Re: [Qemu-devel] [Xen-devel] [PATCH] qemu/xendisk: set maximum number of grants to be used

[Jan Beulich]

>>> On 11.05.12 at 19:07, Stefano Stabellini <address@hidden>
wrote:
> On Fri, 11 May 2012, Jan Beulich wrote:
>> >>> On 11.05.12 at 09:19, "Jan Beulich" <address@hidden> wrote:
>> > Legacy (non-pvops) gntdev drivers may require this to be done when the
>> > number of grants intended to be used simultaneously exceeds a certain
>> > driver specific default limit.
>> > 
>> > Signed-off-by: Jan Beulich <address@hidden>
>> > 
>> > --- a/hw/xen_disk.c
>> > +++ b/hw/xen_disk.c
>> > @@ -536,6 +536,10 @@ static void blk_alloc(struct XenDevice *
>> >      if (xen_mode != XEN_EMULATE) {
>> >          batch_maps = 1;
>> >      }
>> > +    if (xc_gnttab_set_max_grants(xendev->gnttabdev,
>> > +            max_requests * BLKIF_MAX_SEGMENTS_PER_REQUEST + 1) < 0)
>> 
>> In more extensive testing it appears that very rarely this value is still
>> too low:
>> 
>> xen be: qdisk-768: can't map 11 grant refs (Cannot allocate memory, 342 maps)
>> 
>> 342 + 11 = 353 > 352 = 32 * 11
>> 
>> Could someone help out here? I first thought this might be due to
>> use_aio being non-zero, but ioreq_start() doesn't permit more than
>> max_requests struct ioreqs-s to be around.
> 
> Actually 342 + 11 = 353, that should be still OK because it is equal to
> 32 * 11 + 1, where the additional 1 is for the ring, right?

The +1 is for the ring, yes. And the calculation in the driver actually
appears to be fine. It's rather an issue with fragmentation afaict -
the driver needs to allocate 11 contiguous slots, and such may not
be available. I'll send out a v2 of the patch soon, taking fragmentation
into account.

Jan