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

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Date:   Thu, 23 Jan 2020 14:23:59 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Alexander Graf <graf@...zon.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        kvm@...r.kernel.org, mst@...hat.com, linux-kernel@...r.kernel.org,
        willy@...radead.org, mhocko@...nel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        vbabka@...e.cz, "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        yang.zhang.wz@...il.com, nitesh@...hat.com, david@...hat.com,
        pagupta@...hat.com, riel@...riel.com, lcapitulino@...hat.com,
        wei.w.wang@...el.com, aarcange@...hat.com, pbonzini@...hat.com,
        dan.j.williams@...el.com, osalvador@...e.de,
        "Paterson-Jones, Roland" <rolandp@...zon.com>, hannes@...xchg.org,
        hare@...e.com, "Boeuf, Sebastien" <sebastien.boeuf@...el.com>
Subject: Re: [PATCH v16.1 0/9] mm / virtio: Provide support for free page
 reporting

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/