Sep 20 – 24, 2021
US/Pacific timezone

Limitations of tuning glibc malloc on larger systems.

Sep 21, 2021, 8:00 AM
GNU Tools track/Virtual-Room (LPC Virtual)

GNU Tools track/Virtual-Room

LPC Virtual

GNU Tools Track GNU Tools Track


Patrick McGehearty (Oracle)


The malloc library provided by glibc offers considerable flexibilty in deciding when to use mmap for larger allocations and when to use sbrk/trim. The default settings for the decision thresholds are reasonable for many applications. Three tunables are available to adjust these settings. The limits on these settings have not been changed since 2006. Server class systems now have much more memory available and other performance tradeoffs have changed dramatically in the last 15 years. We propose significant increases to the limitations on the MALLOC_MMAP_THRESHOLD_ tunable. (current default = 128K; current maximum 32M). This change will not affect existing usage while allowing select applications to improve their malloc performance, sometimes dramatically.

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Primary author

Patrick McGehearty (Oracle)

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