Re: [PATCH] mlx4: allow for 4K mtu configuration of IB ports

["Jim Schutt" <jaschut-4OHPYypu0djtX7QSmKvirg@public.gmane.org>]

Jim Schutt wrote:
> Or Gerlitz wrote:
>> Roland Dreier wrote:
>>> Bob Pearson <rpearson-klaOcWyJdxkshyMvu7JE4pqQE7yCjDx5@public.gmane.org> wrote:
>>>> With lash+mesh redsky required 6-7 VLs to wire up without deadlocks. 
>>>> I think
>>>> that Jim's version uses 8 SLs but only 2VLs to work.
>>>> If someone was using a torus and also wanted to support QOS and also 
>>>> wanted
>>>> to separate multicast and management on a separate VL to be 
>>>> absolutely sure
>>>> that there is no possibility of a deadlock you might end up with 
>>>> #QOS * 2 +
>>>> 1 which would be 5 using the current algorithm.
>>
>>> But again you don't need all those VLs on the HCAs' links, do you?
>>
>> Jason Gunthorpe wrote:
>>> Routing algorithms only need VLs on interswitch links, not on HCA to
>>> switch links. The only use of the HCA to switch VLs is for QoS. Mesh
>>> topologies can usually be routed with only two VLs, but you need alot
>>> of SLs to make that work.
>>
>> Bob, Jim, Alex
>>
>> I wasn't sure if the SL-to-VL mapping done by open SM is dictated by 
>> the directives @ the user config file or if the routing algorithm is 
>> "VL aware" but the routing engine? if the latter, do interswitch links 
>> use different mapping vs. HCA - switch links?
> 
> FWIW, the torus-2QoS routing engine uses VL bit 0 for torus deadlock
> avoidance, VL bit 1 to route around a missing switch without deadlocks,
> and VL bit 2 to provide two QoS levels.  It needs the port dependence
> of the SL2VL maps to do this in switches.
> 
> The interswitch and HCAs use the same mapping, but only VL bit 2
> is needed on HCAs, to provide the QoS levels.

It occurred to me as soon I sent the above that there's no
good reason to insist that the VL usage is the same for both
interswitch links, and switch-CA links.

Do I need to change this?

-- Jim

> 
> I chose that bit usage because it seemed the proper ordering of
> capabilities if there are fewer than 8 data VLs available - basic
> deadlock avoidance is most important; some QoS is nice to have but
> not that useful if the fabric can deadlock.
> 
> Is that what you were asking, at least WRT. torus-2QoS?
> 
> -- Jim
> 
>>
>> Or.
>>
>>
> 
> 
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