--- title: Spotting and Avoiding Heap Fragmentation in Rust... description: A Rust axum service exhibited a strange 'stair-step' memory profile, gradually climbing to new plateaus and rarely dropping back down, threatening service... canonical: https://daily.dev/posts/spotting-and-avoiding-heap-fragmentation-in-rust-applications-vgglokaqb twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Spotting and Avoiding Heap Fragmentation in Rust Applications | daily.dev og:description: A Rust axum service exhibited a strange 'stair-step' memory profile, gradually climbing to new plateaus and rarely dropping back down, threatening service... og:url: https://daily.dev/posts/spotting-and-avoiding-heap-fragmentation-in-rust-applications-vgglokaqb og:image: https://api.daily.dev/og/posts/VGgLokAQB.png og:image:alt: Spotting and Avoiding Heap Fragmentation in Rust Applications 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. # Spotting and Avoiding Heap Fragmentation in Rust Applications **[Svix Blog](https://daily.dev/sources/svix)** · 9 min read · 0 upvotes · 0 comments ## Summary A Rust axum service exhibited a strange 'stair-step' memory profile, gradually climbing to new plateaus and rarely dropping back down, threatening service stability under load. Investigation ruled out a traditional memory leak and instead pointed to heap fragmentation, exacerbated by large request bodies and high concurrency, likely tied to JSON parsing with serde. A minimal toy project reproduced the issue. Swapping the default allocator for jemalloc, which is designed to reduce fragmentation and support scalable concurrency, resolved the problem: memory usage began rising and falling with load instead of ratcheting upward, both in the toy benchmark and in production. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does my Rust axum service show a stair-step memory pattern that never comes back down? This pattern typically indicates heap fragmentation rather than a memory leak, especially under workloads combining large request bodies with high concurrency. Fragmentation occurs when the allocator creates scattered gaps in the heap that are too small to reuse, forcing new memory to be allocated instead. Unlike a leak, growth plateaus rather than climbing steadily, and JSON parsing with serde was identified as a likely contributing factor in one investigated case. _daily.dev surfaces deep debugging writeups like this for developers chasing tricky Rust memory issues._ ### How do I fix heap fragmentation in a Rust service? Swapping the default allocator for jemalloc can resolve heap fragmentation in Rust services, since jemalloc is explicitly designed to emphasize fragmentation avoidance and scalable concurrency support. Using the tikv/jemallocator crate, the allocator can be swapped with minimal code changes. After the switch, memory usage rose and fell with load instead of permanently ratcheting upward. _Developers weighing allocator choices for Rust services can find practical comparisons like this on daily.dev._ ### What causes allocation failures or memory growth ceilings under high concurrency with large JSON payloads in Rust? Combining larger request bodies with higher concurrency was the key trigger for reproducing severe heap fragmentation in a Rust axum service, causing memory to plateau at unexpectedly high ceilings. Varying payload sizes across requests pushed the ceiling even higher. A minimal reproduction using a single JSON-accepting handler was enough to demonstrate a smaller version of the same effect. _daily.dev helps developers track down reproducible workload patterns behind Rust memory issues like this._ ## Similar posts on daily.dev - [Where Your Bytes Live: Memory Layout in Rust and Java](https://daily.dev/posts/where-your-bytes-live-memory-layout-in-rust-and-java-tjqya8qqa) · Medium · 4 upvotes · 1 comments - [Rust std fs slower than Python\!? No, it's hardware](https://daily.dev/posts/rust-std-fs-slower-than-python-no-it-s-hardware-ywqe52zqu) · Lobsters · 17 upvotes · 0 comments - [Box to save memory](https://daily.dev/posts/box-to-save-memory-foxzggogq) · Dystroy · 66 upvotes · 6 comments - [When Your Hash Becomes a String: Hunting Ruby’s Million-to-One Memory Bug](https://daily.dev/posts/when-your-hash-becomes-a-string-hunting-ruby-s-million-to-one-memory-bug-nhlgsnql3) · Closer to Code · 0 upvotes · 0 comments --- Tags: [#rust](https://daily.dev/tags/rust) [View this post on daily.dev](https://daily.dev/posts/spotting-and-avoiding-heap-fragmentation-in-rust-applications-vgglokaqb) ```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":"Spotting and Avoiding Heap Fragmentation in Rust Applications","url":"https://daily.dev/posts/spotting-and-avoiding-heap-fragmentation-in-rust-applications-vgglokaqb","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/spotting-and-avoiding-heap-fragmentation-in-rust-applications-vgglokaqb"},"datePublished":"2026-08-23T07:20:48.725Z","dateModified":"2026-08-23T07:28:30.229Z","description":"A Rust axum service exhibited a strange 'stair-step' memory profile, gradually climbing to new plateaus and rarely dropping back down, threatening service...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/c3eb3f625610669dbeb83f3c03df9a17?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/c3eb3f625610669dbeb83f3c03df9a17?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Svix Blog","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Svix Blog","logo":"https://media.daily.dev/image/upload/s--Shf9MlOr--/f_auto,q_auto/v1787469609/logos/svix?_a=BAMAMicg0","url":"https://daily.dev/sources/svix"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/spotting-and-avoiding-heap-fragmentation-in-rust-applications-vgglokaqb","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"rust","timeRequired":"PT9M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Svix Blog","item":"https://daily.dev/sources/svix"},{"@type":"ListItem","position":3,"name":"Spotting and Avoiding Heap Fragmentation in Rust Applications"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/spotting-and-avoiding-heap-fragmentation-in-rust-applications-vgglokaqb#faq","mainEntity":[{"@type":"Question","name":"Why does my Rust axum service show a stair-step memory pattern that never comes back down?","acceptedAnswer":{"@type":"Answer","text":"This pattern typically indicates heap fragmentation rather than a memory leak, especially under workloads combining large request bodies with high concurrency. 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