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

[annie li <annie.li@xxxxxxxxxx>]

On 2016/1/20 7:23, 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).
Following are the patches to fix module param, they exist since v4.3.
xen-netfront: respect user provided max_queues
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=32a844056fd43dda647e1c3c6b9983bdfa04d17d
xen-netback: respect user provided max_queues
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4c82ac3c37363e8c4ded6a5fe1ec5fa756b34df3
> 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?
We run into similar issue recently, and guess it is better to suggest 
user to set netback module parameter with the default value as 8? see 
this link,
http://wiki.xenproject.org/wiki/Xen-netback_and_xen-netfront_multi-queue_performance_testing 

Probably more test are needed to get the default number of best experience.

> 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.
Correct.

Thanks
Annie
> Ian.
>
> [0] http://lists.xen.org/archives/html/xen-users/2016-01/msg00100.html
>      http://lists.xen.org/archives/html/xen-users/2016-01/msg00072.html
>      some before hte xmas break too IIRC
>
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