[ofa-general] XmtDiscards

[Bernd Schubert]

Hello Hal,

On Sat, Apr 05, 2008 at 06:19:43AM -0700, Hal Rosenstock wrote:
> Hi Bernd,
> 
> On Sat, 2008-04-05 at 00:12 +0200, Bernd Schubert wrote:
> > Hello,
> > 
> > after I upgraded one of our clusters to opensm-3.2.1 it seems to have gotten 
> > much better there, at least no further RcvSwRelayErrors, even when the 
> > cluster is in idle state and so far also no SymbolErrors, which we also have 
> > seens before.
> > 
> > However, after I just started a lustre stress test on 50 clients (to a lustre 
> > storage system with 20 OSS servers and 60 OSTs), ibcheckerrors reports about 
> > 9000 XmtDiscards within 30 minutes.
> > 
> > Searching for this error I find "This is a symptom of congestion and may 
> > require tweaking either HOQ or switch lifetime values". 
> > Well, I have to admit I neither know what HOQ is, nor do I know how to tweak 
> > it. I also do not have an idea to set switch lifetime values.  I guess this 
> > isn't related to the opensm timeout option, is it?
> > 
> > Hmm, I just found a cisci pdf describing how to set the lifetime on these 
> > switches, but is this also possible on Flextronics switches?
> 
> What routing algorithm are you using ? Rather than play with those
> switch values, if you are not using up/down, could you try that to see
> if it helps with the congestion you are seeing ?

I now configured up/down, but still got XmtDiscards, though, only on one port.

Error check on lid 205 (SW_pfs1_leaf2) port all:  FAILED
#warn: counter XmtDiscards = 6213       (threshold 100) lid 205 port 1
Error check on lid 205 (SW_pfs1_leaf2) port 1:  FAILED
#warn: counter RcvSwRelayErrors = 1431  (threshold 100) lid 205 port 13
Error check on lid 205 (SW_pfs1_leaf2) port 13:  FAILED

I'm also not sure if up/down is the optimal algorithm for a fabric with only 
two switches.

Since describing the connections in words is a bit difficult, I just upload
a drawing here:

http://www.pci.uni-heidelberg.de/tc/usr/bernd/downloads/ib/Interswitch-cabling.pdf

The root-guid for the up/down algorithm is leaf-5 of of the small switch. But
I'm still not sure about up/down at all. Doesn't one need for up/down at least
3 switches? Something like this ascii graphic below?


       root-switch
     /            \
    /              \
 Sw-1 ------------ Sw-2




Thanks for your help,
Bernd


PS: These RcvSwRelayErrors are also back again. I think this occur on some 
operations of Lustre. Even if these RcvSwRelayErrors are not critical, they 
are still a bit annoying, since they make it hard to find other errors in 
the output ob ibcheckerrors. 
If we can really ignore these errors, I will write a patch to not display these 
by default.