[dm-devel] dm-multipath - IO queue dispatch based on FPIN Congestion/Latency notifications.

[Martin Wilck]

On Thu, 2021-04-01 at 12:48 +1000, Erwin van Londen wrote:
> > 
> > Benjamin has added a marginal_path group(multipath marginal
> > pathgroups) in
> > the dm-multipath.
> > https://patchwork.kernel.org/project/dm-devel/cover/1564763622-31752-1-git
> > -send-email-bmarzins at redhat.com/
> > 
> > One of the intention of the Benjamin's patch (support for maginal
> > path) is
> > to support for the FPIN events we receive from fabric.
> > On receiving the fpin-li our intention was to  place all the paths
> > that
> > are affected into the marginal path group.
> I think this should all be done in kernel space as we're talking sub-
> millisecond timings here when it comes to fpins and the reaction time
> expected. I may be wrong but I'll leave that up to you.

Sub-ms latency is impossible with this setup  (kernel -> broadcom FC
daemon -> multipathd -> kernel). It's only suitable for "fatal" FPINs
that would suggest taking a path offline on the time scale of minutes.
I suppose that would work well for LN FPINs, but not for the other
types.

> > 
> > On receiving the congestion notifications our intention is to
> > slowdown the
> > work load gradually from the host until it stops receiving the
> > congestion
> > notifications.
> > We need to validate the same how we can achieve the same of
> > decreasing the
> > workloads with the help of dm-multipath.
> Would it be possible to piggyback on the service time path selector
> in this when it pertains latency?  

Not on service-time itself, but someone could write a new path selector
algorithm. IMO we'd still have the problem that this would be seen as a
layering violation. In the long run dm-mpath may need to add transport-
specific callbacks. But for a proof-of-concept, a selector algorithm
with layering violations would be ok, I believe.

Regards
Martin

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