Add compiler telemetry by jjonescz · Pull Request #84725 · dotnet/roslyn

jjonescz · 2026-07-31T14:49:28Z

Microsoft Reviewers: Open in CodeFlow

azure-pipelines · 2026-07-31T14:49:47Z

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Copilot

Pull request overview

Adds a generic telemetry transport from the compiler server back to the MSBuild Csc/Vbc tasks, and wires up the compilation output cache to emit a structured roslyn/compilercache event that the task forwards to the host via IBuildEngine5.LogTelemetry.

Changes:

Extend the compiler server build protocol to include a list of telemetry events on CompletedBuildResponse.
Add compiler-server-side telemetry infrastructure and emit compilation-cache metrics for C# and VB server compilations.
Forward server-produced telemetry from ManagedCompiler to MSBuild hosts and add unit coverage + design doc updates.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Compilers/Shared/BuildProtocol.cs	Adds `BuildTelemetryEvent` and serializes/deserializes telemetry on `CompletedBuildResponse`.
src/Compilers/Server/VBCSCompilerTests/CompilationCacheTests.cs	Adds tests for cache store result enums + telemetry mapping behavior.
src/Compilers/Server/VBCSCompilerTests/BuildProtocolTest.cs	Adds protocol round-trip test coverage for completed responses with telemetry.
src/Compilers/Server/VBCSCompiler/VisualBasicCompilerServer.cs	Implements telemetry provider and emits cache telemetry for VB server compiles.
src/Compilers/Server/VBCSCompiler/VBCSCompilerCommandLine.projitems	Includes new `CompilerServerTelemetry.cs` in the build.
src/Compilers/Server/VBCSCompiler/CSharpCompilerServer.cs	Implements telemetry provider and emits cache telemetry for C# server compiles.
src/Compilers/Server/VBCSCompiler/CompilerServerTelemetry.cs	Introduces telemetry provider interface + cache telemetry model/enums.
src/Compilers/Server/VBCSCompiler/CompilerRequestHandler.cs	Attaches telemetry events to `CompletedBuildResponse`.
src/Compilers/Server/VBCSCompiler/CompilationCacheUtilities.cs	Captures key/restore/store timings and cache status/store-result for telemetry.
src/Compilers/Server/VBCSCompiler/CompilationCache.cs	Changes cache store API to return a `CompilationCacheStoreResult`.
src/Compilers/Core/MSBuildTaskTests/ManagedCompilerTelemetryTests.cs	Adds unit tests validating telemetry forwarding behavior in the MSBuild task.
src/Compilers/Core/MSBuildTask/ManagedCompiler.cs	Forwards server telemetry events to the host via `IBuildEngine5.LogTelemetry`.
docs/compilers/Design/compiler-output-cache-experiment.md	Documents the new `roslyn/compilercache` telemetry event and its properties.

Suppressed comments (1)

src/Compilers/Shared/BuildProtocol.cs:443

CompletedBuildResponse.Create unconditionally reads TelemetryCount and assumes it is non-negative. If the response body is truncated/corrupt (or comes from an older writer that doesn't include telemetry), this will throw and force a tool fallback. Consider making this parse tolerant by treating missing/negative counts as 0.

            var returnCode = reader.ReadInt32();
            var utf8Output = reader.ReadBoolean();
            var output = ReadLengthPrefixedString(reader);

            var telemetryCount = reader.ReadInt32();
            IReadOnlyList<BuildTelemetryEvent> telemetryEvents = telemetryCount == 0
                ? []
                : readTelemetryEvents(reader, telemetryCount);

azure-pipelines · 2026-08-03T13:45:02Z

Azure Pipelines: Successfully started running 1 pipeline(s). 1 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

Copilot

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/Compilers/Server/VBCSCompilerTests/CompilationCacheTests.cs:480

Using a hard-coded absolute path ("/nonexistent/path/Util.dll") to simulate a missing assembly is brittle across platforms/environments (rooted path semantics differ on Windows, and the path could theoretically exist). Prefer constructing a guaranteed-missing path under a temp directory created for the test.

            var outputFiles = new CompilationOutputFiles { AssemblyPath = "/nonexistent/path/Util.dll" };

            Assert.Equal(CompilationCacheStoreResult.Failed, cache.TryStoreResult("Util.dll", "hash", outputFiles, "key", _logger));

Copilot

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/Compilers/Shared/BuildProtocol.cs:409

The response-body field list in this doc comment is now out of sync with the actual serialization: CompletedBuildResponse reads/writes a Utf8Output boolean between ReturnCode and Output, but the table doesn’t mention it. This makes the protocol documentation misleading for future maintainers.

    ///  Length             UInteger        4
    ///  ReturnCode         Integer         4
    ///  Output             String          Variable
    ///  TelemetryCount     Integer         4
    ///  TelemetryEvents    Variable        Variable

jjonescz · 2026-08-05T09:55:36Z

@dotnet/roslyn-compiler for reviews, thanks

333fred · 2026-08-06T15:58:56Z

+| `cachestatus` | `hit` or `miss` |
+| `storeresult` | `none`, `stored`, `skippedrace`, `skippedexists`, or `failed` |
+| `language` | the compiler language (`C#` / `Visual Basic`) |


Are these optional or always present?

333fred · 2026-08-06T17:58:46Z

+    /// generic <see cref="BuildTelemetryEvent"/>. This is the first contributor to the compiler
+    /// server telemetry channel; additional contributors can be added independently.


Suggested change

/// generic <see cref="BuildTelemetryEvent"/>. This is the first contributor to the compiler

/// server telemetry channel; additional contributors can be added independently.

/// generic <see cref="BuildTelemetryEvent"/>.

333fred · 2026-08-06T17:59:23Z

+namespace Microsoft.CodeAnalysis.CompilerServer
+{
+    /// <summary>
+    /// Implemented by server compilers that can produce telemetry for a build request. The request


Existing wording is awkward, maybe this?

Suggested change

/// Implemented by server compilers that can produce telemetry for a build request. The request

/// Implemented by compiler hosts that can produce telemetry for a build request. The request

333fred · 2026-08-06T18:00:14Z

+    /// <remarks>
+    /// This is intentionally generic: the transport (protocol + task) has no knowledge of any
+    /// particular event, so new server-side telemetry can be added by producing additional
+    /// <see cref="BuildTelemetryEvent"/> instances without changing the protocol or the task.
+    /// </remarks>


This remark seems like noise.

333fred · 2026-08-06T18:36:37Z

    {
        private readonly Func<string, MetadataReferenceProperties, PortableExecutableReference> _metadataProvider;
        private readonly CompilationCache? _cache;
+        private readonly CompilationCacheTelemetry _cacheTelemetry = new();


Please don't use target-typed new in the compiler codebase.

333fred · 2026-08-06T18:41:20Z

    {
        private readonly Func<string, MetadataReferenceProperties, PortableExecutableReference> _metadataProvider;
        private readonly CompilationCache? _cache;
+        private readonly CompilationCacheTelemetry _cacheTelemetry = new();


Same comment.

333fred · 2026-08-06T21:09:05Z


            var dllName = arguments.OutputFileName;
            var outputTimestampUtc = DateTime.UtcNow;
+            var keyStopwatch = Stopwatch.StartNew();


Rather than having the telemetry type manage one timer, and this method manage another, consider just having the telemetry object manage both timers, and share the timer between them.

Add compiler telemetry

03e263f

jjonescz added the Area-Compilers label Jul 31, 2026

jjonescz requested a review from Copilot July 31, 2026 14:50

Copilot started reviewing on behalf of jjonescz July 31, 2026 14:50 View session

Copilot AI reviewed Jul 31, 2026

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Comment thread src/Compilers/Shared/BuildProtocol.cs

Comment thread src/Compilers/Server/VBCSCompiler/CompilerRequestHandler.cs

Improve exception resiliency

3042058

jjonescz mentioned this pull request Aug 3, 2026

Recognize roslyn telemetry dotnet/sdk#55572

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jjonescz marked this pull request as ready for review August 3, 2026 13:44

jjonescz requested a review from a team as a code owner August 3, 2026 13:44

Copilot AI review requested due to automatic review settings August 3, 2026 13:44

Copilot started reviewing on behalf of jjonescz August 3, 2026 13:45 View session

Copilot AI reviewed Aug 3, 2026

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jjonescz added 3 commits August 3, 2026 16:22

Store also compilation time

51d23bd

Simplify code

8587616

Omit inapplicable entries

afab5c4

Copilot AI review requested due to automatic review settings August 3, 2026 14:31

Copilot started reviewing on behalf of jjonescz August 3, 2026 14:32 View session

Copilot AI reviewed Aug 3, 2026

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jjonescz requested a review from baronfel August 5, 2026 09:55

333fred reviewed Aug 6, 2026

View reviewed changes

		/// generic <see cref="BuildTelemetryEvent"/>. This is the first contributor to the compiler
		/// server telemetry channel; additional contributors can be added independently.

	/// Implemented by server compilers that can produce telemetry for a build request. The request
	/// Implemented by compiler hosts that can produce telemetry for a build request. The request

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jjonescz commented Jul 31, 2026 •

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