linux-kernel - Re: [PATCH 1/2] x86: x86_64

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Date:   Fri, 15 Jun 2018 08:23:39 -0400
From:   "Austin S. Hemmelgarn" <ahferroin7@...il.com>
To:     Daniel Díaz <daniel.diaz@...aro.org>,
        x86@...nel.org
Cc:     anders.roxell@...aro.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86: x86_64_defconfig: Enable KSM.

On 2018-06-14 18:50, Daniel Díaz wrote:
> As per the documentation, Kernel Samepage Merging (available
> since 2.6.32) is a memory-saving de-duplication feature,
> enabled by CONFIG_KSM=y and activated via sysfs. More
> information can be found here:
>    https://www.kernel.org/doc/Documentation/vm/ksm.txt
> 
> When enabled in the kernel, the default is to not do anything
> at all, until it is activated at run-time with:
>    echo 1 > /sys/kernel/mm/ksm/run
> 
As pointed out by a couple of others, this doesn't explain why this is a 
good idea.  All you're doing here is giving a reason that it won't have 
a negative impact on most users.

Two points that may be worth adding, but also don't really argue for it 
being a significant improvement:

* Pretty much all of the major distributions that use pre-built kernels 
have it enabled in their kernels (At minimum, Debian, Ubuntu, Fedora 
(and by extension RHEL and CentOS), openSUSE (and by extension SLES), 
Arch, and Alpine have it enabled), so enabling this in defconfig would 
bring it a bit closer to parity with distribution kernels in terms of 
core features.

* Software other than QEMU is starting to take advantage of it if 
available (for example, Netdata [1] can mark it's in-memory TSDB's for 
deduplication, which usually cuts it's memory usage roughly in half).

[1] https://my-netdata.io/