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Re: [Xen-devel] Re: userspace block backend / gntdev problems

Markus Armbruster wrote:
> Derek Murray <Derek.Murray@xxxxxxxxxxxx> writes:
>> Hi Gerd,
>> On 4 Jan 2008, at 13:48, Gerd Hoffmann wrote:
>>> First problem is the fixed limit of 128 slots.  The frontend
>>> submits up
>>> to 32 requests, with up to 11 grants each.  With the shared ring this
>>> sums up to 353 grants per block device.  When is blkbackd running
>>> in aio
>>> mode, thus many requests are in flight at the same time and thus also
>>> many grants mapped at the same time, the 128 limit is easily
>>> reached.  I
>>> don't even need to stress the disk with bonnie or something, just
>>> booting the virtual machine is enougth.  Any chance replace the
>>> fix-sized array with a list to remove that hard-coded limit?  Or at
>>> least raise the limit to -- say -- 1024 grants?
>> The 128-grant limit is fairly arbitrary, and I wanted to see how
>> people were using gntdev before changing this. The reason for using a
>> fixed-size array is that it gives us O(1)-time mapping and unmapping
>> of single grants, which I anticipated would be the most frequently- 
>> used case. I'll prepare a patch that enables the configuration of
>> this limit when the device is opened.
> Any news on this?  I'd like to try converting the PV framebuffer to
> use grants.  I need to map ~2000-5000 pages, depending on the pvfb's
> resolution.
> [...]
In my latest post on "Dynamic modes support for PV xenfb" I am using
grants to map an extended framebuffer. I have a single grant ref that
points to 10 other refs. The other refs contain MFNs. Same technique as
the current framebuffer pd array but avoids the 64bit long issue. Kind
of a hybrid approach. I am able to map a 22MB framebuffer when running a
64 bit guest and 44MB when running a 32 bit guest. When the backend is
done with the mapping it sends a message to the frontend to free up the

I did try to map the whole framebuffers via grants, failed. Like you say
you need a whole bunch of them.

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