Shrink large multidimensional array tests to avoid OOM killer

[tannergooding]

The Copy_LargeMultiDimensionalArray and Clear_LargeMultiDimensionalArray OuterLoop tests each allocated short[2, 2_000_000_000] == 8 GB, but only skipped when TotalAvailableMemoryBytes < 4_000_000_000. On the ~8 GB Debian.10/Debian.11 Helix queues the guard passes and the 8 GB allocation then trips the OOM killer (exit code 137), which is what #58616 is tracking.

The tests exist to exercise the native-sized (nuint/nint) index paths in Array.Copy/Array.Clear/element access, which only requires the flattened element count to exceed Int32.MaxValue. Element size is irrelevant to that boundary since the public API indices are all int. So switch to byte[2, 1_073_741_825] (~2 GB, total 2_147_483_650 elements) and recompute the row/col literals. The allocation now sits at half the unchanged 4 GB guard, so its headroom assumption actually holds, without narrowing where the tests run.

Also removes a stale ~1GB comment and a stray $ in one skip message.

Fixes #58616

---

Validated locally on Windows x64: System.Runtime.Tests compiles clean, and both tests run (not skipped) with outerloop=true -- Total: 2, Failed: 0, Skipped: 0.

Note

This PR description was drafted by Copilot.

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[Copilot]

Pull request overview

Adjusts System.Runtime.Tests OuterLoop coverage for large multi-dimensional arrays to avoid Helix OOM-killer terminations while still exercising the >int.MaxValue flattened-length code paths in Array.Copy/Array.Clear.

Changes:

• Reduce large MD array allocations by switching from short[2, 2_000_000_000] (~8 GB) to byte[2, 1_073_741_825] (~2 GB) while keeping flattened element count > int.MaxValue.
• Recompute index literals used to validate Array.Copy / Array.Clear behavior at the boundary.
• Clean up skip-guard formatting and fix a stray $ in a skip message.

Show a summary per file

File	Description
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/ArrayTests.cs	Shrinks large MD array test allocations, updates boundary index math, and cleans up skip messages to avoid OOM-killer failures.

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/ArrayTests.cs:4955

• Same maintainability concern as Copy_LargeMultiDimensionalArray: the boundary values are correct but easy to accidentally desynchronize (dim1 length vs last-column index). Consider factoring them into named constants derived from int.MaxValue and reusing them throughout the method.

            if (memoryInfo.TotalAvailableMemoryBytes < 4_000_000_000)
            {
                // The array below is ~2 GB; require headroom so the OOM killer
                // doesn't terminate the process before the test completes.
                throw new SkipTestException($"Prone to OOM killer. {memoryInfo.TotalAvailableMemoryBytes} is available.");

• Files reviewed: 1/1 changed files
• Comments generated: 1

[adamsitnik]

LGTM, it's clearly an improvement. Please consider using RemoteExecutor as suggested by @vcsjones (it's supported on every platform except of WASM, tvOS, iOS, android and mac catalyst)

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/ArrayTests.cs:4972

• Same as the Copy_LargeMultiDimensionalArray case: the large allocation now happens inside RemoteExecutor.Invoke without translating OutOfMemoryException (or a Linux SIGKILL/137) into a skip. That changes the failure mode from a skip to a test failure on memory-constrained or fragmented environments.

            // Allocate in a child process so that if the OOM killer does fire it
            // fails just this test instead of taking down the whole test run.
            RemoteExecutor.Invoke(static () =>
            {
                byte[,] a = new byte[2, Dim1Length];

• Files reviewed: 1/1 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0 new

[Copilot]

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1

[tannergooding]

@adamsitnik could this get another sign-off now that the remote executor is used?

[adamsitnik]

LGTM!

@tannergooding could you please kick off the Outerloop CI leg to make sure these tests are passing (running them locally is also acceptable)? RemoteExecutor is sometimes giving me hard time when refactoring the code and using lambdas (IIRC it captures the compiler generated method name and asks its runner to load given dll, type and run this particular method)