[PATCH V2 3/3] nvme pci: introduce module parameter of 'default_queues'

[ming.lei@redhat.com (Ming Lei)]

On Mon, Dec 31, 2018@03:24:55PM -0600, Bjorn Helgaas wrote:
> On Fri, Dec 28, 2018@9:27 PM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On big system with lots of CPU cores, it is easy to consume up irq
> > vectors by assigning defaut queue with num_possible_cpus() irq vectors.
> > Meantime it is often not necessary to allocate so many vectors for
> > reaching NVMe's top performance under that situation.
> 
> s/defaut/default/
> 
> > This patch introduces module parameter of 'default_queues' to try
> > to address this issue reported by Shan Hai.
> 
> Is there a URL to this report by Shan?

http://lists.infradead.org/pipermail/linux-nvme/2018-December/021863.html
http://lists.infradead.org/pipermail/linux-nvme/2018-December/021862.html

http://lists.infradead.org/pipermail/linux-nvme/2018-December/021872.html

> 
> Is there some way you can figure this out automatically instead of
> forcing the user to use a module parameter?

Not yet, otherwise, I won't post this patch out.

> 
> If not, can you provide some guidance in the changelog for how a user
> is supposed to figure out when it's needed and what the value should
> be?  If you add the parameter, I assume that will eventually have to
> be mentioned in a release note, and it would be nice to have something
> to start from.

Ok, that is a good suggestion, how about documenting it via the
following words:

Number of IRQ vectors is system-wide resource, and usually it is big enough
for each device. However, we allocate num_possible_cpus() + 1 irq vectors for
each NVMe PCI controller. In case that system has lots of CPU cores, or there
are more than one NVMe controller, IRQ vectors can be consumed up
easily by NVMe. When this issue is triggered, please try to pass smaller
default queues via the module parameter of 'default_queues', usually
it have to be >= number of NUMA nodes, meantime it needs be big enough
to reach NVMe's top performance, which is often less than num_possible_cpus()
+ 1.


Thanks,
Ming