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.
> 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.
--
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
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