On Mar 7, 2012, at 2:33 AM, Jan Beulich wrote:
>>>> On 06.03.12 at 18:20, Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx> wrote:
>> -> XENBUS_MAX_RING_PAGES - why 2? Why not 4? What is the optimal
>> default size for SSD usage? 16?
>
> What do SSDs have to do with a XenBus definition? Imo it's wrong (and
> unnecessary) to introduce a limit at the XenBus level at all - each driver
> can do this for itself.
>
> As to the limit for SSDs in the block interface - I don't think the number
> of possibly simultaneous requests has anything to do with this. Instead,
> I'd expect the request number/size/segments extension that NetBSD
> apparently implements to possibly have an effect.
>
> Jan
There's another problem here that I brought up during the Xen
Hack-a-thon. The ring macros require that the ring element count
be a power of two. This doesn't mean that the ring will be a power
of 2 pages in size. To illustrate this point, I modified the FreeBSD
blkback driver to provide negotiated ring stats via sysctl.
Here's a connection to a Windows VM running the Citrix PV drivers:
dev.xbbd.2.max_requests: 128
dev.xbbd.2.max_request_segments: 11
dev.xbbd.2.max_request_size: 45056
dev.xbbd.2.ring_elem_size: 108 <= 32bit ABI
dev.xbbd.2.ring_pages: 4
dev.xbbd.2.ring_elements: 128
dev.xbbd.2.ring_waste: 2496
Over half a page is wasted when ring-page-order is 2. I'm sure you
can see where this is going. :-)
Here are the limits published by our backend to the XenStore:
max-ring-pages = "113"
max-ring-page-order = "7"
max-requests = "256"
max-request-segments = "129"
max-request-size = "524288"
Because we allow so many concurrent, large requests in our product,
the ring wastage really adds up if the front end doesn't support
the "ring-pages" variant of the extension. However, you only need
a ring-page-order of 3 with this protocol to start seeing pages of
wasted ring space.
You don't really want to negotiate "ring-pages" either. The backends
often need to support multiple ABIs. I can easily construct a set
of limits for the FreeBSD blkback driver which will cause the ring
limits to vary by a page between the 32bit and 64bit ABIs.
With all this in mind, the backend must do a dance of rounding up,
taking the max of the ring sizes for the different ABIs, and then
validating the front-end published limits taking its ABI into
account. The front-end does some of this too. Its way too messy
and error prone because we don't communicate the ring element limit
directly.
"max-ring-element-order" anyone? :-)
--
Justin
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