Optimize nettrace-to-TraceLog Conversion

[brianrob]

Summary

Optimizes the nettrace-to-TraceLog/ETLX conversion pipeline, reducing conversion time for a 1.68 GB nettrace file from ~22 minutes to ~40 seconds (34x speedup).

Problem

Converting large nettrace files (e.g., from OneCollect Linux traces with many threads) to TraceLog/ETLX format was extremely slow. Profiling revealed three bottlenecks:

1. EventCache.SortAndDispatch consumed ~75% of CPU — O(N×T) linear scan over all thread queues per event block, plus per-call LINQ List allocations.
2. ParsedSymbolMetadata consumed ~10% of CPU — JSON deserialization repeated on every property access.
3. FileName string allocations consumed ~12% of CPU — GetShortUTF8StringAt() called 3 times per mapping event.

Changes

EventCache min-heap (EventCache.cs)

• Replaced the O(N×T) linear scan in SortAndDispatch with an O(N×log T) min-heap merge.
• Introduced a generic MinHeap<TValue> class with full unit test coverage (13 tests).
• Added _activeThreadQueues HashSet to track only threads with pending events, avoiding iteration over the full thread dictionary.

Cache ParsedSymbolMetadata (UniversalSystemTraceEventParser.cs)

• Cached the result of ProcessMappingSymbolMetadataParser.TryParse() to avoid repeated JSON deserialization.
• Reset cache in Dispatch() to prevent stale data when TraceEvent objects are reused.
• Added Clone() override to preserve cached values in cloned events.

Reduce FileName allocations (NettraceUniversalConverter.cs)

• Read data.FileName once into a local variable instead of accessing the property 3 times (each access allocates a new string via GetShortUTF8StringAt).
• Call UniversalMapping(string, ...) overload directly, passing the cached string.

Performance Results

Measured with a 1.68 GB nettrace file (8.6M events, 11.5K processes) using Release builds:

Metric	Before	After
Conversion time	22:26 (1346s)	00:40 (40s)
Speedup	—	34×
Events	8,583,800	8,583,800 ✓
Processes	11,509	11,509 ✓
Unique code addresses	174,476	174,476 ✓
Unique stacks	206,467	206,467 ✓
ETLX file size	3,051,574,460	3,051,574,460 ✓

All semantic statistics match. ETLX files are the same size but not byte-identical due to different tie-breaking order for same-timestamp events (min-heap vs linear scan).

Testing

• All existing tests pass (2,274 on net8.0 + 2,289 on net462).
• 13 new MinHeap unit tests added.

[brianrob]

cc: @zachcmadsen