Re: [Xen-devel] netfront/netback multiqueue exhausting grants

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 20/01/16 12:23, Ian Campbell wrote:
> There have been a few reports recently[0] which relate to a failure of
> netfront to allocate sufficient grant refs for all the queues:
> 
> [    0.533589] xen_netfront: can't alloc rx grant refs
> [    0.533612] net eth0: only created 31 queues
> 
> Which can be worked around by increasing the number of grants on the
> hypervisor command line or by limiting the number of queues permitted by
> either back or front using a module param (which was broken but is now
> fixed on both sides, but I'm not sure it has been backported everywhere
> such that it is a reliable thing to always tell users as a workaround).
> 
> Is there any plan to do anything about the default/out of the box
> experience? Either limiting the number of queues or making both ends cope
> more gracefully with failure to create some queues (or both) might be
> sufficient?
> 
> I think the crash after the above in the first link at [0] is fixed? I
> think that was the purpose of ca88ea1247df "xen-netfront: update num_queues
> to real created" which was in 4.3.

I think the correct solution is to increase the default maximum grant
table size.

Although, unless you're using the not-yet-applied per-cpu rwlock patches
multiqueue is terrible on many (multisocket) systems and the number of
queue should be limited in netback to 4 or even just 2.

David

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