Re: [Qemu-devel] [PATCH] Call MADV_HUGEPAGE for guest RAM allocations

[Aurelien Jarno <aurelien@aurel32.net>]

On Mon, Oct 15, 2012 at 03:57:54PM -0300, Luiz Capitulino wrote:
> On Fri, 5 Oct 2012 16:47:57 -0300
> Luiz Capitulino <lcapitulino@redhat.com> wrote:
> 
> > This makes it possible for QEMU to use transparent huge pages (THP)
> > when transparent_hugepage/enabled=madvise. Otherwise THP is only
> > used when it's enabled system wide.
> > 
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> 
> ping?
> 
> > ---
> >  exec.c  | 1 +
> >  osdep.h | 5 +++++
> >  2 files changed, 6 insertions(+)
> > 
> > diff --git a/exec.c b/exec.c
> > index 1114a09..7504909 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -2584,6 +2584,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
> >      cpu_physical_memory_set_dirty_range(new_block->offset, size, 0xff);
> >  
> >      qemu_ram_setup_dump(new_block->host, size);
> > +    qemu_madvise(new_block->host, size, QEMU_MADV_HUGEPAGE);
> >  
> >      if (kvm_enabled())
> >          kvm_setup_guest_memory(new_block->host, size);
> > diff --git a/osdep.h b/osdep.h
> > index cb213e0..c5fd3d9 100644
> > --- a/osdep.h
> > +++ b/osdep.h
> > @@ -108,6 +108,11 @@ void qemu_vfree(void *ptr);
> >  #else
> >  #define QEMU_MADV_DONTDUMP QEMU_MADV_INVALID
> >  #endif
> > +#ifdef MADV_HUGEPAGE
> > +#define QEMU_MADV_HUGEPAGE MADV_HUGEPAGE
> > +#else
> > +#define QEMU_MADV_HUGEPAGE QEMU_MADV_INVALID
> > +#endif
> >  
> >  #elif defined(CONFIG_POSIX_MADVISE)
> >  
> 

I don't know this part of QEMU very well, so I tried to compare with how
it was done for KSM. I found two main differences:
- In the case of -mem-path QEMU doesn't try to mark the pages as
  mergeable.
- An option (-machine mem-merge=false/true) is provided to enable KSM,
  defaulting to true.

I am not sure if it makes sense for hugepages, but providing a
mem-huge=false/true defaulting to true might be a good idea.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net