pantsbuild / pants / 28692728896

Build # 28692728896

Build Type

push

github

Committed by web-flow

Commit Message

Permit vfork fast-path within local `execute_process` (#23466)

Replaces use of `tokio::process::Command::pre_exec()` to establish a new
process group for the spawned child instead with
`tokio::proces::Command::process_group()`, as the former requires a
"slow" `fork -> (pre_exec closures)* -> exec` codepath, and the latter
permits a `vfork -> exec`-based fast-path.

This improves local build performance relative to both the resident set
size that the Pants process builds up (i.e. through in-memory caching of
rule results), and to the number of processes spawned through
`execute_process()`.

The "slow" path translates to underlying `clone()` syscalls without the
`CLONE_VFORK|CLONE_VM` flags, which suffer latency relative to the
parent process resident set size due to duplication of page tables for
the child, a cost which can become considerable for parent processes
(i.e., `pants`) that have allocated significant memory (e.g., cached
rule results), and can become meaningful for local Pants builds spawning
many processes.

This change permits the runtime to take the "fast" `vfork`-based path
(using `clone()` with `CLONE_VFORK|CLONE_VM`), which has ~constant
latency irrespective of parent memory allocation. "Permits", because
this code change alone does not ensure the fast path is taken;
minimally, the environment must also supply a glibc v2.29+, the earliest
version providing another required ingredient for taking the `vfork`
fast path (`posix_spawn_file_actions_addchdir_np()`).

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