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

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Message-ID: <21444cdc-76f9-1b06-093e-950cbeb4aa1f@intel.com>
Date:   Thu, 23 Jan 2020 09:20:15 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     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>
Cc:     yang.zhang.wz@...il.com, nitesh@...hat.com, konrad.wilk@...cle.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 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 isn't conceptually *that* far away from the "proactive reclaim"
that other folks have proposed:

	https://lwn.net/Articles/787611/