CASSANDRA-21117: Fix unreliable metric "networking cache size" in nodetool info

[EvgeniiR]

nodetool info reported "Network Cache" size using BufferPool.sizeInBytes() which includes memoryAllocated - a monotonically non-decreasing counter that never drops below the pool's high-water mark. Switch to usedSizeInBytes() which tracks only currently checked-out buffers.

Additionally fix BufferPool.LocalPool.putUnusedPortion for non-pooled buffers: the chunk==null branch was decrementing overflowMemoryUsage by the unused portion, but the subsequent put() decremented by the full original capacity, causing overflowMemoryUsage to drift negative over time.

[smiklosovic]

@EvgeniiR can you resolve the conflict? Just nuke that change in changes.txt for now

[Copilot]

Pull request overview

Fixes how Cassandra reports the “Network Cache” size in nodetool info by switching from an allocated-capacity-style metric (Size) to an in-use metric (UsedSize), and corrects overflow memory accounting in BufferPool when putUnusedPortion() is invoked for non-pooled (overflow) buffers to prevent the overflow counter drifting negative over time.

Changes:

• Update nodetool info to report “Network Cache” using UsedSize instead of Size.
• Fix BufferPool.LocalPool.putUnusedPortion() accounting for chunk == null (non-pooled buffers) to avoid double-decrementing overflow usage.
• Add unit tests covering overflow accounting balance and validating that usedSize returns to zero when pooled buffers are returned.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/unit/org/apache/cassandra/metrics/BufferPoolMetricsTest.java	Adds regression/behavior tests for UsedSize and overflow accounting around putUnusedPortion() + put().
src/java/org/apache/cassandra/utils/memory/BufferPool.java	Removes incorrect overflow accounting in LocalPool.putUnusedPortion() for non-pooled buffers and clarifies the comment.
src/java/org/apache/cassandra/tools/nodetool/Info.java	Switches the displayed “Network Cache” size metric from Size to UsedSize (and updates printed wording).
CHANGES.txt	Records the nodetool metric fix under 7.0 changes.

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[EvgeniiR]

@EvgeniiR can you resolve the conflict? Just nuke that change in changes.txt for now

Done.
Also applied update suggested by Copilot.

[smiklosovic]

@EvgeniiR where? You havent pushed or it is same as it was.

[EvgeniiR]

@EvgeniiR where? You havent pushed or it is same as it was.

My bad, sorry. Latest changed pushed now. Output value is updated, tool output kept the same format.

[smiklosovic]

cc @netudima

[EvgeniiR]

Examples from local testing:

Info output with fix applied

bin/nodetool info
ID                     : 6d194555-f6eb-41d0-c000-000000000001
Gossip active          : true
Native Transport active: true
Load                   : 123.67 KiB
Uncompressed load      : 174.96 KiB
Generation No          : 1782235131
Uptime (seconds)       : 426
Heap Memory (MB)       : 7661.99 / 15904.00
Off Heap Memory (MB)   : 0.00
Data Center            : datacenter1
Rack                   : rack1
Exceptions             : 0
Key Cache              : entries 25, size 1.99 KiB, capacity 100 MiB, 1109 hits, 1131 requests, 0.981 recent hit rate, 14400 save period in seconds
Row Cache              : entries 0, size 0 bytes, capacity 0 bytes, 0 hits, 0 requests, NaN recent hit rate, 0 save period in seconds
Counter Cache          : entries 0, size 0 bytes, capacity 50 MiB, 0 hits, 0 requests, NaN recent hit rate, 7200 save period in seconds
Network Cache          : size 8.09 KiB, overflow size: 0 bytes, capacity 128 MiB
Percent Repaired       : 100.0%
Token                  : (invoke with -T/--tokens to see all 16 tokens)
Bootstrap state        : COMPLETED
Bootstrap failed       : false
Decommissioning        : false
Decommission failed    : false

• Network Cache size value now increases and decreases accordingly

Info output on trunk

bin/nodetool info
ID                     : 6d194555-f6eb-41d0-c000-000000000001
Gossip active          : true
Native Transport active: true
Load                   : 230.47 MiB
Uncompressed load      : 230.55 MiB
Generation No          : 1782235801
Uptime (seconds)       : 405
Heap Memory (MB)       : 5359.93 / 15904.00
Off Heap Memory (MB)   : 1.33
Data Center            : datacenter1
Rack                   : rack1
Exceptions             : 0
Key Cache              : entries 11, size 904 bytes, capacity 100 MiB, 149 hits, 160 requests, 0.931 recent hit rate, 14400 save period in seconds
Row Cache              : entries 0, size 0 bytes, capacity 0 bytes, 0 hits, 0 requests, NaN recent hit rate, 0 save period in seconds
Counter Cache          : entries 0, size 0 bytes, capacity 50 MiB, 0 hits, 0 requests, NaN recent hit rate, 7200 save period in seconds
Network Cache          : size 8 MiB, overflow size: 0 bytes, capacity 128 MiB
Percent Repaired       : 100.0%
Token                  : (invoke with -T/--tokens to see all 16 tokens)
Bootstrap state        : COMPLETED
Bootstrap failed       : false
Decommissioning        : false
Decommission failed    : false

• Network Cache size jumped to 8MiB on the first request and does not decrease afterwards