Feature #11377: [PATCH] IO.copy_stream uses poll on Linux

• Status: Closed
• Assignee: normalperson (Eric Wong)
• Target version: -

[normalperson (Eric Wong)]

poll and ppoll have a superior API which doesn't require the
kernel to scan a potentially large bitmap to find a high-numbered
FD [ruby-core:35572]. So favor using poll in case IO.copy_stream
encounters a non-blocking FD.

We cannot reliably use poll on most OSes, because file types (e.g.
FIFOs) which work with select may not work with poll. Fortunately,
Linux uses a common notification mechanism between all
select/poll/epoll variants, so all file types are equally supported
between the notification mechanisms.

Verified by watching strace on the following scripts:

*** maygvl_copy_stream_wait_read ***
require 'io/nonblock'
r, w = IO.pipe
r.nonblock = true
IO.copy_stream(r, "/dev/null")

*** nogvl_copy_stream_wait_write ***
require 'io/nonblock'
r, w = IO.pipe
w.nonblock = true
IO.copy_stream("/dev/zero", w)

• io.c (nogvl_wait_for_single_fd): new function for Linux
  (maygvl_copy_stream_wait_read): Linux-specific version
  (nogvl_copy_stream_wait_write): use nogvl_wait_for_single_fd

• Status changed from Open to Closed

Applied in changeset r51305.

---

io.c: IO.copy_stream uses poll on Linux

poll and ppoll have a superior API which doesn't require the
kernel to scan a potentially large bitmap to find a high-numbered
FD [ruby-core:35572]. So favor using poll in case IO.copy_stream
encounters a non-blocking FD.

We cannot reliably use poll on most OSes, because file types (e.g.
FIFOs) which work with select may not work with poll. Fortunately,
Linux uses a common notification mechanism between all
select/poll/epoll variants, so all file types are equally supported
between the notification mechanisms.

Verified by watching strace on the following scripts:

*** maygvl_copy_stream_wait_read ***
require 'io/nonblock'
r, w = IO.pipe
r.nonblock = true
IO.copy_stream(r, "/dev/null")

*** nogvl_copy_stream_wait_write ***
require 'io/nonblock'
r, w = IO.pipe
w.nonblock = true
IO.copy_stream("/dev/zero", w)

• io.c (nogvl_wait_for_single_fd): new function for Linux
  (maygvl_copy_stream_wait_read): Linux-specific version
  (nogvl_copy_stream_wait_write): use nogvl_wait_for_single_fd
  [ruby-core:70051] [Feature #11377]