Parallelise Report Generator

I've had a go at trying to parallelise the - aimed particularly at helping GitHub Actions / Azure DevOps where the report generator can be quite slow, presumably due to disk IO.  I've read some issues about previous attempts and realised it's not as simple as processing classes in parallel as not all the `IReportBuilder` implements support concurrency.

With this in mind, I believe two parts of the report generation process (`ReportGenerator`) can be parallelised

1. The initial File Analysis can be done concurrently with the report generation
2. Introduced `IParallelisableReportBuilder` - `IReportBuilder`s that also implement `IParallelisableReportBuilder` can then be processed in parallel

this PR has a draft implementation of this, and with the change, I have been able to get report generation (measuring time spent in `Generator.GenerateReport`) down from 21 secs to down as low as 4 seconds with extreme concurrency, and 6secs with more reasonable concurrency levels.

This PR is still a bit of a work in progress - in particular I need to plumb the concurrency level through as a config option, and I also want to do some testing with GitHub Actions / Azure Devops.  However I wanted to raise it in it's current form for some initial thoughts on the approach.

Note, this parallelisation change highly benefits form #TODO

src/ReportGenerator.Core/Reporting/Builders/HtmlBlueRedReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -18,7 +19,7 @@ public class HtmlBlueRedReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlBlueRedReportBuilder" /> class.

src/ReportGenerator.Core/Reporting/Builders/HtmlBlueRedSummaryReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -33,7 +34,7 @@ public override void CreateClassReport(Class @class, IEnumerable<FileAnalysis> f
         /// <param name="summaryResult">The summary result.</param>
         public override void CreateSummaryReport(SummaryResult summaryResult)
         {
-            using (var renderer = new HtmlRenderer(new Dictionary<string, string>(), true, HtmlMode.InlineCssAndJavaScript, new string[] { "custom_adaptive.css", "custom_bluered.css" }, "custom.css"))
+            using (var renderer = new HtmlRenderer(new ConcurrentDictionary<string, string>(), true, HtmlMode.InlineCssAndJavaScript, new string[] { "custom_adaptive.css", "custom_bluered.css" }, "custom.css"))
             {
                 this.CreateSummaryReport(renderer, summaryResult);
             }

src/ReportGenerator.Core/Reporting/Builders/HtmlDarkReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -18,7 +19,7 @@ public class HtmlDarkReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlDarkReportBuilder" /> class.

src/ReportGenerator.Core/Reporting/Builders/HtmlInlineAzurePipelinesDarkReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -13,7 +14,7 @@ public class HtmlInlineAzurePipelinesDarkReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Gets the report type.

src/ReportGenerator.Core/Reporting/Builders/HtmlInlineAzurePipelinesLightReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -13,7 +14,7 @@ public class HtmlInlineAzurePipelinesLightReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Gets the report type.

src/ReportGenerator.Core/Reporting/Builders/HtmlInlineAzurePipelinesReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -13,7 +14,7 @@ public class HtmlInlineAzurePipelinesReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Gets the report type.

src/ReportGenerator.Core/Reporting/Builders/HtmlInlineCssAndJavaScriptReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -13,7 +14,7 @@ public class HtmlInlineCssAndJavaScriptReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Gets the report type.

src/ReportGenerator.Core/Reporting/Builders/HtmlLightReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -18,7 +19,7 @@ public class HtmlLightReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlLightReportBuilder" /> class.

src/ReportGenerator.Core/Reporting/Builders/HtmlReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -18,7 +19,7 @@ public class HtmlReportBuilder : HtmlReportBuilderBase
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass = new Dictionary<string, string>();
+        private readonly ConcurrentDictionary<string, string> fileNameByClass = new ConcurrentDictionary<string, string>();
 
         /// <summary>
         /// Initializes a new instance of the <see cref="HtmlReportBuilder" /> class.

src/ReportGenerator.Core/Reporting/Builders/HtmlReportBuilderBase.cs

@@ -15,7 +15,7 @@ namespace Palmmedia.ReportGenerator.Core.Reporting.Builders
     /// <summary>
     /// Implementation of <see cref="IReportBuilder"/> that uses <see cref="IHtmlRenderer"/> to create reports.
     /// </summary>
-    public abstract class HtmlReportBuilderBase : IReportBuilder
+    public abstract class HtmlReportBuilderBase : IParallelisableReportBuilder
     {
         /// <summary>
         /// The Logger.

src/ReportGenerator.Core/Reporting/Builders/HtmlSummaryReportBuilder.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.IO;
 using Palmmedia.ReportGenerator.Core.Parser.Analysis;
@@ -33,7 +34,7 @@ public override void CreateClassReport(Class @class, IEnumerable<FileAnalysis> f
         /// <param name="summaryResult">The summary result.</param>
         public override void CreateSummaryReport(SummaryResult summaryResult)
         {
-            using (var renderer = new HtmlRenderer(new Dictionary<string, string>(), true, HtmlMode.InlineCssAndJavaScript))
+            using (var renderer = new HtmlRenderer(new ConcurrentDictionary<string, string>(), true, HtmlMode.InlineCssAndJavaScript))
             {
                 this.CreateSummaryReport(renderer, summaryResult);
             }

src/ReportGenerator.Core/Reporting/Builders/Rendering/HtmlRenderer.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Globalization;
 using System.IO;
@@ -58,7 +59,7 @@ internal class HtmlRenderer : IHtmlRenderer, IDisposable
         /// <summary>
         /// Dictionary containing the filenames of the class reports by class.
         /// </summary>
-        private readonly IDictionary<string, string> fileNameByClass;
+        private readonly ConcurrentDictionary<string, string> fileNameByClass;
 
         /// <summary>
         /// Indicates that only a summary report is created (no class reports).
@@ -109,7 +110,7 @@ internal class HtmlRenderer : IHtmlRenderer, IDisposable
         /// <param name="cssFileResource">Optional CSS file resource.</param>
         /// <param name="additionalCssFileResource">Optional additional CSS file resource.</param>
         internal HtmlRenderer(
-            IDictionary<string, string> fileNameByClass,
+            ConcurrentDictionary<string, string> fileNameByClass,
             bool onlySummary,
             HtmlMode htmlMode,
             string cssFileResource = "custom.css",
@@ -132,7 +133,7 @@ internal HtmlRenderer(
         /// <param name="additionalCssFileResources">Optional additional CSS file resources.</param>
         /// <param name="cssFileResource">Optional CSS file resource.</param>
         internal HtmlRenderer(
-            IDictionary<string, string> fileNameByClass,
+            ConcurrentDictionary<string, string> fileNameByClass,
             bool onlySummary,
             HtmlMode htmlMode,
             string[] additionalCssFileResources,
@@ -1557,7 +1558,7 @@ private string GetClassReportFilename(Assembly assembly, string className)
                     while (this.fileNameByClass.Values.Any(v => v.Equals(fileName, StringComparison.OrdinalIgnoreCase)));
                 }
 
-                this.fileNameByClass.Add(key, fileName);
+                this.fileNameByClass[key] = fileName;
             }
 
             return fileName;

src/ReportGenerator.Core/Reporting/IParallelisableReportBuilder.cs

@@ -0,0 +1,7 @@
+namespace Palmmedia.ReportGenerator.Core.Reporting
+{
+    /// <summary>
+    /// Interface indicating that an <see cref="IReportBuilder"/> can build multiple reports concurrently.
+    /// </summary>
+    public interface IParallelisableReportBuilder : IReportBuilder { }
+}

src/ReportGenerator.Core/Reporting/ReportGenerator.cs

@@ -1,6 +1,8 @@
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 using Palmmedia.ReportGenerator.Core.Common;
 using Palmmedia.ReportGenerator.Core.Logging;
@@ -16,6 +18,9 @@ namespace Palmmedia.ReportGenerator.Core.Reporting
     /// </summary>
     internal class ReportGenerator
     {
+        // TODO: Make this configurable
+        private const int MaxConcurrency = 8;
+
         /// <summary>
         /// The Logger.
         /// </summary>
@@ -34,7 +39,7 @@ internal class ReportGenerator
         /// <summary>
         /// The renderers.
         /// </summary>
-        private readonly IEnumerable<IReportBuilder> renderers;
+        private readonly List<IReportBuilder> renderers;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ReportGenerator" /> class.
@@ -46,7 +51,7 @@ internal ReportGenerator(IFileReader fileReader, ParserResult parserResult, IEnu
         {
             this.fileReader = fileReader ?? throw new ArgumentNullException(nameof(fileReader));
             this.parserResult = parserResult ?? throw new ArgumentNullException(nameof(parserResult));
-            this.renderers = renderers ?? throw new ArgumentNullException(nameof(renderers));
+            this.renderers = renderers?.ToList() ?? throw new ArgumentNullException(nameof(renderers));
         }
 
         /// <summary>
@@ -58,54 +63,73 @@ internal ReportGenerator(IFileReader fileReader, ParserResult parserResult, IEnu
         /// <param name="tag">The custom tag (e.g. build number).</param>
         internal void CreateReport(bool addHistoricCoverage, List<HistoricCoverage> overallHistoricCoverages, DateTime executionTime, string tag)
         {
+            // TODO: Can we change overallHistoricCoverages to a ConcurrentBag to avoid this?
+            object overallHistoricCoveragesLock = new object();
+
+            // TODO: This probably belongs in the ReportGenerator Console and/or Global Tool
+            //       AND can probably be a bit smarter.
+            //
+            // If there aren't enough threads in the thread pool, the Parallelism here can deadlock until the pool grows large enough
+            // With all avaialble threads being used in class analysis, but with ConcurrentReportBuilder threads having
+            // to wait for the thread pool to grow.  By default, .Net core adds 2 threads every 0.5 secs
+            // App will become responsive within a few seconds, but given that report generator can complete in tens of seconds,
+            // these stalls become significant
+            ThreadPool.SetMinThreads(200, 200);
+
+            var allClasses = this.parserResult.Assemblies.SelectMany(a => a.Classes);
+            var classAnalysis = Partitioner.Create(allClasses, EnumerablePartitionerOptions.NoBuffering)
+                .AsParallel()
+                .AsOrdered()
+                .WithDegreeOfParallelism(MaxConcurrency)
+                .Select(AnalyseClass)
+                .AsSequential();
+
             int numberOfClasses = this.parserResult.Assemblies.SafeSum(a => a.Classes.Count());
 
             Logger.DebugFormat(Resources.AnalyzingClasses, numberOfClasses);
 
             int counter = 0;
+            var concurrentRenderers = this.renderers.OfType<IParallelisableReportBuilder>().ToList();
+            var sequentialRenderers = this.renderers.Except(concurrentRenderers).ToList();
 
-            foreach (var assembly in this.parserResult.Assemblies)
+            var concurrentRenderQueue = new BlockingCollection<(IReportBuilder renderer, Class @class, List<FileAnalysis> analysis)>(MaxConcurrency);
+            Task concurrentRendererTask = Task.CompletedTask;
+
+            if (concurrentRenderers.Any())
             {
-                foreach (var @class in assembly.Classes)
+                concurrentRendererTask = Task.Factory.StartNew(() =>
                 {
-                    counter++;
-
-                    Logger.DebugFormat(
-                        Resources.CreatingReport,
-                        counter,
-                        numberOfClasses,
-                        @class.Assembly.ShortName,
-                        @class.Name);
-
-                    var fileAnalyses = @class.Files.Select(f => f.AnalyzeFile(this.fileReader)).ToArray();
-
-                    if (addHistoricCoverage)
-                    {
-                        var historicCoverage = new HistoricCoverage(@class, executionTime, tag);
-                        @class.AddHistoricCoverage(historicCoverage);
-                        overallHistoricCoverages.Add(historicCoverage);
-                    }
+                    Partitioner.Create(concurrentRenderQueue.GetConsumingEnumerable(), EnumerablePartitionerOptions.NoBuffering)
+                        .AsParallel()
+                        .WithDegreeOfParallelism(MaxConcurrency)
+                        .ForAll(x => RenderReport(x.renderer, x.@class, x.analysis));
+                });
+            }
 
-                    Parallel.ForEach(
-                        this.renderers,
-                        renderer =>
-                        {
-                            try
-                            {
-                                renderer.CreateClassReport(@class, fileAnalyses);
-                            }
-                            catch (Exception ex)
-                            {
-                                Logger.ErrorFormat(
-                                    Resources.ErrorDuringRenderingClassReport,
-                                    @class.Name,
-                                    renderer.ReportType,
-                                    ex.GetExceptionMessageForDisplay());
-                            }
-                        });
+            foreach (var (@class, analysis) in classAnalysis)
+            {
+                counter++;
+                Logger.DebugFormat(
+                    Resources.CreatingReport,
+                    counter,
+                    numberOfClasses,
+                    @class.Assembly.ShortName,
+                    @class.Name);
+
+                foreach (var renderer in concurrentRenderers)
+                {
+                    concurrentRenderQueue.Add((renderer, @class, analysis));
                 }
+
+                sequentialRenderers
+                    .AsParallel()
+                    .WithMergeOptions(ParallelMergeOptions.NotBuffered)
+                    .ForAll(x => RenderReport(x, @class, analysis));
             }
 
+            concurrentRenderQueue.CompleteAdding();
+            concurrentRendererTask.Wait();
+
             Logger.Debug(Resources.CreatingSummary);
             SummaryResult summaryResult = new SummaryResult(this.parserResult);
 
@@ -128,6 +152,40 @@ internal void CreateReport(bool addHistoricCoverage, List<HistoricCoverage> over
                     }
                 }
             }
+
+            (Class @class, List<FileAnalysis> analysis) AnalyseClass(Class @class)
+            {
+                var fileAnalyses = @class.Files.Select(f => f.AnalyzeFile(this.fileReader)).ToList();
+
+                if (addHistoricCoverage)
+                {
+                    var historicCoverage = new HistoricCoverage(@class, executionTime, tag);
+                    @class.AddHistoricCoverage(historicCoverage);
+
+                    lock (overallHistoricCoveragesLock)
+                    {
+                        overallHistoricCoverages.Add(historicCoverage);
+                    }
+                }
+
+                return (@class, fileAnalyses);
+            }
+        }
+
+        private static void RenderReport(IReportBuilder renderer, Class @class, List<FileAnalysis> analysis)
+        {
+            try
+            {
+                renderer.CreateClassReport(@class, analysis);
+            }
+            catch (Exception ex)
+            {
+                Logger.ErrorFormat(
+                    Resources.ErrorDuringRenderingClassReport,
+                    @class.Name,
+                    renderer.ReportType,
+                    ex.GetExceptionMessageForDisplay());
+            }
         }
     }
 }