Re: [Xen-devel] xen-blkfront hang

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Aug 04, 2017 at 05:36:29PM +0800, Dongli Zhang wrote:
> 
> 
> On 08/04/2017 05:13 PM, Roger Pau Monné wrote:
> > On Fri, Aug 04, 2017 at 10:00:09AM +0200, Valentin Vidic wrote:
> >> On Mon, Jul 31, 2017 at 09:09:19AM +0800, Dongli Zhang wrote:
> >>> To verify whether the above patch would help, please check the 
> >>> nr_grant_frames
> >>> value in guest domU. If this value is exactly the same of maximum grant 
> >>> frames
> >>> (by default, xen mainline uses 32) and the number of free grant 
> >>> references is
> >>> very small, the above patch might help.
> >>
> >> You are right, this is what I get after the machine hangs:
> >>
> >>   crash> print nr_grant_frames 
> >>   $1 = 32
> >>   crash> print gnttab_free_count 
> >>   $2 = 9
> >>
> >>> The best way is to increase the gnttab_max_frames to larger value (e.g.,  
> >>> 256)
> >>> in dom0 xen.gz grub.
> >>
> >> Thank you, this seems to help.  The test machine does not hang now and
> >> the numbers are looking better now:
> >>
> >>   crash> print nr_grant_frames 
> >>   $1 = 59
> >>   crash> print gnttab_free_count 
> >>   $2 = 356
> > 
> > At some point I've already expressed my opinion that having a
> > per-queue list of persistent grants was not a good idea. Now I think
> > the only solution is to remove persistent grants, or to lower the
> > default number of per-queue persistent grants to a ridiculously low
> > value.
> 
> It would be more efficient if (1) persistent grants are shared by all queues 
> and

There was a complain that having a shared pool of persistent grants
introduced too much contention.

> (2) there is a new mechanism to allow frontend to actively ask backend to 
> unmap
> existing persistent grants. So far, it is backend's responsibility to decide
> when to unmap persistent grants.

Hm, I would rather remove them than make the protocol more complex.

Roger.

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