Re: [Qemu-devel] [PATCH 1/4] multifd: Change page count default to 128

[Juan Quintela <quintela@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> wrote:
> On Wed, Feb 06, 2019 at 02:23:28PM +0100, Juan Quintela wrote:
>> I haven't seend any problem about using 64 or 128.  And it make much
>> less contention on the locks.  Just change it.
>
> Isn't there a issue with having a fixed page count given the
> very different default page sizes across architectures ?
>
> x86 is 4kb pages, while ppc64 uses 64kb pages IIUC.
>
> This would mean current value of 64 pages, would correspond
> to 1/4 MB on x86, and 4 MB on ppc64.  The new value would
> be 1/2 MB on x86 and 8 MB on ppc64.

I saw no difference (on x86 between 64 and 128 pages).  Bigger pages
means half the contention on the locks and better for compression (see
next series).


> Should we instead be measuring this tunable in units that
> are independant of page size ? eg meansure in KB, with a
> requirement that the value is a multiple of the page size.
> Then set the default to 512 KB ?

See next patch, I just dropped the tunable altogether.  Libvirt don't
want to support it (difficult to explain), and in the past you asked me
to choose a sane value and live with it O:-)
It was good for testing, though.

Once there, is there a good value for a network packet?
I put it in pages because it facilitates the coding, but doing a:

CONSTANT/qemu_target_page_size() is not going to complicate anything
either.

Later, Juan.