Date:   Thu, 8 Jun 2017 09:59:36 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 0/8] scheduler tinification


Also, let me make it clear at the outset that we do care about RAM footprint all 
the time, and I've applied countless data structure and .text reducing patches to 
the kernel. But there's a cost/benefit analysis to be made, and this series fails 
that test in my view, because it increases the complexity of an already complex 
code base:

* Nicolas Pitre <nicolas.pitre@...aro.org> wrote:

> Most IOT targets are so small that people are rewriting new operating systems 
> from scratch for them. Lots of fragmentation already exists.

Let me offer a speculative if somewhat cynical prediction: 90% of those ghastly 
IOT hardware hacks won't survive the market. The remaining 10% will be successful 
financially, despite being ghastly hardware hacks and will eventually, in the next 
iteration or so, get a proper OS.

As users ask for more features the the hardware capabilities will increase 
dramatically and home-grown microcontroller derived code plus minimal OSes will be 
replaced by a 'real' OS. Because both developers and users will demand IPv6 
compatibility, or Bluetooth connectivity, or storage support, or any random range 
of features we have in the Linux kernel.

With the stroke of a pen from the CFO: "yes, we can spend more on our next 
hardware design!" the problem goes away, overnight, and nobody will look back at 
the hardware hack that had only 1MB of RAM.

> [...] We're talking about systems with less than one megabyte of RAM, sometimes 
> much less.

Two data points:

Firstly, by the time any Linux kernel change I commit today gets to a typical 
distro it's at least 0.5-1 years, 2 years for it to get widely used by hardware 
shops - 5 years to get used by enterprises. More latency in more conservative 
places.

Secondly, I don't see Moore's Law reversing:

   http://nerdfever.com/wp-content/uploads/2015/06/2015-06_Moravec_MIPS.png

If you combine those two time frames, the consequence of this:

Even taking the 1MB size at face value (which I don't: a networking enabled system 
can probably not function very well with just 1MB of RAM) - the RAM-starved 1 MB 
system today will effectively be a 2 MB system in 2 years.

And yes, I don't claim Moore's law will go on forever and I'm oversimplifying - 
maybe things are slowing down and it will only be 1.5 MB, but the point remains: 
the importance of your 20kb .text savings will become a 10-15k .text savings in 
just 2 years. In 8 years today's 1 MB system will be a 32 MB system if that trend 
holds up.

You can already fit a mostly full Linux system into 32 MB just fine, i.e. the 
problem has solved itself just by waiting a bit or by increasing the hardware 
capabilities a bit.

But the kernel complexity you introduce with this series stays with us! It will be 
an additional cost added to many scheduler commits going forward. It's an added 
cost for all the other usecases.

Also, it's not like 20k .text savings will magically enable Linux to fit into 1MB 
of RAM - it won't. The smallest still practical more or less generic Linux system 
in existence today is around 16 MB. You can shrink it more, but the effort 
increases exponentially once you go below a natural minimum size.

> [...]  Still, those things are being connected to the internet. [...]

So while I believe small size has its value, I think it's far more important to be 
able to _trust_ those devices than to squeeze the last kilobyte out of the kernel.

In that sense these qualities:

 - reducing complexity,
 - reducing actual line count,
 - increasing testability,
 - increasing reviewability,
 - offering behavioral and ABI uniformity

are more important than 1% of RAM of very, very RAM starved system which likely 
won't use Linux to begin with...

So while it obviously the "complexity vs. kernel size" trade-off will always be a 
judgement call, for the scheduler it's not really an open question what we need to 
do at this stage: we need to reduce complexity and #ifdef variants, not increase 
it.

Thanks,

	Ingo

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