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SubjectRe: Transparent Hugepage Nit
On 01/15/2013 03:26:58 PM, Andi Kleen wrote:
> John McCorquodale <mcq@rockgeek.org> writes:
>
> > Suppose a hugepage-aligned mmap(MAP_ANONYMOUS) mapping has been
> madvise()d
> > HUGEPAGE. If a subeqeuent call to mremap() grows the mapping and
> has to
> > move the mapping, the hugepage-alignment is not preserved in the
> choice of
> > new address (in 3.7.2).
> >
> > I can workaround this by doing a 1-hugepage-oversized remap to find
> a new
> > aligned address and then size it back down MREMAP_FIXED, but that's
> probably
> > a lot of frags to 4k pages and back that aren't necessary.
> >
> > Should it not be the case that mremap(MAYMOVE) on something advised
> hugepage
> > ALWAYS chooses a hugepage-aligned address? This would be handy
> when doing the
> > initial allocation too: mmap, madvise, mremap (to the same size) to
> get
> > alignment.
>
> The hole searching currently doesn't know anything about transparent
> huge pages. There were some discussions on fixing it. But it's
> essentially a trade off between memory fragmentation and huge page
> optimization: aggressively aligning to 2MB can lose address space
> in holes.
>
> Usually if the program uses large enough mappings and enough memory
> it shouldn't be a problem.

Possibly we should just document that doing mremap() on a hugepage
loses the hugepageness, and consider it pilot error to do that?

Rob

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