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Pavel
Asked: 2019-05-23 12:40:36 +0800 CST

How does Linux kernel decide when to free pages allocated via mmap?

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I have an application that mmaps a large read-only data structure into memory and reads parts of it. After some time, around 1 GB of physical memory is filled with pages loaded from this data structure. When the amount of free memory on that machine drops below a certain threshold (~750 MB or so), the kernel seems to release all (or almost all) pages loaded through mmap.

How does Kernel decide when to start unloading mmapped pages? Is there a way to control that threshold? (e.g. anything in /proc/sys/vm or cgroups)

memory-usage

2 Answers

  1. From manual, the munmap() system call deletes the mappings for the specified address range, causing further references to addresses within the range to generate invalid memory references.

    DIRTY PAGE HANDLING How munmap() handles a dirty page, depends on what type of memory is being unmapped:

     [Anonymous]        If the memory is anonymous memory and if the last reference is going away, then the contents are discarded by definition of anonymous memory.
 [System V Shared]  If the memory mapping was created using System V shared memory, then the contents persist until the System V memory region is destroyed or the sys-                        tem is rebooted.
 [File mapping]     If the mapping maps data from a file (MAP_SHARED), then the memory will eventually be written back to disk if it's dirty.  This will happen automat-                        ically at some point in the future (implementation dependent).  Note: to force the memory to be written back to the disk, use msync(2).
 If there are still other references to the memory when the munmap is done, then nothing is done to the memory itself and it may be swapped out if need be. The memory
 will continue to persist until the last reference goes away
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  2. Tweaking the thresholds is a bit tricky. There always needs to be some buffer of free memory, for good performance. And personally, I don't fully understand virtual memory tuning with regards to mmap pages vs anonymous.

    For more of the working set in memory, add RAM to the host. Or maybe a faster storage system, to read parts of the file in.

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