Re: Memory mapped pages not being swapped out

[Rafael Telles <rafaelt@simbioseventures.com>]

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Thank you so much for your attention Michal,

Are there any settings (such as sysctl parameters) that I can use to better
control the memory reclaiming? Such as: defining the max. amount of mmap
pages allocated or max. amount of memory used by mmap pages?

Or will the system start reclaiming only when it needs more memory?

I found that I could use madvise with MADV_DONTNEED in order to actively
free RSS memory used by mmap pages, but it would add more complexity on my
software.

On Wed, Jun 6, 2018 at 9:43 AM Michal Hocko <mhocko@kernel.org> wrote:

> On Tue 05-06-18 16:14:02, Rafael Telles wrote:
> > Hi there, I am running a program where I need to map hundreds of
> thousands
> > of files and each file has several kilobytes (min. of 4kb per file). The
> > program calls mmap() for every 4096 bytes on each file, ending up with
> > millions of memory mapped pages, so I have ceil(N/4096) pages for each
> > file, where N is the file size.
> >
> > As the program runs, more files are created and the older files get
> bigger,
> > then I need to remap those pages, so it's always adding more pages.
> >
> > I am concerned about when and how Linux is going to swap out pages in
> order
> > to get more memory, the program seems to only increase memory usage
> overall
> > and I am afraid it runs out of memory.
>
> We definitely do reclaim mmaped memory - be it a page cache or anonymous
> memory. The code doing that is mostly in shrink_page_list (resp.
> page_check_references for aging decisions) - somehow non-trivial to
> follow but you know where to start looking at least ;)
>
> > I tried setting these sysctl parameters so it would swap out as soon as
> > possible (just to understand how Linux memory management works), but it
> > didn't change anything:
> >
> > vm.zone_reclaim_mode = 1
>
> This will make difference only for NUMA machines and it will try to
> keep allocations to local nodes. It can lead to a more extensive
> reclaim but I would definitely not recommend setting it up unless you
> want a strong NUMA locality payed by reclaiming more while the rest of
> the memory might be sitting idle.
>
>
> > vm.min_unmapped_ratio = 99
>
> This one is active only for the zone/node reclaim and tells whether to
> reclaim the specific node based on how much of memory is mapped. Your
> setting would tell that the node is not worth to be reclaimed unless 99%
> of it is clean page cache (the behavior depends on the zone_reclaim_mode
> because zone_reclaim_mode = 1 excludes mapped pages AFAIR).
>
> So this will most likely not do what you think.
>
> > How can I be sure the program won't run out of memory?
>
> The default overcommit setting should not allow you to mmap too much in
> many cases.
>
> > Do I have to manually unmap pages to free memory?
>
> No.
> --
> Michal Hocko
> SUSE Labs
>

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