WTinyLFU: Evict main_cache_victim when victim from window cache wins

[manitofigh]

From #162.
I tested some pre/post miss ration comparisons (txt/vscsi/oracle):
The miss ratios are identical for txt (0.4373 vs 0.4373) and vscsi (same).
For oracle, develop is 0.8246 and this branch is 0.8233 (tiny improvement of -0.0013).

[gemini-code-assist]

Summary of Changes

Hello @manitofigh, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a subtle oversight in the WTinyLFU eviction policy by ensuring that the evicted flag is properly updated when an object from the window cache successfully replaces an item in the main cache. This change aims to improve the precision of eviction tracking within the cache simulation, which has shown minor positive impacts on miss ratios for specific workloads.

Highlights

• WTinyLFU Eviction Logic: The evicted flag is now correctly set to true when an object from the window cache wins the eviction battle and is subsequently admitted into the main cache. This ensures accurate tracking of eviction events within the WTinyLFU algorithm.

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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in the WTinyLFU_evict function. When a victim from the window cache has a higher frequency than a victim from the main cache, an eviction from the main cache occurs, and the window victim is promoted. Previously, the evicted flag was not set in this scenario, which could lead to an infinite loop or multiple evictions. By adding evicted = true;, this change ensures the eviction loop terminates correctly. The fix is sound and improves the correctness of the W-TinyLFU implementation. While reviewing this function, I noticed another potential issue. In the case where the main cache has enough space to admit an object from the window cache (lines 278-290), the object is moved, but no space is freed in the overall cache system, and the evicted flag is not set. Since WTinyLFU_evict is called in a loop by cache_get_base to free up space, this could lead to an infinite loop. I recommend investigating this as a potential critical bug in a follow-up.