Nuno Santos created OAK-12299:
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Summary: PersistentDiskCache: precompute elementCount and
currentWeight to avoid expensive filesystem traversal on metrics reads
Key: OAK-12299
URL: https://issues.apache.org/jira/browse/OAK-12299
Project: Jackrabbit Oak
Issue Type: Improvement
Components: segment-azure
Reporter: Nuno Santos
{{PersistentDiskCache}} exposes {{elementCount}} and {{currentWeight}} through
{{{}SegmentCacheStats{}}}, but both metrics are computed by traversing the file
store on every read:
* {{elementCount}} calls {{directory.listFiles().length}} — a syscall that
allocates an array of all cache files.
* {{currentWeight}} calls {{FileUtils.sizeOfDirectory(directory)}} — a full
recursive directory traversal that reads every file's size from the filesystem.
Both operations are O(n) in the number of cached segments and are called each
time metrics are collected, adding significant overhead to every metrics scrape.
*Solution:* Replace both suppliers with reads of in-memory {{AtomicLong}}
fields. At construction time, scan the directory once to initialize
{{cacheSize}} (already maintained in {{{}AbstractPersistentCache{}}}) and a new
{{elementCount}} field. Increment/decrement {{elementCount}} alongside
{{cacheSize}} in the existing write and eviction paths. Metric reads then
become O(1) field reads.
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