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

[Markus Armbruster <armbru@xxxxxxxxxx>]

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.

[...]

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