pageserver,safekeeper: increase heap profiling frequency to 2 MB

[erikgrinaker]

Problem

Currently, the heap profiling frequency is every 1 MB allocated. Taking a profile stack trace takes about 1 µs, and allocating 1 MB takes about 15 µs, so the overhead is about 6.7% which is a bit high. This is a fixed cost regardless of whether heap profiles are actually accessed.

Summary of changes

Increase the heap profiling sample frequency from 1 MB to 2 MB, which reduces the overhead to about 3.3%. This seems acceptable, considering performance-sensitive code will avoid allocations as far as possible anyway.

[github-actions]

7282 tests run: 6918 passed, 0 failed, 364 skipped (full report)

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Flaky tests (2)

Postgres 17

• test_timeline_archival_chaos: release-arm64

Postgres 14

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-arm64

Code coverage* (full report)

• functions: 32.7% (8051 of 24640 functions)
• lines: 47.7% (66844 of 140041 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9c4317c at 2025-01-12T15:56:05.952Z :recycle:}