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SubjectRe: RFC: design for new VM


On Fri, 4 Aug 2000, Matthew Dillon wrote:
> :
> :There are architecture-specific special cases, of course. On ia64, the
> :..
>
> I spent a weekend a few months ago trying to implement page table
> sharing in FreeBSD -- and gave up, but it left me with the feeling
> that it should be possible to do without polluting the general VM
> architecture.
>
> For IA32, what it comes down to is that the page table generated by
> any segment-aligned mmap() (segment == 4MB) made by two processes
> should be shareable, simply be sharing the page directory entry (and thus
> the physical page representing 4MB worth of mappings). This would be
> restricted to MAP_SHARED mappings with the same protections, but the two
> processes would not have to map the segments at the same VM address, they
> need only be segment-aligned.

I agree that from a page table standpoint you should be correct.

I don't think that the other issues are as easily resolved, though.
Especially with address space ID's on other architectures it can get
_really_ interesting to do TLB invalidates correctly to other CPU's etc
(you need to keep track of who shares parts of your page tables etc).

> This would be a transparent optimization wholely invisible to the process,
> something that would be optionally implemented in the machine-dependant
> part of the VM code (with general support in the machine-independant
> part for the concept). If the process did anything to create a mapping
> mismatch, such as call mprotect(), the shared page table would be split.

Right. But what about the TLB?

It's not a problem on the x86, because the x86 doesn't have ASN's anyway.
But fo rit to be a valid notion, I feel that it should be able to be
portable too.

You have to have some page table locking mechanism for SMP eventually: I
think you miss some of the problems because the current FreeBSD SMP stuff
is mostly still "big kernel lock" (outdated info?), and you'll end up
kicking yourself in a big way when you have the 300 processes sharing the
same lock for that region..

(Not that I think you'd necessarily have much contention on the lock - the
problem tends to be more in the logistics of keeping track of the locks of
partial VM regions etc).

> (Linux falls on its face for other reasons, mainly the fact that it
> maps all of physical memory into KVM in order to manage it).

Not true any more.. Trying to map 64GB of RAM convinced us otherwise ;)

> I think the loss of MP locking for this situation is outweighed by the
> benefit of a huge reduction in page faults -- rather then see 300
> processes each take a page fault on the same page, only the first process
> would and the pte would already be in place when the others got to it.
> When it comes right down to it, page faults on shared data sets are not
> really an issue for MP scaleability.

I think you'll find that there are all these small details that just
cannot be solved cleanly. Do you want to be stuck with a x86-only
solution?

That said, I cannot honestly say that I have tried very hard to come up
with solutions. I just have this feeling that it's a dark ugly hole that I
wouldn't want to go down..

Linus


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