB pages requires copying
102,400 page structs. On Linux, a page struct in the kernel is at least 72
bytes so this requires a copy of 7.2MiB.

This scales linearly with heap size, after some personal testing on Linux I
found forking with almost nothing allocated to take ~60 microseconds, 100 MiB
~3 milliseconds, and 1GiB to take 30-45 milliseconds. vfork took a constant 20
microseconds no matter the heap size.

Solution: where available, use posix_spawn, vfork, etc. These do not require
copying page tables. std.process does not require the page tables since it
immediately replaces itself with another process.

I marked this all OSes, but really it's all POSIX OSes I guess.

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