# ParparVM Reduces HashMap Miss Costs with a New Probe Sequence

DevFeed: [ParparVM Reduces HashMap Miss Costs with a New Probe Sequence](<https://devfeed.tech/articles/faster-maps-chasing-swiss-speed-19318.md>)

Original publisher: [Read original article](<https://www.codenameone.com/blog/hashmap-misses-probe-sequence/>)

Author: Shai Almog

Published: 2026-09-13T00:00:00Z

Content type: article

Language: en

Sources: [CodeName One](<https://devfeed.tech/sources/codename-one.md>)

Topics: [Java](<https://devfeed.tech/topics/java.md>), [Data structures](<https://devfeed.tech/topics/data-structures.md>), [Benchmark](<https://devfeed.tech/topics/benchmark.md>)

Tags: [benchmark](<https://devfeed.tech/tags/benchmark.md>), [compare](<https://devfeed.tech/tags/compare.md>), [java](<https://devfeed.tech/tags/java.md>)

## AI overview

This article examines pathological missing-key performance in ParparVM's Java HashMap. It identifies linear probing through dense key runs and tombstones as the cause, then describes a fix that preserves the first probe while using a CPython-style recurrence for subsequent probes. The change improved misses but slowed some dense-key construction and scan workloads by 1.8 to 2.2 times in the measured cases.

## Source excerpt

ParparVM fixes pathological map misses and extends tagged boxed values. Probe counts, allocation coverage, and regressions show where ordinary Java collections became cheaper.