Re: [PR] Emit missing `ingest/merge/time`, `ingest/merge/cpu` and `ingest/persists/cpu` metrics (druid)

["jtuglu1 (via GitHub)" <gi...@apache.org>]

jtuglu1 commented on code in PR #18866:
URL: https://github.com/apache/druid/pull/18866#discussion_r2643650156


##########
server/src/main/java/org/apache/druid/segment/realtime/appenderator/StreamAppenderator.java:
##########
@@ -748,14 +750,15 @@ public Object call() throws IOException
             }
             finally {
               metrics.incrementNumPersists();

Review Comment:
   nit: can we move the increments below the time-sensitive metric emissions? Given we are reporting at nano-scale, chance of interrupt and/or delay due to atomic load+store can skew the nano count by X-XXns.



##########
server/src/main/java/org/apache/druid/segment/realtime/appenderator/StreamAppenderator.java:
##########
@@ -991,8 +998,8 @@ private DataSegmentWithMetadata mergeAndPush(
           identifier,
           segment.getSize(),
           indexes.size(),
-          (mergeFinishTime - startTime) / 1000000,

Review Comment:
   👍 We should probably make sure this is not being done elsewhere. For future readers, `System.nanoTime()` is actually more expensive than `System.getCurrentTimeMillis()` so if you need milli-level precision, just use the latter from the start.



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