Re: [Qemu-devel] Performance regression using KVM/ARM

[Andrew Jones]

On Fri, Apr 22, 2016 at 12:26:52PM +0200, Christoffer Dall wrote:
> On Fri, Apr 22, 2016 at 11:17:47AM +0100, Peter Maydell wrote:
> > On 22 April 2016 at 11:15, Christoffer Dall <address@hidden> wrote:
> > > Peter just pointed me to a change I remember doing for ARM, so perhaps
> > > this fix is the right one?
> > >
> > >
> > > diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> > > index d25f671..a36e734 100644
> > > --- a/util/oslib-posix.c
> > > +++ b/util/oslib-posix.c
> > > @@ -35,7 +35,7 @@
> > >  extern int daemon(int, int);
> > >  #endif
> > >
> > > -#if defined(__linux__) && (defined(__x86_64__) || defined(__arm__))
> > > +#if defined(__linux__) && (defined(__x86_64__) || defined(__arm__)) || 
> > > defined(__aarch64__)
> > >     /* Use 2 MiB alignment so transparent hugepages can be used by KVM.
> > >        Valgrind does not support alignments larger than 1 MiB,
> > >        therefore we need special code which handles running on Valgrind. 
> > > */
> > 
> > I hadn't realised AArch64 didn't define __arm__. Your extra clause
> > wants to be inside the parens for the ||, not outside.
> > 
> > So was the problem just that we weren't passing 2MB as the align
> > parameter to qemu_ram_mmap(), and if we do pass 2MB then it does the
> > right thing ?
> > 
> Yes, that was essentially the problem.  I'll send a proper patch.
> 
> However, Marc pointed out, that we should probably think about improving
> on this in the future, because if you're running on a 64K page system
> and want huge pages, you have to align at a higher boundary, but I'm on
> the other hand not sure such a boundary is always practical on a 64K
> system.  Which would suggest that we either need to:
> 
> 1) Probe the kernel for the page size and always align to something that
> allows huge pages, or
> 
> 2) Let the user specify an option that says it wants to be able to use
> THP and only then align to the huge page boundary.
> 
> Not sure...

Probably (1), otherwise the T in THP becomes t, i.e. the use of THP
gets slightly less transparent. Though I agree that using THP on 64k
page systems isn't always a good choice, I think, in those cases,
that users would likely just disable THP for the whole system.

drew