MBUF Cluster Network freeze problem

Eric Norum eric.norum at usask.ca
Tue Nov 28 15:18:38 UTC 2000

bob wrote:
> I have come up against an interesting problem when the local BSP is
> moderately task loaded and the local network is also very active talking to
> the BSP. We are using the Mot 68360 and RTEMS 4.5 Beta
> The symptoms are that the MBUF Clusters fill up slowly over time - as if the
> CPU can't keep up with the processing of the data. Eventually, the message
> "Still waiting for mbuf cluster." pops out from m_clalloc in rtems_glue.c.
> If the other network traffic to the BSP is stopped, to reduce the external
> network load to virtually nothing, the BSP still never recovers and seems to
> have a no free clusters for ever (still has plenty of free MBUFS).
> To generate this condition we run our normal applications - a PC which polls
> a task in the BSP and a flood ping aimed at the BSP. It takes up to a couple
> of minutes to fall-over. However, we think a similar situation may have
> occurred in real life without needing the flood pings.
> I believe the network should recover and the MBUF Clusters should free up
> after the load is reduced.
> Anyone got any theories on what's happening, or ideas on where to start
> looking / work-arounds. I believe increasing MBUFs/Clusters will do no good
> as I think it's the CPU overload, that causes them to fill up.
> What we need is a graceful recovery?
> Any help/ideas much appreciated, thanks.

There have been *lots* of network changes since 4.5 beta.  One of the
biggest was cleaning up a problem where things would lock up if a packet
were fragmented into more parts than the number of 68360 transmit buffer
descriptors.  I recall others having problems with flood pings and that
we made some changes to prevent these problems as well, but I don't
remember the exact details.

Eric Norum                                 eric.norum at usask.ca
Department of Electrical Engineering       Phone: (306) 966-5394
University of Saskatchewan                 FAX:   (306) 966-5407
Saskatoon, Canada.

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