Too much string.Concat in generated code with large blocks of HTML

[rynowak]

Take a gander at the generated code for a page with a lot of 'literal text'

https://github.com/rynowak/mvc-sandbox/blob/master/workloads/RazorCodeGenerator/LargePage.cshtml

namespace Test
{
    using System;
    using System.Linq;
    using System.Collections.Generic;
    using Microsoft.AspNet.Mvc;
    using Microsoft.AspNet.Mvc.Rendering;
    using System.Threading.Tasks;

    public class LargePage : Microsoft.AspNet.Mvc.Razor.RazorPage<dynamic>
    {
        #line hidden
        public LargePage()
        {
        }
        #line hidden
        [Microsoft.AspNet.Mvc.Razor.Internal.RazorInjectAttribute]
        public Microsoft.AspNet.Mvc.IUrlHelper Url { get; private set; }
        [Microsoft.AspNet.Mvc.Razor.Internal.RazorInjectAttribute]
        public Microsoft.AspNet.Mvc.IViewComponentHelper Component { get; private set; }
        [Microsoft.AspNet.Mvc.Razor.Internal.RazorInjectAttribute]
        public Microsoft.AspNet.Mvc.Rendering.IJsonHelper Json { get; private set; }
        [Microsoft.AspNet.Mvc.Razor.Internal.RazorInjectAttribute]
        public Microsoft.AspNet.Mvc.Rendering.IHtmlHelper<dynamic> Html { get; private set; }

        #line hidden

        #pragma warning disable 1998
        public override async Task ExecuteAsync()
        {
            BeginContext(0, 160056, true);
            WriteLiteral("<p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
"p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
...

Yeah, that's a string.Concat that's going to make a 160056 character string.

We need to make Razor smarter.

[Eilon]

I'm surprised that this is using string.Concat() at all. Why does the compiler not fold these into one large string literal? Is this about debug/release? An optimization that doesn't exist in Roslyn perhaps?

[Eilon]

For example, this code:

            var x = "a" + "b";
            Console.WriteLine(x);

Compiled into this (via ILSpy):

    string value = "ab";
    Console.WriteLine(value);

And similar code is in the IL:

    IL_0008: ldstr "ab"

[Eilon]

I tried with strings that add up to ~300KB and I got the same results each time: all the string literals were folded (feld? felled? felded? follied?) into one giant string.

The form of my strings was the same as in the example:

            var x =
                "p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
                "p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><p>Paragraph</p><" +
    ...

[rynowak]

Per @NTaylorMullen - this semantic was copied from CodeDOM, and was intended to avoid an LOH allocation. It doesn't sound like that's doing what it's supposed to do. We might need to measure more to see what's better for GC perf here - allowing the LOH string vs breaking it up into separate writes.

Ok with pushing this item out until we're in a better position to test this.

[Eilon]

My only point was there there's no string.Concat at all in the code shown. Just one ginormous string (which has to end up somewhere in random access RAM memory).

[rynowak]

Yeah, you're totes bringing the pain. We should still look at this though.

[rynowak]

@Eilon @NTaylorMullen - this causes an OOM sometimes during runtime compilation:

>   mscorlib.dll!string.Concat(string str0, string str1)    Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.FoldStringConcatenation(Microsoft.CodeAnalysis.CSharp.BinaryOperatorKind kind, Microsoft.CodeAnalysis.ConstantValue valueLeft, Microsoft.CodeAnalysis.ConstantValue valueRight, ref int compoundStringLength)    Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.FoldBinaryOperator(Microsoft.CodeAnalysis.CSharp.CSharpSyntaxNode syntax, Microsoft.CodeAnalysis.CSharp.BinaryOperatorKind kind, Microsoft.CodeAnalysis.CSharp.BoundExpression left, Microsoft.CodeAnalysis.CSharp.BoundExpression right, Microsoft.CodeAnalysis.SpecialType resultType, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, ref int compoundStringLength) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindSimpleBinaryOperator(Microsoft.CodeAnalysis.CSharp.Syntax.BinaryExpressionSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, Microsoft.CodeAnalysis.CSharp.BoundExpression left, Microsoft.CodeAnalysis.CSharp.BoundExpression right, ref int compoundStringLength)  Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindSimpleBinaryOperator(Microsoft.CodeAnalysis.CSharp.Syntax.BinaryExpressionSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindExpressionInternal(Microsoft.CodeAnalysis.CSharp.Syntax.ExpressionSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, bool invoked, bool indexed) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindArgumentAndName(Microsoft.CodeAnalysis.CSharp.AnalyzedArguments result, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, bool hadError, Microsoft.CodeAnalysis.CSharp.CSharpSyntaxNode argumentSyntax, Microsoft.CodeAnalysis.CSharp.Syntax.ExpressionSyntax argumentExpression, Microsoft.CodeAnalysis.CSharp.Syntax.NameColonSyntax nameColonSyntax, Microsoft.CodeAnalysis.RefKind refKind, bool allowArglist)   Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindArgumentAndName(Microsoft.CodeAnalysis.CSharp.AnalyzedArguments result, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, bool hadError, Microsoft.CodeAnalysis.CSharp.Syntax.ArgumentSyntax argumentSyntax, bool allowArglist, bool isDelegateCreation) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindInvocationExpression(Microsoft.CodeAnalysis.CSharp.Syntax.InvocationExpressionSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindExpressionInternal(Microsoft.CodeAnalysis.CSharp.Syntax.ExpressionSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, bool invoked, bool indexed) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindExpression(Microsoft.CodeAnalysis.CSharp.Syntax.ExpressionSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, bool invoked, bool indexed) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindExpressionStatement(Microsoft.CodeAnalysis.CSharp.CSharpSyntaxNode node, Microsoft.CodeAnalysis.CSharp.Syntax.ExpressionSyntax syntax, bool allowsAnyExpression, Microsoft.CodeAnalysis.DiagnosticBag diagnostics)   Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindStatement(Microsoft.CodeAnalysis.CSharp.Syntax.StatementSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics)   Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Binder.BindBlock(Microsoft.CodeAnalysis.CSharp.Syntax.BlockSyntax node, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, Microsoft.CodeAnalysis.CSharp.Binder blockBinder) Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.BindMethodBody(Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol method, Microsoft.CodeAnalysis.CSharp.TypeCompilationState compilationState, Microsoft.CodeAnalysis.DiagnosticBag diagnostics, out Microsoft.CodeAnalysis.CSharp.ImportChain importChain)  Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethod(Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol methodSymbol, int methodOrdinal, ref Microsoft.CodeAnalysis.CSharp.Binder.ProcessedFieldInitializers processedInitializers, Microsoft.CodeAnalysis.CSharp.SynthesizedSubmissionFields previousSubmissionFields, Microsoft.CodeAnalysis.CSharp.TypeCompilationState compilationState)    Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol containingType)   Unknown
    Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedTypeAsTask.AnonymousMethod__0()  Unknown
    Microsoft.CodeAnalysis.dll!Roslyn.Utilities.UICultureUtilities.WithCurrentUICulture.AnonymousMethod__0()    Unknown
    mscorlib.dll!System.Threading.Tasks.Task.InnerInvoke()  Unknown
    mscorlib.dll!System.Threading.Tasks.Task.Execute()  Unknown
    mscorlib.dll!System.Threading.Tasks.Task.ExecutionContextCallback(object obj)   Unknown
    mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)   Unknown
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx)   Unknown
    mscorlib.dll!System.Threading.Tasks.Task.ExecuteWithThreadLocal(ref System.Threading.Tasks.Task currentTaskSlot)    Unknown
    mscorlib.dll!System.Threading.Tasks.Task.ExecuteEntry(bool bPreventDoubleExecution) Unknown
    mscorlib.dll!System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() Unknown
    mscorlib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch()    Unknown
    mscorlib.dll!System.Threading._ThreadPoolWaitCallback.PerformWaitCallback() Unknown

We need to split this up into smaller chunks

[Eilon]

@rynowak should we also be filing a Roslyn bug? The memory quantities in play here are not really that much. A few hundred KB is a drop in the bucket even in a 32bit process.

[rynowak]

I think we should file a Roslyn bug if we're still seeing this on dotnet cli. We already dump a ton of stuff into LOH, and it's fragmentation that's likely causing this.

Also, this is likely going on in addition to the other allocations Razor is doing.

Derp

I think I'll go file that bug now.

[Eilon]

Nice memory stress test 😄