clintropolis opened a new pull request, #19396:
URL: https://github.com/apache/druid/pull/19396
### Description
This PR switches the `SegmentLocalCacheManager` on-demand load executor
(used in virtual-storage mode) from a fixed platform-thread pool to one virtual
thread per task with a `Semaphore` for backpressure, and converts
`SegmentCacheEntry` from synchronized to ReentrantLock so virtual threads park
rather than pinning their carrier during the long-running mount path.
The on-demand load work is overwhelmingly socket wait against deep storage
with a small CPU portion at the end (factorize/mmap setup). Virtual threads let
us fan out hundreds of in-flight loads cheaply, with the semaphore providing a
similar kind of backpressure the pool size used to give implicitly. This
becomes especially relevant once partial loading lands and the pool starts
handling many smaller per-internal-segment-file range read calls instead of one
big mount per segment.
The `ReentrantLock` conversion in `SegmentCacheEntry` is what makes the
virtual threads switch actually pay off on Java 21. Without it, the entire
`mount()` body runs inside `synchronized (this)`, pinning the carrier thread
and effectively
capping concurrent mounts at the carrier-pool size regardless of how many
virtual threads are spawned. `ReentrantLock` parks the virtual thread properly.
Once Druid's minimum is Java 24+, [JEP 491](https://openjdk.org/jeps/491) makes
this conversion redundant, but it's a mechanical minimum-risk change in the
meantime.
changes:
* switch default `SegmentLocalCacheManager.virtualStorageLoadOnDemandExec`
to virtual threads with a `Semaphore` for backpressure
* added `SegmentLoaderConfig.virtualStorageUseVirtualThreads`
(`druid.segmentCache.virtualStorageUseVirtualThreads`) config that defaults to
true, but allows opt-out via setting to false
* raise default `SegmentLoaderConfig.virtualStorageLoadThreads` default to
Math.max(32, 4 * cores), sized as ~4x lookahead per processing thread
* convert `SegmentCacheEntry` from `synchronized` to `ReentrantLock` so
virtual threads park instead of pinning the carrier during mount
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