Re: shared pagetable benchmarking

[Andrew Morton <akpm@digeo.com>]

Linus Torvalds wrote:
> 
> On Fri, 27 Dec 2002, Andrew Morton wrote:
> >
> > I think we can do a few things still in the 2.6 context.  The fact that
> > my "apply seventy patches with patch-scripts" test takes 350,000 pagefaults
> > in 13 seconds makes one go "hmm".
> 
> Hmm.. Whatever happened to the MAP_POPULATE tests?
> 
> The current "filemap_populate()" function is extremely stupid (it takes
> advantage neither of the locality of the page tables _nor_ of the radix
> tree layout), but even so it would probably be a win to pre-populate at
> mmap time.

Yup.  Ingo said at the time:

  It would be faster to iterate the pagecache mapping's radix tree
  and the pagetables at once, but it's also *much* more complex. I have
  tried to implement it and had to unroll the change - mixing radix tree
  walking and pagetable walking and getting all the VM details right is
  really complex - especially considering all the re-lookup race checks
  that have to occur upon IO.

But find_get_pages() is well-suited to this, and was not in place when
he did this work.

> But having a better "populate()" function that actually does multiple
> pages at once by just accessing the radix trees and page table trees
> directly should really be very low-overhead for the normal case, and be a
> _big_ win in avoiding page faults.
> 
> Even with the existing stupid populate function, it might be interesting
> seeing what would happen just from doing something silly like
> 
> ===== arch/i386/kernel/sys_i386.c 1.10 vs edited =====
> --- 1.10/arch/i386/kernel/sys_i386.c    Sat Dec 21 08:24:45 2002
> +++ edited/arch/i386/kernel/sys_i386.c  Fri Dec 27 19:08:30 2002
> @@ -54,6 +54,8 @@
>                 file = fget(fd);
>                 if (!file)
>                         goto out;
> +               if (prot & PROT_EXEC)
> +                       flags |= MAP_POPULATE | MAP_NONBLOCK;

Yes, this could be used to prototype it, I think.

It doesn't work as-is, because remap_file_pages() requires a shared
mapping.  Disabling that check results in a scrogged ld.so and a
non-booting system.  remap_file_pages() plays games with the vma
protection in ways which I do not understand.

So hum.  I'll finish off some other stuff, take a more detailed look
at this soon.
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