Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend

[Paolo Bonzini]

Il 10/06/2014 13:35, Michael S. Tsirkin ha scritto:
> > > > Why?  For example it would be useful for testing on machines that you don't
> > > > have root for, and that do not have a hugetlbfs mount point.  For example
> > > > you could run the test case from the vhost-user's patches.
> > >
> > > Sounds useful, I guess we could allow this when running under qtest.
> >
> > Or TCG, or Xen.  At this point, why single out KVM?
> >
> > (Also, "--enable-kvm -mem-path /dev/shm" works on 2.0, and it would be a
> > regression in 2.1).
>
> It prints
>         fprintf(stderr, "Warning: path not on HugeTLBFS: %s\n", path);
>
> Correct?

Yes.

> I guess I agree then, hopefully the warning is enough.
> Maybe add an extra warning that performance will suffer.
>
> > > > THP is not a magic wand and you can get slowness from memory fragmentation
> > > > at any time.
> > >
> > > Right but there's a difference between "can get slowness when memory
> > > is overcommitted" and "will get slowness even on a mostly idle box".
> >
> > I would like to see the slowness on a real-world benchmark though.  I
> > suspect in most scenarios it would not matter.
>
> Weird.  Things like kernel build time are known to be measureably
> improved by using THP.

Even measurable speedups in most scenarios would not matter.  I don't 
care if a kernel compile takes 2 minutes vs. 110 seconds (for a 10% 
speedup), even though it's great that THP can speed up such a common task.

Paolo