Use custom BlockingQueue to significantly improve F5 perf with SDCA without caching

src/Microsoft.ML.Data/DataLoadSave/Text/BlockingQueue.cs

@@ -0,0 +1,229 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+
+namespace Microsoft.ML.Data
+{
+    // NOTE:
+    // This is a temporary workaround for https://github.com/dotnet/corefx/issues/34602 or until TextLoader is rearchitected.
+    // BlockingCollection is fairly efficient for blocking producer/consumer scenarios, but it was optimized for scenarios where
+    // they're not created/destroyed quickly.  Its CompleteAdding mechanism is implemented internally in such a way that if a
+    // taker is currently blocked when CompleteAdding is called, that taker thread will incur an OperationCanceledException that's
+    // eaten internally.  If such an exception is only happening rarely, it's not a big deal, but the way TextLoader uses
+    // BlockingCollections, they can end up being created and destroyed very frequently, tens of thousands of times during the
+    // course of an algorithm like SDCA (when no caching is employed).  That in turn can result in tens of thousands of exceptions
+    // getting thrown and caught. While in normal processing even that number of exceptions results in an overhead that's not
+    // particularly impactful, things change when a debugger is attached, as that makes the overhead of exceptions several orders
+    // of magnitude higher (e.g. 1000x). Until either TextLoader is rearchitected to not create so many BlockingCollections in these
+    // situations, or until this implementation detail in BlockingCollection is changed, we use a replacement BlockingQueue implementation,
+    // that's similar in nature to BlockingCollection (albeit without many of its bells and whistles) but that specifically doesn't
+    // rely on cancellation for its CompleteAdding mechanism, and thus doesn't incur this impactful exception. Other than type name, this
+    // type is designed to expose the exact surface area required by the existing usage of BlockingCollection<T>, no more, no less,
+    // making it easy to swap in and out.
+
+    /// <summary>Provides a thread-safe queue that supports blocking takes when empty and blocking adds when full.</summary>
+    /// <typeparam name="T">Specifies the type of data contained.</typeparam>
+    internal sealed class BlockingQueue<T> : IDisposable
+    {
+        /// <summary>The underlying queue storing all elements.</summary>
+        private readonly ConcurrentQueue<T> _queue;
+        /// <summary>A semaphore that can be waited on to know when an item is available for taking.</summary>
+        private readonly CompletableSemaphore _itemsAvailable;
+        /// <summary>A semaphore that can be waited on to know when space is available for adding.</summary>
+        private readonly CompletableSemaphore _spaceAvailable;
+
+        /// <summary>Initializes the blocking queue.</summary>
+        /// <param name="boundedCapacity">The maximum number of items the queue may contain.</param>
+        public BlockingQueue(int boundedCapacity)
+        {
+            Contracts.Assert(boundedCapacity > 0);
+
+            _queue = new ConcurrentQueue<T>();
+            _itemsAvailable = new CompletableSemaphore(0);
+            _spaceAvailable = new CompletableSemaphore(boundedCapacity);
+        }
+
+        /// <summary>Cleans up all resources used by the blocking collection.</summary>
+        public void Dispose()
+        {
+            // This method/IDisposable implementation is here for API compat with BlockingCollection<T>,
+            // but there's nothing to actually dispose.
+        }
+
+        /// <summary>Adds an item to the blocking collection.</summary>
+        /// <param name="item">The item to add.</param>
+        /// <param name="millisecondsTimeout">The time to wait, in milliseconds, or -1 to wait indefinitely.</param>
+        /// <returns>
+        /// true if the item was successfully added; false if the timeout expired or if the collection were marked
+        /// as complete for adding before the item could be added.
+        /// </returns>
+        public bool TryAdd(T item, int millisecondsTimeout = 0)
+        {
+            Contracts.Assert(!_itemsAvailable.Completed);
+
+            // Wait for space to be available, then once it is, enqueue the item,
+            // and notify anyone waiting that another item is available.
+            if (_spaceAvailable.Wait(millisecondsTimeout))
+            {
+                _queue.Enqueue(item);
+                _itemsAvailable.Release();
+                return true;
+            }
+
+            return false;
+        }
+
+        /// <summary>Tries to take an item from the blocking collection.</summary>
+        /// <param name="item">The item removed, or default if none could be taken.</param>
+        /// <param name="millisecondsTimeout">The time to wait, in milliseconds, or -1 to wait indefinitely.</param>
+        /// <returns>
+        /// true if the item was successfully taken; false if the timeout expired or if the collection is empty
+        /// and has been marked as complete for adding.
+        /// </returns>
+        public bool TryTake(out T item, int millisecondsTimeout = 0)
+        {
+            // Wait for an item to be available, and once one is, dequeue it,
+            // and assuming we got one, notify anyone waiting that space is available.
+            if (_itemsAvailable.Wait(millisecondsTimeout))
+            {
+                bool gotItem = _queue.TryDequeue(out item);
+                Contracts.Assert(gotItem || _itemsAvailable.Completed);
+                if (gotItem)
+                {
+                    _spaceAvailable.Release();
+                    return true;
+                }
+            }
+
+            item = default;
+            return false;
+        }
+
+        /// <summary>
+        /// Gets an enumerable for taking all items out of the collection until
+        /// the collection has been marked as complete for adding and is empty.
+        /// </summary>
+        public IEnumerable<T> GetConsumingEnumerable()
+        {
+            // Block waiting for each additional item, yielding each as we take it,
+            // and exiting only when the collection is and will forever be empty.
+            while (TryTake(out T item, Timeout.Infinite))
+            {
+                yield return item;
+            }
+        }
+
+        /// <summary>Mark the collection as complete for adding.</summary>
+        /// <remarks>After this is called, no calls made on this queue will block.</remarks>
+        public void CompleteAdding()
+        {
+            _itemsAvailable.Complete();
+            _spaceAvailable.Complete();
+        }
+
+        /// <summary>
+        /// A basic monitor-based semaphore that, in addition to standard Wait/Release semantics,
+        /// also supports marking the semaphore as completed, in which case all waiters immediately
+        /// fail if there's no count remaining.
+        /// </summary>
+        private sealed class CompletableSemaphore
+        {
+            /// <summary>The remaining count in the semaphore.</summary>
+            private int _count;
+            /// <summary>The number of threads currently waiting in Wait.</summary>
+            private int _waiters;
+
+            /// <summary>Initializes the semaphore with the specified initial count.</summary>
+            /// <param name="initialCount">The initial count.</param>
+            public CompletableSemaphore(int initialCount)
+            {
+                Contracts.Assert(initialCount >= 0);
+                _count = initialCount;
+            }
+
+            /// <summary>Gets whether the semaphore has been marked as completed.</summary>
+            /// <remarks>
+            /// If completed, no calls to Wait will block; if no count remains, regardless of timeout, Waits will
+            /// return immediately with a result of false.
+            /// </remarks>
+            public bool Completed { get; private set; }
+
+            /// <summary>Releases the semaphore once.</summary>
+            public void Release()
+            {
+                lock (this)
+                {
+                    // Increment the count, and if anyone is waiting, notify one of them.
+                    _count++;
+                    if (_waiters > 0)
+                    {
+                        Monitor.Pulse(this);
+                    }
+                }
+            }
+
+            /// <summary>Blocks the current thread until it can enter the semaphore once.</summary>
+            /// <param name="millisecondsTimeout">The maximum amount of time to wait to enter the semaphore, or -1 to wait indefinitely.</param>
+            /// <returns>true if the semaphore was entered; otherwise, false.</returns>
+            public bool Wait(int millisecondsTimeout = Timeout.Infinite)
+            {
+                lock (this)
+                {
+                    while (true)
+                    {
+                        // If the count is greater than 0, take one, and we're done.
+                        Contracts.Assert(_count >= 0);
+                        if (_count > 0)
+                        {
+                            _count--;
+                            return true;
+                        }
+
+                        // If the count is 0 but we've been marked as completed, fail.
+                        if (Completed)
+                        {
+                            return false;
+                        }
+
+                        // Wait until either there's a count available or the timeout expires.
+                        // In practice we should never have a case where the timeout occurs
+                        // and we need to wait again, so we don't bother doing any manual
+                        // tracking of the timeout.
+                        _waiters++;
+                        try
+                        {
+                            if (!Monitor.Wait(this, millisecondsTimeout))
+                            {
+                                return false;
+                            }
+                        }
+                        finally
+                        {
+                            _waiters--;
+                            Contracts.Assert(_waiters >= 0);
+                        }
+                    }
+                }
+            }
+
+            /// <summary>Marks the semaphore as completed, such that no further operations will block.</summary>
+            public void Complete()
+            {
+                lock (this)
+                {
+                    // Mark the semaphore as completed and wake up all waiters.
+                    Completed = true;
+                    if (_waiters > 0)
+                    {
+                        Monitor.PulseAll(this);
+                    }
+                }
+            }
+        }
+    }
+}

src/Microsoft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs

@@ -395,7 +395,7 @@ private sealed class LineReader
 
                 // The line reader can be referenced by multiple workers. This is the reference count.
                 private int _cref;
-                private BlockingCollection<LineBatch> _queue;
+                private BlockingQueue<LineBatch> _queue;
                 private Task _thdRead;
                 private volatile bool _abort;
 
@@ -415,7 +415,7 @@ public LineReader(IMultiStreamSource files, int batchSize, int bufSize, bool has
                     _files = files;
                     _cref = cref;
 
-                    _queue = new BlockingCollection<LineBatch>(bufSize);
+                    _queue = new BlockingQueue<LineBatch>(bufSize);
                     _thdRead = Utils.RunOnBackgroundThread(ThreadProc);
                 }
 
@@ -638,7 +638,7 @@ private sealed class ParallelState : IDisposable
                 private readonly OrderedWaiter _waiterPublish;
 
                 // A small capacity blocking collection that the main cursor thread consumes.
-                private readonly BlockingCollection<RowBatch> _queue;
+                private readonly BlockingQueue<RowBatch> _queue;
 
                 private readonly Task[] _threads;
 
@@ -673,7 +673,7 @@ public ParallelState(Cursor curs, out RowSet rows, int cthd)
 
                     // The size limit here ensures that worker threads are never writing to
                     // a range that is being served up by the cursor.
-                    _queue = new BlockingCollection<RowBatch>(2);
+                    _queue = new BlockingQueue<RowBatch>(2);
 
                     _threads = new Task[cthd];
                     _threadsRunning = cthd;