From 529cae35e93975ba16955541aab4fd029d9d1e6f Mon Sep 17 00:00:00 2001
From: Badrish Chandramouli <badrishc@microsoft.com>
Date: Mon, 15 Nov 2021 11:25:27 -0800
Subject: [PATCH 1/2] Make PendingFlushList never error out in stressful
 situations. Current implementation uses lock for ~5-10% perf degradation over
 prior structure.

---
 cs/src/core/Allocator/AllocatorBase.cs    | 20 +++-------
 cs/src/core/Allocator/PendingFlushList.cs | 47 +++++++++--------------
 2 files changed, 24 insertions(+), 43 deletions(-)

diff --git a/cs/src/core/Allocator/AllocatorBase.cs b/cs/src/core/Allocator/AllocatorBase.cs
index 3a089d3f8..49ff3e28e 100644
--- a/cs/src/core/Allocator/AllocatorBase.cs
+++ b/cs/src/core/Allocator/AllocatorBase.cs
@@ -1752,22 +1752,12 @@ public void AsyncFlushPages(long fromAddress, long untilAddress)
                     }
 
                     // Enqueue work in shared queue
-                    if (PendingFlush[index].Add(asyncResult))
-                    {
-                        // Perform work from shared queue if possible
-                        if (PendingFlush[index].RemoveNextAdjacent(FlushedUntilAddress, out PageAsyncFlushResult<Empty> request))
-                        {
-                            WriteAsync(request.fromAddress >> LogPageSizeBits, AsyncFlushPageCallback, request);
-                        }
-                    }
-                    else
+                    PendingFlush[index].Add(asyncResult);
+
+                    // Perform work from shared queue if possible
+                    if (PendingFlush[index].RemoveNextAdjacent(FlushedUntilAddress, out PageAsyncFlushResult<Empty> request))
                     {
-                        // Because we are invoking the callback away from the usual codepath, need to externally
-                        // ensure that flush address are updated in order
-                        while (FlushedUntilAddress < asyncResult.fromAddress) Thread.Yield();
-                        // Could not add to pending flush list, treat as a failed write
-                        // Use a special errorCode to convey that this is not from a syscall
-                        AsyncFlushPageCallback(16000, 0, asyncResult);
+                        WriteAsync(request.fromAddress >> LogPageSizeBits, AsyncFlushPageCallback, request);
                     }
                 }
                 else
diff --git a/cs/src/core/Allocator/PendingFlushList.cs b/cs/src/core/Allocator/PendingFlushList.cs
index 565ee9fc0..c38d964cc 100644
--- a/cs/src/core/Allocator/PendingFlushList.cs
+++ b/cs/src/core/Allocator/PendingFlushList.cs
@@ -1,38 +1,25 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
-using System.Threading;
+using System.Collections.Generic;
 
 namespace FASTER.core
 {
     class PendingFlushList
     {
-        const int maxSize = 8;
-        const int maxRetries = 10;
-        public PageAsyncFlushResult<Empty>[] list;
+        public readonly LinkedList<PageAsyncFlushResult<Empty>> list;
 
         public PendingFlushList()
         {
-            list = new PageAsyncFlushResult<Empty>[maxSize];
+            list = new();
         }
 
-        public bool Add(PageAsyncFlushResult<Empty> t)
+        public void Add(PageAsyncFlushResult<Empty> t)
         {
-            int retries = 0;
-            do
+            lock (list)
             {
-                for (int i = 0; i < maxSize; i++)
-                {
-                    if (list[i] == default)
-                    {
-                        if (Interlocked.CompareExchange(ref list[i], t, default) == default)
-                        {
-                            return true;
-                        }
-                    }
-                }
-            } while (retries++ < maxRetries);
-            return false;
+                list.AddFirst(t);
+            }
         }
 
         /// <summary>
@@ -40,15 +27,17 @@ public bool Add(PageAsyncFlushResult<Empty> t)
         /// </summary>
         public bool RemoveNextAdjacent(long address, out PageAsyncFlushResult<Empty> request)
         {
-            for (int i=0; i<maxSize; i++)
+            lock (list)
             {
-                request = list[i];
-                if (request?.fromAddress == address)
+                for (var it = list.First; it != null;)
                 {
-                    if (Interlocked.CompareExchange(ref list[i], null, request) == request)
+                    request = it.Value;
+                    if (request.fromAddress == address)
                     {
+                        list.Remove(it);
                         return true;
                     }
+                    it = it.Next;
                 }
             }
             request = null;
@@ -60,15 +49,17 @@ public bool RemoveNextAdjacent(long address, out PageAsyncFlushResult<Empty> req
         /// </summary>
         public bool RemovePreviousAdjacent(long address, out PageAsyncFlushResult<Empty> request)
         {
-            for (int i = 0; i < maxSize; i++)
+            lock (list)
             {
-                request = list[i];
-                if (request?.untilAddress == address)
+                for (var it = list.First; it != null;)
                 {
-                    if (Interlocked.CompareExchange(ref list[i], null, request) == request)
+                    request = it.Value;
+                    if (request.untilAddress == address)
                     {
+                        list.Remove(it);
                         return true;
                     }
+                    it = it.Next;
                 }
             }
             request = null;

From b7fb2f770eb451c875d7b5d3480d0db6106bfd7f Mon Sep 17 00:00:00 2001
From: Badrish Chandramouli <badrishc@microsoft.com>
Date: Mon, 15 Nov 2021 11:25:43 -0800
Subject: [PATCH 2/2] nit

---
 cs/src/core/Allocator/PendingFlushList.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/cs/src/core/Allocator/PendingFlushList.cs b/cs/src/core/Allocator/PendingFlushList.cs
index c38d964cc..a3776dddd 100644
--- a/cs/src/core/Allocator/PendingFlushList.cs
+++ b/cs/src/core/Allocator/PendingFlushList.cs
@@ -5,7 +5,7 @@
 
 namespace FASTER.core
 {
-    class PendingFlushList
+    sealed class PendingFlushList
     {
         public readonly LinkedList<PageAsyncFlushResult<Empty>> list;