Re: [Xen-devel] [RFC/PATCH] Improve speed of mapping guest memory into Dom0

[Mats Petersson <mats.petersson@xxxxxxxxxx>]

On 14/11/12 16:39, David Vrabel wrote:
> On 14/11/12 13:08, David Vrabel wrote:
> > On 14/11/12 11:13, Mats Petersson wrote:
> >
> > > I have also found that the munmap() call used to unmap the guest memory
> > > from Dom0 is about 35% slower in 3.7 kernel than in the 2.6 kernel (3.8M
> > > cycles vs 2.8M cycles).
> > This performance reduction only occurs with 32-bit guests is the Xen
> > then traps-and-emulates both halves of the PTE write.
> >
> > > I think this could be made quicker by using a
> > > direct write of zero rather than the compare exchange operation that is
> > > currently used [which traps into Xen, performs the compare & exchange] -
> > This is something I noticed but never got around to producing a patch.
> > How about this (uncomplied!) patch?
> >
> > -- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1146,8 +1146,16 @@ again:
> >                                      page->index > details->last_index))
> >                                         continue;
> >                         }
> > -                       ptent = ptep_get_and_clear_full(mm, addr, pte,
> > -                                                       tlb->fullmm);
> > +                       /*
> > +                        * No need for the expensive atomic get and
> > +                        * clear for anonymous mappings as the dirty
> > +                        * and young bits are not used.
> > +                        */
> > +                       if (PageAnon(page))
> The mapping might not be backed by pages (e.g., foreign mappings) so:
>
> if (!page || PageAnon(page))
Indeed, this works fine - it now takes just under 500K cycles to "unmap" 
1024 pages - compared to 3800k cycles with the original code.
--
Mats
> > +                               pte_clear(mm, addr, pte);
> > +                       else
> > +                               ptent = ptep_get_and_clear_full(mm, addr, pte,
> > +                                                               tlb->fullmm);
> >                         tlb_remove_tlb_entry(tlb, pte, addr);
> >                         if (unlikely(!page))
> >                                 continue;
> David

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