--- title: ArenaAllocators don't play nicely with ArrayLists description: When using Zig's ArenaAllocator, ArrayList growth can silently waste memory. When an ArrayList needs to grow beyond its current capacity and can't remap... canonical: https://daily.dev/posts/arenaallocators-don-t-play-nicely-with-arraylists-cormqlo0y twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: ArenaAllocators don't play nicely with ArrayLists | daily.dev og:description: When using Zig's ArenaAllocator, ArrayList growth can silently waste memory. When an ArrayList needs to grow beyond its current capacity and can't remap... og:url: https://daily.dev/posts/arenaallocators-don-t-play-nicely-with-arraylists-cormqlo0y og:image: https://api.daily.dev/og/posts/CorMQLo0y.png og:image:alt: ArenaAllocators don't play nicely with ArrayLists 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. # ArenaAllocators don't play nicely with ArrayLists **[openmymind](https://daily.dev/sources/Karl Seguin)** · 3 min read · 2 upvotes · 0 comments ## Summary When using Zig's ArenaAllocator, ArrayList growth can silently waste memory. When an ArrayList needs to grow beyond its current capacity and can't remap in-place, it allocates new memory, copies data, and frees the old allocation — but since the freed memory is no longer the last allocation, the ArenaAllocator cannot reclaim it. This can result in up to ~3x memory usage in the worst case. Mitigations include pre-sizing the ArrayList with initCapacity or ensureTotalCapacityPrecise, and avoiding interleaving other allocations with appends to maximize in-place remap success. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why doesn't ArenaAllocator actually free memory when an ArrayList grows in Zig? When an ArrayList grows, it allocates new larger memory, copies the old contents over, and then frees the old memory - meaning the old memory is never the arena's most recently allocated block. Since ArenaAllocator only returns memory to the pool when the freed block was the last allocation on the current node, this allocate-copy-free sequence guarantees the old memory can't be reclaimed, leaving stale memory allocated until the whole arena is destroyed. _daily.dev surfaces practical Zig memory-management gotchas like this for developers optimizing allocator usage._ ### How can I reduce wasted memory when using ArrayLists with an ArenaAllocator in Zig? Pre-size the ArrayList using initCapacity or ensureTotalCapacityPrecise to avoid repeated reallocation, and avoid interleaving other allocations with appends or writes so the allocator hits its remap (grow-in-place) branch more often instead of allocating, copying, and freeing. Without these precautions, worst-case memory usage with an ArenaAllocator can reach about 3x what's actually needed. _teams tuning Zig allocator performance can track memory-safety patterns like these through daily.dev._ ## Similar posts on daily.dev - [Making \`Arena.ofConfined\(\)\` Even Cheaper in JDK 28](https://daily.dev/posts/making-arena-ofconfined-even-cheaper-in-jdk-28-jp5eqpooo) · Inside Java · 3 upvotes · 0 comments - [Native Memory in Java: Arenas, Malloc, and Pools](https://daily.dev/posts/native-memory-in-java-arenas-malloc-and-pools-hyaefjfpb) · Foojay.io · 1 upvotes · 0 comments - [std.Io.Writer.Allocating ate all my memory](https://daily.dev/posts/std-io-writer-allocating-ate-all-my-memory-furqt8avu) · openmymind · 2 upvotes · 0 comments --- Tags: [#zig](https://daily.dev/tags/zig) [View this post on daily.dev](https://daily.dev/posts/arenaallocators-don-t-play-nicely-with-arraylists-cormqlo0y) ```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":"ArenaAllocators don't play nicely with ArrayLists","url":"https://daily.dev/posts/arenaallocators-don-t-play-nicely-with-arraylists-cormqlo0y","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/arenaallocators-don-t-play-nicely-with-arraylists-cormqlo0y"},"datePublished":"2026-08-03T23:51:37.458Z","dateModified":"2026-09-14T06:03:20.493Z","description":"When using Zig's ArenaAllocator, ArrayList growth can silently waste memory. 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Since ArenaAllocator only returns memory to the pool when the freed block was the last allocation on the current node, this allocate-copy-free sequence guarantees the old memory can't be reclaimed, leaving stale memory allocated until the whole arena is destroyed. daily.dev surfaces practical Zig memory-management gotchas like this for developers optimizing allocator usage."}},{"@type":"Question","name":"How can I reduce wasted memory when using ArrayLists with an ArenaAllocator in Zig?","acceptedAnswer":{"@type":"Answer","text":"Pre-size the ArrayList using initCapacity or ensureTotalCapacityPrecise to avoid repeated reallocation, and avoid interleaving other allocations with appends or writes so the allocator hits its remap (grow-in-place) branch more often instead of allocating, copying, and freeing. Without these precautions, worst-case memory usage with an ArenaAllocator can reach about 3x what's actually needed. teams tuning Zig allocator performance can track memory-safety patterns like these through daily.dev."}}]} ```