Re: [Xen-devel] Poor network performance between DomU with multiqueue support

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 27/02/15 10:59, Wei Liu wrote:
> 
> Persistent grant is not silver bullet. There is email thread on the
> list discussing whether it should be removed in block driver.

Persistent grants for to-guest network traffic is a flawed idea.  It
either requires:

a) the backend to memcpy into the mapped grant /and/ the frontend to
memcpy out of the persistently mapped pool.  This is clearly going to be
worse for memory bandwidth than a single grant copy.

or

b) the backend to accumulate more and more mappings of guest memory,
which is bad for security and it uses too many grant and map track
resources hence it does not scale to many VIFs.

David

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