On Fri, Nov 16, 2012 at 1:13 AM, David Gibson
<da...@gibson.dropbear.id.au>wrote:
> On Thu, Nov 15, 2012 at 04:31:44PM -0500, Eric B Munson wrote:
> > On 2012-11-15 14:44, telenn barz wrote:
> > > On Thu, Nov 15, 2012 at 3:34 PM, Eric B Munson <emun...@mgebm.net
> > > [1]>
> > > wrote:
> > >
> > >> On 2012-11-15 03:42, telenn barz wrote:
> > >>
> > >>> Making some tests with libhugetlbfs on kernel 2.6.34, backing
> > >>> automatically the heap, we realized that the fallback mechanism
> > >>> is
> > >>> all-or-nothing : if theres not enough hugepages, the whole asked
> > >>>
> > >>> memory is backed by base pages.
> > >>>
> > >>> We expected libhugetlbfs to mix hugepages / basepages in such a
> > >>> case.
> > >>> Would there be any issue with this requirement ?
> > >
> > >>> Thanks,
> > >>> Telenn
> > >>
> > >> Telenn,
> > >>
> > >> This may be possible on x86/x86_64 but it isnt possible on POWER
> > >> due to memory segment page size restrictions. To avoid having arch
> > >> dependent morecore hooks we have not attempted this.
> > >
> > > Page size restrictions on POWER ? Could you please tell more about
> > > this - in a few words ?
> >
> >
> > Sure, PPC view memory in segments, for memory below 4GB these segments
> > are 256MB, and I don't recall the segment size over 4GB.
>
> 1TB. From a hardware PoV, the OS can use 256MB or 1TB segments as it
> chooses (as long as they're naturally aligned). But Linux (at
> present) always uses 256MB segments below 4GB and 1TB segments above.
>
This segment notion, wouldn't it be for hash page table based powerpc only ?
What about Freescale e500 family cores (bookE) ? (I don't see any segment
notion for them - maybe I missed something)
> > Inside a
> > segment the page size must be uniform. So if morecore wanted to handle
> > your version of fallback it would also have to know where it can place
> > huge pages still.
>
> In addition I don't think the glibc morecore callback lets us control
> the fallback that closely. IIRC, we just fail the morecore, and glibc
> falls back on an mmap(). So even on x86 to do partial fallback we'd
> need a different (and much more complex) implementation of hugepage
> malloc() backing than just intercepting the morecore hook.
>
> Are you talking about a full re-implementation of malloc within
libhugetlbfs ?
> --
> David Gibson | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_
> _other_
> | _way_ _around_!
> http://www.ozlabs.org/~dgibson
>
Telenn
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