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

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.

Well we already have a bunch of spin locks in Linux's page allocator.
This spin lock protects a very small critical section which looks quite
acceptable to me.

> 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.

Please consider we might have some sort of mapping mechanism in the
future, that's when page pool becomes able to actually limit number of
pages used by vifs.

> 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.

Please note that 256 is only the current status, we might need to
tune this number in the future.

I would like to have more input on this.


> David

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