J-HowHuang commented on code in PR #19861:
URL: https://github.com/apache/pinot/pull/19861#discussion_r4280464879


##########
pinot-core/src/main/java/org/apache/pinot/core/data/manager/BaseTableDataManager.java:
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@@ -1085,6 +1104,100 @@ protected void 
setZkOperationTimeIfAvailable(ImmutableSegment segment, @Nullable
     }
   }
 
+  /// Returns the subset of `segmentDataManagers` that [#isReloadNeeded] 
reports as out of date, evaluating the check
+  /// in parallel on the reload executor. Skipped segments are counted in the 
reload job status (as successful) when
+  /// `reloadJobId` is set. A segment is kept (i.e. reloaded as usual) when 
its check fails, so an error never turns
+  /// into a silently skipped reload.
+  ///
+  /// The check runs against the currently loaded copy without holding the 
segment lock. Skipping on that snapshot is
+  /// safe: anything that later changes the segment (a replace or another 
reload) loads it against the then-current
+  /// config, and a segment found to need reload still goes through the 
regular reload path, which re-evaluates
+  /// everything under the lock.
+  @VisibleForTesting
+  List<SegmentDataManager> 
selectSegmentsNeedingReload(List<SegmentDataManager> segmentDataManagers,
+      IndexLoadingConfig indexLoadingConfig, @Nullable String reloadJobId)
+      throws Exception {
+    boolean[] needReload = new boolean[segmentDataManagers.size()];
+    CompletableFuture<?>[] futures = new 
CompletableFuture<?>[segmentDataManagers.size()];
+    for (int i = 0; i < segmentDataManagers.size(); i++) {
+      int index = i;
+      SegmentDataManager segmentDataManager = segmentDataManagers.get(i);
+      futures[i] = CompletableFuture.runAsync(() -> {
+        String segmentName = segmentDataManager.getSegmentName();
+        // Hold a reference so the segment cannot be destroyed while its 
directory is inspected. If it is already
+        // being destroyed it was replaced or offloaded concurrently; keep it 
so the regular reload path handles it.
+        if (!segmentDataManager.increaseReferenceCount()) {
+          needReload[index] = true;
+          return;
+        }
+        try {
+          needReload[index] = isReloadNeeded(segmentDataManager, 
indexLoadingConfig);
+        } catch (Exception e) {
+          _logger.warn("Caught exception while checking whether segment: {} 
needs reload, reloading it", segmentName,
+              e);
+          needReload[index] = true;
+        } finally {
+          releaseSegment(segmentDataManager);
+        }
+      }, _segmentReloadExecutor);
+    }
+    CompletableFuture.allOf(futures).get();
+
+    List<SegmentDataManager> segmentsToReload = new ArrayList<>();
+    List<String> skippedSegments = new ArrayList<>();
+    for (int i = 0; i < segmentDataManagers.size(); i++) {
+      if (needReload[i]) {
+        segmentsToReload.add(segmentDataManagers.get(i));
+      } else {
+        skippedSegments.add(segmentDataManagers.get(i).getSegmentName());
+      }
+    }
+    if (!skippedSegments.isEmpty()) {
+      _logger.info("Skipping reload for {}/{} segments that are already up to 
date", skippedSegments.size(),
+          segmentDataManagers.size());
+      _logger.debug("Skipped reload for up-to-date segments: {}", 
skippedSegments);
+      if (reloadJobId != null) {
+        _reloadJobStatusCache.recordSkipped(reloadJobId, 
skippedSegments.size());
+      }
+    }
+    return segmentsToReload;
+  }
+
+  /// Returns whether reloading the segment would change what is loaded, i.e. 
whether the regular reload path would
+  /// download a new copy (completed segment whose CRC differs from ZK), move 
it to a different tier, or preprocess it
+  /// under the given index loading config. Consuming segments are never 
considered as needing reload, as reloading
+  /// them means force committing.
+  protected boolean isReloadNeeded(SegmentDataManager segmentDataManager, 
IndexLoadingConfig indexLoadingConfig)

Review Comment:
   Done. `isSegmentStale` is left unchange, it has quite its own logic



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