/// <summary> /// Load a text and a version of the document in the workspace. /// </summary> /// <exception cref="IOException"></exception> /// <exception cref="InvalidDataException"></exception> public override async Task <TextAndVersion> LoadTextAndVersionAsync(Workspace workspace, DocumentId documentId, CancellationToken cancellationToken) { ValidateFileLength(workspace, _path); DateTime prevLastWriteTime = FileUtilities.GetFileTimeStamp(_path); TextAndVersion textAndVersion; // In many .NET Framework versions (specifically the 4.5.* series, but probably much earlier // and also later) there is this particularly interesting bit in FileStream.BeginReadAsync: // // // [ed: full comment clipped for brevity] // // // // If we did a sync read to fill the buffer, we could avoid the // // problem, and any async read less than 64K gets turned into a // // synchronous read by NT anyways... // if (numBytes < _bufferSize) // { // if (_buffer == null) _buffer = new byte[_bufferSize]; // IAsyncResult bufferRead = BeginReadCore(_buffer, 0, _bufferSize, null, null, 0); // _readLen = EndRead(bufferRead); // // In English, this means that if you do a asynchronous read for smaller than _bufferSize, // this is implemented by the framework by starting an asynchronous read, and then // blocking your thread until that read is completed. The comment implies this is "fine" // because the asynchronous read will actually be synchronous and thus EndRead won't do // any blocking -- it'll be an effective no-op. In theory, everything is fine here. // // In reality, this can end very poorly. That read in fact can be asynchronous, which means the // EndRead will enter a wait and block the thread. If we are running that call to ReadAsync on a // thread pool thread that completed a previous piece of IO, it means there has to be another // thread available to service the completion of that request in order for our thread to make // progress. Why is this worse than the claim about the operating system turning an // asynchronous read into a synchronous one? If the underlying native ReadFile completes // synchronously, that would mean just our thread is being blocked, and will be unblocked once // the kernel gets done with our work. In this case, if the OS does do the read asynchronously // we are now dependent on another thread being available to unblock us. // // So how does ths manifest itself? We have seen dumps from customers reporting hangs where // we have over a hundred thread pool threads all blocked on EndRead() calls as we read this stream. // In these cases, the user had just completed a build that had a bunch of XAML files, and // this resulted in many .g.i.cs files being written and updated. As a result, Roslyn is trying to // re-read them to provide a new compilation to the XAML language service that is asking for it. // Inspecting these dumps and sampling some of the threads made some notable discoveries: // // 1. When there was a read blocked, it was the _last_ chunk that we were reading in the file in // the file that we were reading. This leads me to believe that it isn't simply very slow IO // (like a network drive), because in that case I'd expect to see some threads in different // places than others. // 2. Some stacks were starting by the continuation of a ReadAsync, and some were the first read // of a file from the background parser. In the first case, all of those threads were if the // files were over 4K in size. The ones with the BackgroundParser still on the stack were files // less than 4K in size. // 3. The "time unresponsive" in seconds correlated with roughly the number of threads we had // blocked, which makes me think we were impacted by the once-per-second hill climbing algorithm // used by the thread pool. // // So what's my analysis? When the XAML language service updated all the files, we kicked off // background parses for all of them. If the file was over 4K the asynchronous read actually did // happen (see point #2), but we'd eventually block the thread pool reading the last chunk. // Point #1 confirms that it was always the last chunk. And in small file cases, we'd block on // the first chunk. But in either case, we'd be blocking off a thread pool thread until another // thread pool thread was available. Since we had enough requests going (over a hundred), // sometimes the user got unlucky and all the threads got blocked. At this point, the CLR // started slowly kicking off more threads, but each time it'd start a new thread rather than // starting work that would be needed to unblock a thread, it just handled an IO that resulted // in another file read hitting the end of the file and another thread would get blocked. The // CLR then must kick off another thread, rinse, repeat. Eventually it'll make progress once // there's no more pending IO requests, everything will complete, and life then continues. // // To work around this issue, we set bufferSize to 1, which means that all reads should bypass // this logic. This is tracked by https://github.com/dotnet/corefx/issues/6007, at least in // corefx. We also open the file for reading with FileShare mode read/write/delete so that // we do not lock this file. using (var stream = FileUtilities.RethrowExceptionsAsIOException(() => new FileStream(_path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete, bufferSize: 1, useAsync: true))) { var version = VersionStamp.Create(prevLastWriteTime); // we do this so that we asynchronously read from file. and this should allocate less for IDE case. // but probably not for command line case where it doesn't use more sophisticated services. using (var readStream = await SerializableBytes.CreateReadableStreamAsync(stream, cancellationToken: cancellationToken).ConfigureAwait(false)) { var text = CreateText(readStream, workspace); textAndVersion = TextAndVersion.Create(text, version, _path); } } // Check if the file was definitely modified and closed while we were reading. In this case, we know the read we got was // probably invalid, so throw an IOException which indicates to our caller that we should automatically attempt a re-read. // If the file hasn't been closed yet and there's another writer, we will rely on file change notifications to notify us // and reload the file. DateTime newLastWriteTime = FileUtilities.GetFileTimeStamp(_path); if (!newLastWriteTime.Equals(prevLastWriteTime)) { var message = string.Format(WorkspacesResources.File_was_externally_modified_colon_0, _path); throw new IOException(message); } return(textAndVersion); }
internal override TextAndVersion LoadTextAndVersionSynchronously(Workspace workspace, DocumentId documentId, CancellationToken cancellationToken) { return(TextAndVersion.Create(_container.CurrentText, _version, _filePath)); }
internal TextDocumentLoader(TextAndVersion textAndVersion) { _textAndVersion = textAndVersion; }
public virtual async Task <VersionStamp> GetTopLevelChangeTextVersionAsync(CancellationToken cancellationToken) { TextAndVersion textAndVersion = await this.textAndVersionSource.GetValueAsync(cancellationToken).ConfigureAwait(false); return(textAndVersion.Version); }
protected static ValueSource <TextAndVersion> CreateRecoverableText(TextAndVersion text, SolutionServices services) { return(new RecoverableTextAndVersion(CreateStrongText(text), services.TemporaryStorage)); }
protected static ValueSource <TextAndVersion> CreateStrongText(TextAndVersion text) { return(new ConstantValueSource <TextAndVersion>(text)); }