viirya commented on code in PR #1108:
URL: https://github.com/apache/datafusion-comet/pull/1108#discussion_r1852759262
##########
docs/source/user-guide/tuning.md:
##########
@@ -105,8 +105,15 @@ then any shuffle operations that cannot be supported in
this mode will fall back
## Metrics
-Comet metrics are not directly comparable to Spark metrics in some cases.
+Some Comet metrics are not directly comparable to Spark metrics in some cases.
`CometScanExec` uses nanoseconds for total scan time. Spark also measures scan
time in nanoseconds but converts to
-milliseconds _per batch_ which can result in a large loss of precision. In one
case we saw total scan time
-of 41 seconds reported as 23 seconds for example.
+milliseconds _per batch_ which can result in a large loss of precision.
+
+Comet also adds some custom metrics:
+
+### ShuffleWriterExec
+
+| Metric | Description
|
+| ---------------- |
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
+| `jvm_fetch_time` | Measure the time it takes for `ShuffleWriterExec` to
fetch an existing batch from the JVM. Note that this does not include the
execution time of the query that produced the input batch. |
Review Comment:
Looks like the new metric measures the time on fetching all batches, not
just a batch?
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