Re: [PATCH net-next 1/3] xen-netback: page pool support

[Wei Liu <wei.liu2@citrix.com>]

On Fri, May 24, 2013 at 01:44:31PM +0100, David Vrabel wrote:
> On 24/05/13 11:32, Wei Liu wrote:
> > This patch implements a page pool for all vifs. It has two functionalities:
> >  a) to limit the amount of pages used by all vifs
> >  b) to track pages belong to vifs
> 
> This adds a global spin lock.  This doesn't seem very scalable.
> 
> It's also not clear how this is usefully limiting the memory usage by
> guest network traffic.  It limits the number of pages that netback can
> use during the grant copy from the guest pages but this is only short
> time compared to the lifetime of the network packet within the rest of
> the network stack.
> 
> If you didn't have this page pool stuff then each thread/VIF is limited
> to at most 256 pages anyway and I think 1 MiB of memory per VIF is
> perfectly acceptable.
> 

Oh I forgot to mention another important function of page pool -- to
track pages.

With this tracking facility it is really tricky to make thread-per-vif
work.

If you look at the original code, you can see we steal page->mapping
field for tracking purpose. The netback ref is stored in that field.

When switching to 1:1 model, the information embedded into page->mapping
is the index to pool element. If we don't have this pool, tracking
becomes more difficult.


Wei.

> David