[ruby-core:42743] [ruby-trunk - Feature #5138] Add nonblocking IO that does not use exceptions for EOF and EWOULDBLOCK

[Charles Nutter <headius@headius.com>]

Issue #5138 has been updated by Charles Nutter.


I never got to weigh in on this thread back in the day, and I'm working on IO stuff more recently, so a few thoughts...

Eric Wong wrote:
>  * Exceptions with the current *_nonblock methods are expensive.
>  
>    - extending with IO::Wait* increments the global VM state version

This is really poor form. I'm not sure why it hasn't been fixed yet. JRuby master solves this in a neat, compatible way: instead of raising singleton EAGAIN with WaitReable or WaitWritable mixed in, we raise EAGAINReadable or EAGAINWritable, subclasses that have already done the mixing. No other changes are needed.

>    - backtrace generation cost

In JRuby, EAGAIN does not generate a backtrace unless you pass a flag, since generally it's an "expected" exception and you don't want the backtrace anyway.

How does this translate to perf?

system ~/projects $ jruby eagain_bench.rb 
  1.553000   0.000000   1.553000 (  1.553000)
  0.673000   0.000000   0.673000 (  0.673000)
  0.667000   0.000000   0.667000 (  0.667000)
  0.669000   0.000000   0.669000 (  0.669000)
  0.696000   0.000000   0.696000 (  0.697000)

system ~/projects $ rvm 1.9.3 do ruby eagain_bench.rb 
  1.250000   0.150000   1.400000 (  1.396295)
  1.250000   0.150000   1.400000 (  1.408975)
  1.260000   0.150000   1.410000 (  1.405947)
  1.250000   0.150000   1.400000 (  1.405391)
  1.260000   0.150000   1.410000 (  1.407389)

system ~/projects $ rvm ruby-head do ruby eagain_bench.rb 
  1.310000   0.120000   1.430000 (  1.428627)
  1.300000   0.120000   1.420000 (  1.430652)
  1.310000   0.130000   1.440000 (  1.426011)
  1.310000   0.120000   1.430000 (  1.430642)
  1.310000   0.120000   1.430000 (  1.436441)

Rather well, I think.
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Feature #5138: Add nonblocking IO that does not use exceptions for EOF and EWOULDBLOCK
https://bugs.ruby-lang.org/issues/5138

Author: Yehuda Katz
Status: Open
Priority: Normal
Assignee: Yukihiro Matsumoto
Category: core
Target version: 1.9.4


The current Ruby I/O classes have non-blocking methods (read_nonblock and write_nonblock). These methods will never block, and if they would block, they raise an exception instead (IO::WaitReadable or IO::WaitWritable). In addition, if the IO is at EOF, they raise an EOFError.

These exceptions are raised repeatedly in virtually every use of the non-blocking methods. This patch adds a pair of methods (try_read_nonblock and try_write_nonblock) that have the same semantics as the existing methods, but they return Symbols instead of raising exceptions for these routine cases:

* :read_would_block
* :write_would_block
* :eof

The patch contains updates for IO, StringIO, and OpenSSL. The updates are fully documented and tested.


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