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

[Paolo Bonzini]

Il 10/06/2014 13:23, Michael S. Tsirkin ha scritto:
> On Tue, Jun 10, 2014 at 01:12:04PM +0200, Paolo Bonzini wrote:
> > Il 10/06/2014 11:59, Michael S. Tsirkin ha scritto:
> > > > > What's the point compared to memory-backend-ram?
> > > >
> > > > That you can use shared memory, for example together with vhost-user.
> > >
> > > I don't think it's a good idea until THP supports shared memory.
> >
> > 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).

> > 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.

> > We should not limit ourselves due to kernel bugs.
>
> Why not?  Practically people do have to run this on some kernel,
> we should not use kernel in a way that it can't support well.
> Old firefox doing a ton of fsync commands and slowing
> the box to a crawl comes to mind as another example of this.

Yes, and firefox doesn't say "no sorry can't do it" when running on such 
a kernel (which is much worse than more expensive TLB misses).

Paolo