Re: [RFC][PATCH] avoid swapping out with swappiness==0

[Johannes Weiner]

On Fri, Mar 02, 2012 at 12:36:40PM -0500, Satoru Moriya wrote:
> Sometimes we'd like to avoid swapping out anonymous memory
> in particular, avoid swapping out pages of important process or
> process groups while there is a reasonable amount of pagecache
> on RAM so that we can satisfy our customers' requirements.
> 
> OTOH, we can control how aggressive the kernel will swap memory pages
> with /proc/sys/vm/swappiness for global and
> /sys/fs/cgroup/memory/memory.swappiness for each memcg.
> 
> But with current reclaim implementation, the kernel may swap out
> even if we set swappiness==0 and there is pagecache on RAM.
> 
> This patch changes the behavior with swappiness==0. If we set
> swappiness==0, the kernel does not swap out completely
> (for global reclaim until the amount of free pages and filebacked
> pages in a zone has been reduced to something very very small
> (nr_free + nr_filebacked < high watermark)).
> 
> Any comments are welcome.

Last time I tried that (getting rid of sc->may_swap, using
!swappiness), it was rejected it as there were users who relied on
swapping very slowly with this setting.

KOSAKI-san, do I remember correctly?  Do you still think it's an
issue?

Personally, I still think it's illogical that !swappiness allows
swapping and would love to see this patch go in.
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