Re: [PATCH v16.1 0/9] mm / virtio: Provide support for free page reporting

[Konrad Rzeszutek Wilk]

On Thu, Jan 23, 2020 at 09:20:15AM -0800, Dave Hansen wrote:
> On 1/23/20 8:26 AM, Alexander Duyck wrote:
> >> The big piece I'm missing is the page cache. Linux will by default try 
> >> to keep the free list as small as it can in favor of page cache, so most 
> >> of the benefit of this patch set will be void in real world scenarios.
> > Agreed. This is a the next piece of this I plan to work on once this is
> > accepted. For now the quick and dirty approach is to essentially make use
> > of the /proc/sys/vm/drop_caches interface in the guest by either putting
> > it in a cronjob somewhere or to have it after memory intensive workloads.
> 
> There was an implementation in "Clear Linux" that used this sysctl:
> 
> > https://github.com/Conan-Kudo/omv-kernel-rc/blob/master/0154-sysctl-vm-Fine-grained-cache-shrinking.patch
> 
> (I can't find it in the Clear repos at the moment, must not be used
> currently).  But the idea was to have a little daemon in the host that
> periodically applied some artificial pressure with this sysctl.  This
> sysctl is a smaller hammer than /proc/sys/vm/drop_caches and lets you
> drop small amounts of cache.
> 
> The right way to do it is probably to do real, generic reclaim instead
> of drop_caches.

This  sounds like Transcendent Memory (https://www.linux-kvm.org/images/d/d7/TmemNotVirt-Linuxcon2011-Final.pdf)
which has (which is in the Linux kernel) a driver to push on the swapper
and everything else to evict pages to the hypervisor. 

Look at cleancache and frontswap and xen-selfballoon.c (was removed by
by 814bbf49dcd0ad642e7ceb8991e57555c5472cce)

Avi Kivity pointed out one big issue with all of this - customers have
to be nicely behaved - which they don't seem to be.

But I would recommend you look at cleancache for the page cache.

> 
> This isn't conceptually *that* far away from the "proactive reclaim"
> that other folks have proposed:
> 
> 	https://lwn.net/Articles/787611/