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

[Michael S. Tsirkin]

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.

> 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".

> We should not limit ourselves due to kernel bugs.
> 
> Paolo

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.

-- 
MST