Re: [Xen-devel] [Xen-users] Grant reference batch transmission

[Gareth Stockwell <Gareth.Stockwell@xxxxxxx>]

On Wed, Mar 11, 2015 at 10:24:48, Ian Campbell wrote:
> > I think this gives us two options for grant reference batch transmission:
> >
> > 1. Send the grant references via the ring buffer.
> > This doesn't require any additional allocations, but means that if
> > the number of grant references in the batch is greater than
> > O(sizeof(ringbuffer) / sizeof(grant_ref_t)), cycling through the
> > ring will be required.
>
> Correct. In fact it's a bit worse because the ring pointers steal a
> bit of space out of the shared page. You might also find that in
> practice you want an id in the request which is echoed back in the response 
> e.g.
> to handle out of order completion (depends on your use case though),
> which would increase sizeof(grant_ref_t) (which is really sizeof(my_req_t)).
>
> What sorts of batch sizes are you expecting to see in your use case?

We need to share hundreds of MB, so (assuming a 4kB guest page size) the batch 
size can be thousands of grant references.

> > 2. Allocate and share one or more "index" page(s) which hold the
> > grant
> references.
> > This means that only a single grant_ref_t needs to be sent via the
> > ring, but at the cost of allocating additional memory for the index.
> > If multiple index pages are required, they could be chained together
> > by appending to index page N a grant reference pointing to page N+1.
>
> There have been patches for both net and blk at various points which
> implemented "multipage rings" in order to get around the limitation on
> the number of requests/responses which can git in a single page. I'm
> not sure what the status of those is (Roger, Wei?) but I would
> assume/hope that they also included some improvements to the common
> infrastructure to make it simpler to arrange for this.
>
> The other approach, which I believe is used by the blk protocol today,
> is "indirect descriptors" where the ring request contains a gref which
> references another page which can then be packed full of grant references.
> That has the downside of one more map call per request (although these
> can still be batched by the backend) but lets you pack a lot more "bandwidth"
> into a single ring.

The approach I was thinking about sounds similar to "indirect descriptors" - 
I'll have a closer look at the blk drivers.


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