Re: [f-cpu] Do we really need virtual memory?

[Yann Guidon <whygee@f-cpu.org>]

hi,

Beat Steiner wrote:

> The question might be stupid, but I ask it anyway:
> Do we need virtual memory?
it depends on whether you mean "we" = "I" or not.
if you think that VM is pointless, just drop it.
however, as F-CPU is meant to be a workstation-class CPU,
virtual memory addressing to hardware memory addressing mapping
is doubtlessly necessary.

> In a huge address space,
> every program could be physical memory aware.
but, you know, programs "have a life" (they grow, shrink, etc)
and they might want to allocate more memor than physically available.

In an embedded system, it might make sense to tune the whole
software (OS+application) in order to limit the required memory
to the available SDRAM and use ucLinux for example.

But when you want to execute desktop-class or server-class
applications, you can't drop the virtual memory.

> The only case I see now that VM makes sense is memory fragmentation:
> Example: I need to expand a memory block, but there is already one 
> allocated
> next to it. VM could map the new block from anywhere to the adjacent 
> address...
> With VM, we save the work of moving around the memory block to be 
> enlarged.
hmmm you start to see the light ;-)

> Any further examples welcome.
malloc() a huge area and only access some sparse elements from time to time.
without VM, your allocated memory must be smaller than
SDRAM size - already allocated memory.
with VM, you are not limited, so you can physically allocate a memory
page on first access and  swap the unused pages, at the cost of VM traps
(page faults triggered by unallocated pages or pages that are swpped out).

> Getting rid of VM would simplify the "TLB" (only permission checks) and
> significantly reduce its footprint.We may get rid of VM for the FC0 
> only and add it again later.
the first "working" versions of FC0 are do not necessary implement the TLB,
it will come around F-CPU v0.7 or 0.8.

YG



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