--- title: Read Locks Are Not Your Friends | daily.dev description: A hands-on investigation into why RwLock performed ~5× worse than Mutex for a read-heavy LRU tensor cache in Rust. The root cause is cache-line ping-pong: even... canonical: https://daily.dev/posts/read-locks-are-not-your-friends-rfms5918e twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Read Locks Are Not Your Friends | daily.dev og:description: A hands-on investigation into why RwLock performed ~5× worse than Mutex for a read-heavy LRU tensor cache in Rust. The root cause is cache-line ping-pong: even... og:url: https://daily.dev/posts/read-locks-are-not-your-friends-rfms5918e og:image: https://api.daily.dev/og/posts/rFMs5918e.png og:image:alt: Read Locks Are Not Your Friends og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Read Locks Are Not Your Friends **[Lobsters](https://daily.dev/sources/lobsters)** · 5 min read · 0 upvotes · 0 comments ## Summary A hands-on investigation into why RwLock performed ~5× worse than Mutex for a read-heavy LRU tensor cache in Rust. The root cause is cache-line ping-pong: even a read lock requires an atomic increment of an internal reader counter, causing every core to fight for exclusive ownership of that cache line. For very short critical sections (nanosecond-scale hashmap lookups), this contention overhead dominates actual work time. Key takeaways: prefer Mutex for tiny critical sections, use sharding to reduce contention, and always profile with tools like perf or flamegraph before assuming RwLock is faster for read-heavy workloads. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Spinlocks Considered Harmful \(2020\)](https://daily.dev/posts/spinlocks-considered-harmful-2020--xqfzzgjdk) · Lobsters · 0 upvotes · 0 comments - [Read-Copy-Update \(RCU\): The Secret to Lock-Free Performance](https://daily.dev/posts/read-copy-update-rcu-the-secret-to-lock-free-performance-ekig5oxyy) · InfoQ · 0 upvotes · 0 comments - [Rust Prevents Data Races, Not Race Conditions](https://daily.dev/posts/rust-prevents-data-races-not-race-conditions-fjfi1lvd3) · corrode · 9 upvotes · 0 comments - [Spinning around: Please don't\!](https://daily.dev/posts/spinning-around-please-don-t--cekmueark) · Hacker News · 0 upvotes · 0 comments - [ReadWriteLock allows wrong-thread write release and stray read-release counter corruption](https://daily.dev/posts/readwritelock-allows-wrong-thread-write-release-and-stray-read-release-counter-corruption-nsztjc8hx) · RUBYLAND · 0 upvotes · 0 comments --- Tags: [#performance](https://daily.dev/tags/performance), [#rust](https://daily.dev/tags/rust) [View this post on daily.dev](https://daily.dev/posts/read-locks-are-not-your-friends-rfms5918e) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Read Locks Are Not Your Friends","url":"https://daily.dev/posts/read-locks-are-not-your-friends-rfms5918e","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/read-locks-are-not-your-friends-rfms5918e"},"datePublished":"2026-02-23T05:56:29.122Z","dateModified":"2026-02-23T05:56:49.973Z","description":"A hands-on investigation into why RwLock performed ~5× worse than Mutex for a read-heavy LRU tensor cache in Rust. 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