From df634904b60f0aa17a99afd225c58a7605c9ee37 Mon Sep 17 00:00:00 2001
From: Eric Wong <normalperson@yhbt.net>
Date: Fri, 27 Dec 2013 08:08:50 +0000
Subject: merge some select calls for splice users

Avoid an additional select syscall in the splice path by injecting
the target checks into the main loop.  We can do this because we
always process writers before readers.  This adds additional
userspace processing, but it avoids one potentially expensive (and
potentially task-switching) syscall in many cases; so it should be
worth it to avoid skipping with small buffer.

This should avoid buffer underuns with mixed-sized buffers when
using multiple sinks.

This could work for the read_write path, too, but I don't
use that enough and this change may still be buggy even
for splice users.
---
 lib/dtas/buffer/splice.rb |  4 +++-
 lib/dtas/player.rb        |  8 ++++++++
 lib/dtas/writable_iter.rb | 16 ++++++++++++++++
 3 files changed, 27 insertions(+), 1 deletion(-)

(limited to 'lib/dtas')

diff --git a/lib/dtas/buffer/splice.rb b/lib/dtas/buffer/splice.rb
index 9093860..2a7e9a4 100644
--- a/lib/dtas/buffer/splice.rb
+++ b/lib/dtas/buffer/splice.rb
@@ -69,7 +69,9 @@ module DTAS::Buffer::Splice # :nodoc:
   end
 
   def broadcast_inf(targets)
-    if targets.none?(&:nonblock?)
+    if targets.all?(&:ready_write_optimized?)
+      blocked = []
+    elsif targets.none?(&:nonblock?)
       # if all targets are blocking, don't start until they're all writable
       r = IO.select(nil, targets, nil, 0) or return targets
       blocked = targets - r[1]
diff --git a/lib/dtas/player.rb b/lib/dtas/player.rb
index 56fbdac..04c03b0 100644
--- a/lib/dtas/player.rb
+++ b/lib/dtas/player.rb
@@ -300,6 +300,13 @@ class DTAS::Player # :nodoc:
     end
   end
 
+  def _optimize_write_prepare(targets)
+    targets.each do |dst|
+      dst.wait_writable_prepare
+      @srv.wait_ctl(dst, :wait_writable)
+    end
+  end
+
   # returns a wait_ctl arg for self
   def broadcast_iter(buf, targets)
     case rv = buf.broadcast(targets)
@@ -315,6 +322,7 @@ class DTAS::Player # :nodoc:
       # via DTAS::Sink#writable_iter
       :ignore
     else # :wait_readable or nil
+      _optimize_write_prepare(targets)
       rv
     end
   end
diff --git a/lib/dtas/writable_iter.rb b/lib/dtas/writable_iter.rb
index 1fd65eb..2355352 100644
--- a/lib/dtas/writable_iter.rb
+++ b/lib/dtas/writable_iter.rb
@@ -4,9 +4,24 @@ require_relative '../dtas'
 
 module DTAS::WritableIter # :nodoc:
   attr_accessor :on_writable
+  # we may use the ready_write flag to avoid an extra IO.select
+  attr_accessor :ready_write
 
   def writable_iter_init
+    @mark_writable = proc { @ready_write = true }
     @on_writable = nil
+    @ready_write = true
+  end
+
+  def ready_write_optimized?
+    rv = @ready_write
+    @ready_write = false
+    rv
+  end
+
+  def wait_writable_prepare
+    @ready_write = false
+    @on_writable ||= @mark_writable
   end
 
   # this is used to exchange our own writable status for the readable
@@ -14,6 +29,7 @@ module DTAS::WritableIter # :nodoc:
   def writable_iter
     if owr = @on_writable
       @on_writable = nil
+      @ready_write = true
       owr.call # this triggers readability watching of DTAS::Buffer
     end
     :ignore
-- 
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