From b70f62a64a455ab0ef6f1df02ef14ae52388950b Mon Sep 17 00:00:00 2001
From: Julien Portalier <julien@portalier.com>
Date: Wed, 8 Jan 2025 15:04:59 +0100
Subject: [PATCH 1/3] Use Crystal::PointerLinkedList instead of Deque in Mutex

The flat array also doesn't have much impact since it's never traversed:
we only reach the head or the tail once (to dequeue or dequeue one).

This spares a number of GC allocations since the Deque needs to be
allocated and also a buffer that will have to be reallocated sometimes
(and will only ever grow).
---
 src/fiber/waiting.cr | 13 +++++++++++++
 src/mutex.cr         | 20 ++++++++++++--------
 src/wait_group.cr    | 16 +++-------------
 3 files changed, 28 insertions(+), 21 deletions(-)
 create mode 100644 src/fiber/waiting.cr

diff --git a/src/fiber/waiting.cr b/src/fiber/waiting.cr
new file mode 100644
index 000000000000..8c4d0138a9f1
--- /dev/null
+++ b/src/fiber/waiting.cr
@@ -0,0 +1,13 @@
+class Fiber
+  # :nodoc:
+  struct Waiting
+    include Crystal::PointerLinkedList::Node
+
+    def initialize(@fiber : Fiber)
+    end
+
+    def enqueue : Nil
+      @fiber.enqueue
+    end
+  end
+end
diff --git a/src/mutex.cr b/src/mutex.cr
index 780eac468201..ad9bc1ac2ca5 100644
--- a/src/mutex.cr
+++ b/src/mutex.cr
@@ -1,3 +1,4 @@
+require "fiber/waiting"
 require "crystal/spin_lock"
 
 # A fiber-safe mutex.
@@ -22,9 +23,9 @@ class Mutex
   @state = Atomic(Int32).new(UNLOCKED)
   @mutex_fiber : Fiber?
   @lock_count = 0
-  @queue = Deque(Fiber).new
+  @queue = Crystal::PointerLinkedList(Fiber::Waiting).new
   @queue_count = Atomic(Int32).new(0)
-  @lock = Crystal::SpinLock.new
+  @spin = Crystal::SpinLock.new
 
   enum Protection
     Checked
@@ -59,7 +60,9 @@ class Mutex
     loop do
       break if try_lock
 
-      @lock.sync do
+      waiting = Fiber::Waiting.new(Fiber.current)
+
+      @spin.sync do
         @queue_count.add(1)
 
         if @state.get(:relaxed) == UNLOCKED
@@ -71,7 +74,7 @@ class Mutex
           end
         end
 
-        @queue.push Fiber.current
+        @queue.push pointerof(waiting)
       end
 
       Fiber.suspend
@@ -116,17 +119,18 @@ class Mutex
       return
     end
 
-    fiber = nil
-    @lock.sync do
+    waiting = nil
+    @spin.sync do
       if @queue_count.get == 0
         return
       end
 
-      if fiber = @queue.shift?
+      if waiting = @queue.shift?
         @queue_count.add(-1)
       end
     end
-    fiber.enqueue if fiber
+
+    waiting.try(&.value.enqueue)
   end
 
   def synchronize(&)
diff --git a/src/wait_group.cr b/src/wait_group.cr
index cf1ca8900e8f..efa19f15e1b4 100644
--- a/src/wait_group.cr
+++ b/src/wait_group.cr
@@ -1,4 +1,5 @@
 require "fiber"
+require "fiber/waiting"
 require "crystal/spin_lock"
 require "crystal/pointer_linked_list"
 
@@ -31,17 +32,6 @@ require "crystal/pointer_linked_list"
 # wg.wait
 # ```
 class WaitGroup
-  private struct Waiting
-    include Crystal::PointerLinkedList::Node
-
-    def initialize(@fiber : Fiber)
-    end
-
-    def enqueue : Nil
-      @fiber.enqueue
-    end
-  end
-
   # Yields a `WaitGroup` instance and waits at the end of the block for all of
   # the work enqueued inside it to complete.
   #
@@ -59,7 +49,7 @@ class WaitGroup
   end
 
   def initialize(n : Int32 = 0)
-    @waiting = Crystal::PointerLinkedList(Waiting).new
+    @waiting = Crystal::PointerLinkedList(Fiber::Waiting).new
     @lock = Crystal::SpinLock.new
     @counter = Atomic(Int32).new(n)
   end
@@ -128,7 +118,7 @@ class WaitGroup
   def wait : Nil
     return if done?
 
-    waiting = Waiting.new(Fiber.current)
+    waiting = Fiber::Waiting.new(Fiber.current)
 
     @lock.sync do
       # must check again to avoid a race condition where #done may have

From 098cb66f7fced244de48040196cd5187bd3ba29e Mon Sep 17 00:00:00 2001
From: Julien Portalier <julien@portalier.com>
Date: Tue, 14 Jan 2025 09:13:55 +0100
Subject: [PATCH 2/3] fix: revert `@lock` to `@spin` rename

---
 src/mutex.cr | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/mutex.cr b/src/mutex.cr
index ad9bc1ac2ca5..956485a4cb3c 100644
--- a/src/mutex.cr
+++ b/src/mutex.cr
@@ -25,7 +25,7 @@ class Mutex
   @lock_count = 0
   @queue = Crystal::PointerLinkedList(Fiber::Waiting).new
   @queue_count = Atomic(Int32).new(0)
-  @spin = Crystal::SpinLock.new
+  @lock = Crystal::SpinLock.new
 
   enum Protection
     Checked
@@ -62,7 +62,7 @@ class Mutex
 
       waiting = Fiber::Waiting.new(Fiber.current)
 
-      @spin.sync do
+      @lock.sync do
         @queue_count.add(1)
 
         if @state.get(:relaxed) == UNLOCKED
@@ -120,7 +120,7 @@ class Mutex
     end
 
     waiting = nil
-    @spin.sync do
+    @lock.sync do
       if @queue_count.get == 0
         return
       end

From 895345b963df341077585a3323a9866c7de95c57 Mon Sep 17 00:00:00 2001
From: Julien Portalier <julien@portalier.com>
Date: Tue, 14 Jan 2025 09:16:47 +0100
Subject: [PATCH 3/3] Rename Fiber::Waiting as Fiber::PointerLinkedListNode

---
 src/fiber/{waiting.cr => pointer_linked_list_node.cr} | 4 +++-
 src/mutex.cr                                          | 6 +++---
 src/wait_group.cr                                     | 7 +++----
 3 files changed, 9 insertions(+), 8 deletions(-)
 rename src/fiber/{waiting.cr => pointer_linked_list_node.cr} (71%)

diff --git a/src/fiber/waiting.cr b/src/fiber/pointer_linked_list_node.cr
similarity index 71%
rename from src/fiber/waiting.cr
rename to src/fiber/pointer_linked_list_node.cr
index 8c4d0138a9f1..45994fe5c489 100644
--- a/src/fiber/waiting.cr
+++ b/src/fiber/pointer_linked_list_node.cr
@@ -1,6 +1,8 @@
+require "crystal/pointer_linked_list"
+
 class Fiber
   # :nodoc:
-  struct Waiting
+  struct PointerLinkedListNode
     include Crystal::PointerLinkedList::Node
 
     def initialize(@fiber : Fiber)
diff --git a/src/mutex.cr b/src/mutex.cr
index 956485a4cb3c..14d1aedf7923 100644
--- a/src/mutex.cr
+++ b/src/mutex.cr
@@ -1,4 +1,4 @@
-require "fiber/waiting"
+require "fiber/pointer_linked_list_node"
 require "crystal/spin_lock"
 
 # A fiber-safe mutex.
@@ -23,7 +23,7 @@ class Mutex
   @state = Atomic(Int32).new(UNLOCKED)
   @mutex_fiber : Fiber?
   @lock_count = 0
-  @queue = Crystal::PointerLinkedList(Fiber::Waiting).new
+  @queue = Crystal::PointerLinkedList(Fiber::PointerLinkedListNode).new
   @queue_count = Atomic(Int32).new(0)
   @lock = Crystal::SpinLock.new
 
@@ -60,7 +60,7 @@ class Mutex
     loop do
       break if try_lock
 
-      waiting = Fiber::Waiting.new(Fiber.current)
+      waiting = Fiber::PointerLinkedListNode.new(Fiber.current)
 
       @lock.sync do
         @queue_count.add(1)
diff --git a/src/wait_group.cr b/src/wait_group.cr
index efa19f15e1b4..003921bd9f46 100644
--- a/src/wait_group.cr
+++ b/src/wait_group.cr
@@ -1,7 +1,6 @@
 require "fiber"
-require "fiber/waiting"
+require "fiber/pointer_linked_list_node"
 require "crystal/spin_lock"
-require "crystal/pointer_linked_list"
 
 # Suspend execution until a collection of fibers are finished.
 #
@@ -49,7 +48,7 @@ class WaitGroup
   end
 
   def initialize(n : Int32 = 0)
-    @waiting = Crystal::PointerLinkedList(Fiber::Waiting).new
+    @waiting = Crystal::PointerLinkedList(Fiber::PointerLinkedListNode).new
     @lock = Crystal::SpinLock.new
     @counter = Atomic(Int32).new(n)
   end
@@ -118,7 +117,7 @@ class WaitGroup
   def wait : Nil
     return if done?
 
-    waiting = Fiber::Waiting.new(Fiber.current)
+    waiting = Fiber::PointerLinkedListNode.new(Fiber.current)
 
     @lock.sync do
       # must check again to avoid a race condition where #done may have