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fix(nodestore): sample writer depth over one population

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Depth was observed at insert entry but its sample was only counted at
insert exit, so an insert still in flight contributed nothing to the
mean while completed inserts -- disproportionately the fast, shallow
ones -- all did. The reported mean understated queueing exactly when
queueing was worst: with every writer inside its first insert the gauge
was omitted entirely, while writers-in-flight correctly showed them all.

Depth and its sample count are now both folded in at entry, so the mean
is taken over one population. Mean depth is the L in Little's Law, so a
biased L understated the derived queueing share of each insert.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Pratik Mankawde
2026-07-28 18:24:07 +01:00
parent cfa2fd06ca
commit 61b101e147
3 changed files with 41 additions and 5 deletions
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13
include/xrpl/nodestore/WriteStats.h
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@@ -25,7 +25,7 @@ namespace xrpl::node_store {
* |
* concurrentWriters = queue length here
* insertTotalUs / insertCount = time in the whole system (W)
* depthSum / insertCount = mean depth (L)
* depthSum / depthSamples = mean depth (L)
*
* @note All fields except @ref concurrentWriters are cumulative for the
* life of the backend, so a reader must difference successive
@@ -38,7 +38,7 @@ namespace xrpl::node_store {
* if (auto const s = backend->getWriteStats(); s && s->insertCount)
* {
* double const meanUs = double(s->insertTotalUs) / s->insertCount;
* double const meanDepth = double(s->depthSum) / s->insertCount;
* double const meanDepth = double(s->depthSum) / s->depthSamples;
* double const serviceUs = meanUs / meanDepth; // Little's Law
* }
* @endcode
@@ -76,6 +76,15 @@ struct WriteStats
* @ref insertCount this gives mean depth.
*/
std::uint64_t depthSum = 0;
/**
* Number of depth samples summed into @ref depthSum.
*
* Divide @ref depthSum by this, not by @ref insertCount: a sample is
* taken when an insert starts, while insertCount only rises when one
* finishes, so the two populations differ while inserts are in flight.
*/
std::uint64_t depthSamples = 0;
};
} // namespace xrpl::node_store