Update metrics tracking and enhance cache eviction strategies

- Added metrics for bytes saved from cache to improve performance insights.
- Updated cache eviction strategies in MemoryFS and DiskFS to include metrics tracking for hits and evictions.
- Enhanced README.md with updated garbage collection algorithm descriptions and recommendations for cache usage.
- Introduced new madviseSequential functionality for improved memory access hints on Unix systems.
- Adjusted validation configuration in examples to better reflect realistic usage scenarios.

docs/examples/validate-config.yaml

@@ -6,11 +6,14 @@
 # "benchmark" commands.
 #
 # Why these values?
-# - Both memory and disk tiers are enabled but deliberately small.
-#   A modest benchmark workload (a few hundred MB to low GB) will exercise:
-#     * Memory tier promotions
-#     * Disk tier attach + writes
-#     * GC / eviction pressure on at least one tier
+# - Both tiers enabled. Memory is sized large enough to survive the disk attach
+#   window in mixed mode (see steamcache.go: the goroutine that blocks on d.Size()
+#   before SetSlow). With a realistic SteamPrefill benchmark (high rate of unique
+#   ~1MB chunks) the old tiny 128MB mem + 512MB disk caused almost all early
+#   content to live only in memory, get evicted by its GC, and never reach disk.
+#   Result: "never hitting", hit_rate 0, memory_size 0, despite files appearing
+#   on disk for late-arriving chunks. Larger mem + disk makes the validation
+#   actually exercise hits, promotions, disk tier, and GC as intended.
 # - Conservative concurrency limits suitable for a developer laptop.
 # - trusted_proxies set for 127.0.0.0/8 so that an external benchmark tool
 #   can simulate multiple distinct clients via X-Forwarded-For if desired.
@@ -31,7 +34,7 @@
 
 listen_address: :80
 
-max_concurrent_requests: 100
+max_concurrent_requests: 1000
 max_requests_per_client: 10
 
 max_object_size: "0"          # unlimited for validation (real Steam files can be large)
@@ -39,10 +42,10 @@ trusted_proxies: ["127.0.0.0/8"]
 
 cache:
   memory:
-    size: 128MB
-    gc_algorithm: lru      # lru is somewhat recommeded in the project README and is better for validation since its not just using the same again
+    size: 1GB
+    gc_algorithm: largest
   disk:
-    size: 512MB
+    size: 2GB
     path: ./validate-disk     # ephemeral; clean between runs if you want a fresh test
     gc_algorithm: hybrid      # recommended for disk in the project README