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Re: Why does Linux need memory management units and cache?

Cross-posted to 3 groups.

Followups directed to: comp.os.linux.advocacy, comp.os.linux.embedded, uk.comp.os.linux

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Piergiorgio Sartor wrote:

> On 05/19/2012 07:34 PM, 7 wrote:
> [...]
>> Virtual address space in a nut shell means program counter relative
>> operation. So if I were careful to keep supervisory
>> programs separate from user programs, and manage my address
>> space of user programs with PC relative addressing,
>> I won't need memory management hardware. All I need is a
>> fast program that keeps the virtual memory management functions
>> going because that should still be faster than setting up MMU
>> and managing its operations.
> 
> Actually, that's more or less what Amiga was doing
> and many Real Time OSes.
> 
> It is faster, also faster to crash, since programs
> could, -intentionally or unintentionally-, address
> memory which they should not.

That is trivial to address.
The crashing can be prevented if PC relative
addressing had bound checking hardware that executes trap
whenever address range is out of bounds.

So MMU is not needed if Linux could manage its
memory with fast software, and if CPUs came equipped
with bound checking for PC relative addressing.

 
>> So a cache could be implemented in software. And if done properly
> 
> I doubt it is possible, unless the CPU supports such
> things, having, for example, some scratch pad RAM.
> 
> Since cache is cache, i.e. fast but small memory, it
> has to be an hardware component.
> How this hardware is handled, is CPU dependent.

A cache could be more effective if smart
software precisely keeps tabs on what
the DMA controller is doing; and programs that
are running have fine grained messaging that lets
the cache know what is happening with them.
That way cache fetching could be aborted and flushed
more rapidly than letting the cache decide what
to cache. Aborting a cache operation can save tons
of time on a CPU because typically it can take
hundreds of nanoseconds to microseconds
to set up another fetch from somewhere else,
whilst during that time, 100+ instructions
could have executed making an intelligent
decision as to what best to cache.

So in my opinion, hardware based
caching is a waste of time.

So to the best of my opinion,
with these ultra modern CPUs that are running at
gigahertz speeds, the most important advancements
for an operating system and speed is better software
for software implementation of MMU and better software for caching
and to remove the hardware MMU and hardware cache
(and in its place put in more DMA controllers and local RAM).

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Thread

Why does Linux need memory management units and cache? 7 <email_at_www_at_enemygadgets_dot_com@enemygadgets.com> - 2012-05-19 13:40 +0100
  Re: Why does Linux need memory management units and cache? Niklas Holsti <niklas.holsti@tidorum.invalid> - 2012-05-19 19:40 +0300
    Re: Why does Linux need memory management units and cache? 7 <email_at_www_at_enemygadgets_dot_com@enemygadgets.com> - 2012-05-19 18:34 +0100
      Re: Why does Linux need memory management units and cache? Piergiorgio Sartor <piergiorgio.sartor.this.should.not.be.used@nexgo.REMOVETHIS.de> - 2012-05-19 19:44 +0200
        Re: Why does Linux need memory management units and cache? 7 <email_at_www_at_enemygadgets_dot_com@enemygadgets.com> - 2012-05-19 19:57 +0100
          Re: Why does Linux need memory management units and cache? Piergiorgio Sartor <piergiorgio.sartor.this.should.not.be.used@nexgo.REMOVETHIS.de> - 2012-05-19 21:17 +0200
            Re: Why does Linux need memory management units and cache? 7 <email_at_www_at_enemygadgets_dot_com@enemygadgets.com> - 2012-05-20 22:53 +0100
      Re: Why does Linux need memory management units and cache? Grant Edwards <invalid@invalid.invalid> - 2012-05-20 14:42 +0000
        Re: Why does Linux need memory management units and cache? Hadron<hadronquark@gmail.com> - 2012-05-20 16:48 +0200
          Re: Why does Linux need memory management units and cache? Nix <nix-razor-pit@esperi.org.uk> - 2012-05-20 22:19 +0100
            Re: Why does Linux need memory management units and cache? 7 <email_at_www_at_enemygadgets_dot_com@enemygadgets.com> - 2012-05-20 22:55 +0100
              Re: Why does Linux need memory management units and cache? Nix <nix-razor-pit@esperi.org.uk> - 2012-05-21 16:24 +0100
                Re: Why does Linux need memory management units and cache? chrisv <chrisv@nospam.invalid> - 2012-05-21 10:44 -0500
                Re: Why does Linux need memory management units and cache? GreyCloud <mist@cumulus.com> - 2012-05-21 10:51 -0600
                Re: Why does Linux need memory management units and cache? GreyCloud <mist@cumulus.com> - 2012-05-21 10:51 -0600
                Re: Why does Linux need memory management units and cache? Foster <frankfoster50@yahoo.com> - 2012-05-21 12:53 -0400

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