Add cache stripe lock contention metric

[bryancall]

Summary

Adds two new metrics to track cache lock contention:

1. proxy.process.cache.stripe.lock_contention - counts stripe mutex contention
2. proxy.process.cache.writer.lock_contention - counts writer VC mutex contention during read aggregation

Also available per-volume as proxy.process.cache.volume_N.stripe.lock_contention.

Background

When ATS is configured with more threads than cache volumes, threads contend heavily for the stripe mutex, causing throughput degradation. These metrics make contention visible so operators can tune their configuration.

Benchmark Results

Testing on a 16-core system with 100 cached URLs:

Threads	Volumes	Throughput	Contentions/s
16	1	476k req/s	12,095k
16	16	1,160k req/s	177k
24	32	1,260k req/s	161k

With only 1 volume, 16 threads is slower than 4 threads due to contention. Adding volumes eliminates the bottleneck.

Usage

# Stripe lock contention (global)
traffic_ctl metric get proxy.process.cache.stripe.lock_contention

# Stripe lock contention (per-volume)
traffic_ctl metric match volume.*stripe.lock_contention

# Writer lock contention
traffic_ctl metric get proxy.process.cache.writer.lock_contention

Implementation

Stripe Lock Contention Call Sites (VC_SCHED_LOCK_RETRY / VC_LOCK_RETRY_EVENT)

All for stripe->mutex:

CacheRead.cc:

• L210 - openReadClose
• L428 - openReadReadDone
• L456 - openReadReadDone
• L653 - openReadMain
• L705 - openReadMain
• L766 - openReadStartEarliest
• L932 - openReadVecWrite
• L988 - openReadStartHead
• L1210 - openReadDirDelete

CacheVC.cc:

• L355 - openReadClose
• L553 - die
• L938 - scanOpenWrite

CacheWrite.cc:

• L78 - handleWriteLock
• L84 - handleWriteLock
• L278 - openWriteCloseDir
• L331 - openWriteCloseHeadDone
• L410 - openWriteCloseDataDone
• L504 - openWriteWriteDone
• L648 - openWriteOverwrite
• L681 - openWriteOverwrite
• L794 - openWriteMain

Writer Lock Contention Call Site (VC_SCHED_WRITER_LOCK_RETRY)

For write_vc->mutex (not stripe):

CacheRead.cc:

• L278 - openReadFromWriter (read aggregation)

Files Changed

• P_CacheStats.h: Add stripe_lock_contention and writer_lock_contention counters
• CacheProcessor.cc: Register both metrics
• P_CacheInternal.h: Add metric increments to retry macros, add VC_SCHED_WRITER_LOCK_RETRY()
• CacheRead.cc: Use VC_SCHED_WRITER_LOCK_RETRY() for writer mutex case