[Beowulf] tcp error: Need ideas!
gerry.creager at tamu.edu
Sun Jan 25 06:55:41 PST 2009
We've run some forensics with a real testset. It's not the HP Procurve
We've also seen good jumbo results with some of the managed Linksys
48-port gigabit switches.
In other words, it's not the switch. I tend to "think out loud" to
expose all possible failure modes, a process I learned at NASA/Johnson
when I worked on Space Station's Medical Operations. In manned
spaceflight, you have one exercise where you sit around and try to
determine everything that could possibly go wrong, and how such a
failure would manifest itself. That tends to be useful in other
Paulo Afonso Lopes wrote:
>> I wonder if the switch could be implicated. We have seen some (cheap)
>> GbE switches not support (in practice) jumbo frames (irrespective of
> I got the SMC 8624T because it advertised both Jumbo and link aggregation.
> Is this one of the "cheap" you have seen that does not work with Jumbo?
>> Nifty Tom Mitchell wrote:
>>> On Sat, Jan 24, 2009 at 09:36:09AM -0600, Gerry Creager wrote:
>>>> Couple of follow-up notes.
>>>> MTU=4500: Had one node fall over with the same overflow errors.
>>>> MTU=3000: A WRF model is running, but single timesteps are executing
>>>> 2.5x slower than MTU=1500
>> Segment offload? Is TSO on or off?
>> ethtool -k eth0
>> will tell you. You might also have one very reluctant machine, in the
>> sense of being unwilling to switch their mtu. Could you do an
>> ifconfig eth0 | grep MTU
>> on each machine and verify that everyone is using the right MTU?
>>>> I'll go snag the new driver and compile it. After all: What can it
>>>> Thanks, Guy!
>>>> Regards, Gerry
>>>> Guy Coates wrote:
>>>>> We have also seen problems with the bnx2 drivers.
>>>>> I got a more recent set of bnx2 drivers from Broadcom:
>>> Has the data been snooped for this data to see if all
>>> is as expected.
>>> If you are seeing a natural MTU running faster than a jumbo MTU
>>> then something is fragmenting or causing fragmentation of the data.
>>> Should the MTU=4500 causes overflow errors it might be related to
>>> Both the sender and receiver have to keep all the bits on a reliable
>>> transfer until the data has been acknowledged. At one time
>>> could only be done once to a minimum MTU in the life of a packet.
>>> In addition to snooping packets try "tracepath" to and from all
>>> the involved boxes to discover what is going on.
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Gerry Creager -- gerry.creager at tamu.edu
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