[release/8.0] Port dump collection perf improvements

[hoyosjs]

Fixes #122459

main PRs:

• Use SHash as DAC instance hash #125631
• Cache debugger patches to speed up x64 stackwalk epilogue/prologue scanning #125459
• Change heap dumps to use HEAP2 as the default #127321

Description

Backports three DAC performance improvements for minidump collection:

1. Use SHash as DAC instance hash (Use SHash as DAC instance hash #125631): Replaces the hand-rolled hash table in DacInstanceManager with an SHash-based implementation. The previous fixed-bucket hash degraded quickly for Find and insertion operations under high load. Measured ~9.5x speedup for minidump collection against a repro app with 2.5k-frame deep stacks over 50 threads.

2. Cache debugger patches (Cache debugger patches to speed up x64 stackwalk epilogue/prologue scanning #125459): Caches the list of debugger breakpoint patches so that x64 stack unwinding doesn't re-scan the 1,000-bucket patch hash table on every frame. The cache is populated once on first access and invalidated on Flush(). Measured reduction from 55s to ~7s for minidump collection (10,000 iterations across 10 threads).

3. Enable CLRDATA_ENUM_MEM_HEAP2 via environment variable: When the target process has DOTNET_EnableFastHeapDumps set, the DAC promotes CLRDATA_ENUM_MEM_HEAP to CLRDATA_ENUM_MEM_HEAP2, which dumps loader heap pages in bulk instead of walking individual runtime structures.

Customer Impact

Customers collecting minidumps of large .NET applications (many threads, deep stacks) experience extremely slow dump collection times - on the order of minutes for what should take seconds. This directly impacts incident response time in production environments. Without these fixes, dump collection through Watson/dotnet-dump/createdump remains unacceptably slow for large workloads.

Regression

Yes, with respect to framework. Customers doing migrations have noticed them - framework used non-portable variants of the MSVC library.

Testing

• Both PRs (Use SHash as DAC instance hash #125631, Cache debugger patches to speed up x64 stackwalk epilogue/prologue scanning #125459) were merged to main and have been stable.
• Performance was measured against a repro app exercising deep stacks across many threads, confirming the expected speedups.
• The env var change is gated behind an explicit opt-in (DOTNET_EnableFastHeapDumps), so no change in default behavior. This is the riskier change since it makes heap dump match our expectations but might yield unknown!unknown if the modules aren't indexed properly.

Risk

Low.

• The SHash and patch cache changes are confined to DAC-only code paths (daccess.cpp, dacfn.cpp, dacimpl.h) that execute only during diagnostic operations (dump collection, debugging). They do not affect runtime execution.
• The DOTNET_EnableFastHeapDumps env var is opt-in and does not change default behavior.
• All three changes have been running in main since March without reported issues.

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[Copilot]

Pull request overview

This PR backports three CoreCLR DAC-focused changes intended to reduce minidump/heap-dump collection time for large workloads (many threads, deep stacks) by improving DAC caching behavior and optionally enabling a faster heap enumeration mode.

Changes:

• Add an opt-in knob (DOTNET_EnableFastHeapDumps) that promotes heap dump enumeration from CLRDATA_ENUM_MEM_HEAP to CLRDATA_ENUM_MEM_HEAP2.
• Introduce a DAC-side cache of debugger breakpoint patches to avoid re-scanning the patch table during x64 unwinding.
• Replace the non-scaling DAC instance hash implementation with an SHash-based implementation (intended).

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/coreclr/vm/vars.hpp	Declares g_EnableFastHeapDumps global.
src/coreclr/vm/vars.cpp	Defines/initializes g_EnableFastHeapDumps.
src/coreclr/vm/ceemain.cpp	Reads EXTERNAL_EnableFastHeapDumps at startup into g_EnableFastHeapDumps.
src/coreclr/inc/dacvars.h	Exposes g_EnableFastHeapDumps to the DAC via DEFINE_DACVAR.
src/coreclr/inc/clrconfigvalues.h	Adds EXTERNAL_EnableFastHeapDumps config entry.
src/coreclr/debug/daccess/enummem.cpp	Promotes heap dump enumeration to HEAP2 when enabled.
src/coreclr/debug/daccess/dacimpl.h	Updates instance-hash implementation (intended SHash) and adds DacPatchCache to ClrDataAccess.
src/coreclr/debug/daccess/dacfn.cpp	Implements patch cache population and uses cached patches during host-memory patch replacement.
src/coreclr/debug/daccess/daccess.cpp	Updates DacInstanceManager map operations for the SHash-based implementation and flushes the patch cache on ClrDataAccess::Flush().

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[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated no new comments.

[hoyosjs]

/ba-g the failures are known on a release branch or unrelated timeouts