linux-kernel - Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

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Date:   Wed, 15 May 2019 09:37:23 +0200
From:   Oleksandr Natalenko <oleksandr@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Timofey Titovets <nefelim4ag@...il.com>,
        Aaron Tomlin <atomlin@...hat.com>,
        Grzegorz Halat <ghalat@...hat.com>, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH RFC v2 0/4] mm/ksm: add option to automerge VMAs

Hi.

On Wed, May 15, 2019 at 08:53:11AM +0200, Michal Hocko wrote:
> On Wed 15-05-19 08:25:23, Oleksandr Natalenko wrote:
> [...]
> > > > Please make sure to describe a usecase that warrants adding a new
> > > > interface we have to maintain for ever.
> > 
> > I think of two major consumers of this interface:
> > 
> > 1) hosts, that run containers, especially similar ones and especially in
> > a trusted environment;
> > 
> > 2) heavy applications, that can be run in multiple instances, not
> > limited to opensource ones like Firefox, but also those that cannot be
> > modified.
> 
> This is way too generic. Please provide something more specific. Ideally
> with numbers. Why those usecases cannot use an existing interfaces.
> Remember you are trying to add a new user interface which we will have
> to maintain for ever.

For my current setup with 2 Firefox instances I get 100 to 200 MiB saved
for the second instance depending on the amount of tabs.

1 FF instance with 15 tabs:

$ echo "$(cat /sys/kernel/mm/ksm/pages_sharing) * 4 / 1024" | bc
410

2 FF instances, second one has 12 tabs (all the tabs are different):

$ echo "$(cat /sys/kernel/mm/ksm/pages_sharing) * 4 / 1024" | bc
592

At the very moment I do not have specific numbers for containerised
workload, but those should be similar in case the containers share
similar/same runtime (like multiple Node.js containers etc).

Answering your question regarding using existing interfaces, since
there's only one, madvise(2), this requires modifying all the
applications one wants to de-duplicate. In case of containers with
arbitrary content or in case of binary-only apps this is pretty hard if
not impossible to do properly.

> I will try to comment on the interface itself later. But I have to say
> that I am not impressed. Abusing sysfs for per process features is quite
> gross to be honest.

Sure, please do.

Thanks for your time and inputs.

-- 
  Best regards,
    Oleksandr Natalenko (post-factum)
    Senior Software Maintenance Engineer