Re: Is HT faster infiniband?

Del Cecchi wrote:
> Robert Myers wrote:

> >
> > As it is, the Stone and Partridge stuff doesn't seem to have created
> > much more than some interesting exchanges on comp.arch. Does anybody
> > care anymore? I'm sure that IBM does, but can it afford to?
> >
>
> I don't know about their work, and I know little about ethernet. I do
> know from experience in the lab that a 32 bit crc, properly chosen, with
> retry can cope with quite high error rates without any problem with the
> system. And I would believe that the systems in question would not
> tolerate very many undetected errors because the disks for the virtual
> memory, and the coherence traffic if any was carried over the network in
> question along with all the other I/O traffic.
>

I think you did participate in the discussion of this subject on
comp.arch:

Stone, J., Partridge, C.: "When The CRC and TCP Checksum Disagree",
Proceedings of the ACM conference on Applications, Technologies,
Architectures, and Protocols for Computer Communication (SIGCOMM'00),
Stockholm, Sweden, August/September 2000, pp. 309-319

Abstract

"Traces of Internet packets from the past two years show that between 1
packet in 1,100 and 1 packet in 32,000 fails the TCP checksum, even on
links where link-level CRCs should catch all but 1 in 4 billion errors.
For certain situations, the rate of checksum failures can be even
higher: in one hour-long test we observed a checksum failure of 1
packet in 400. We investigate why so many errors are observed, when
link-level CRCs should catch nearly all of them.We have collected
nearly 500,000 packets which failed the TCP or UDP or IP checksum. This
dataset shows the Internet has a wide variety of error sources which
can not be detected by link-level checks. We describe analysis tools
that have identified nearly 100 different error patterns. Categorizing
packet errors, we can infer likely causes which explain roughly half
the observed errors. The causes span the entire spectrum of a network
stack, from memory errors to bugs in TCP.After an analysis we conclude
that the checksum will fail to detect errors for roughly 1 in 16
million to 10 billion packets. From our analysis of the cause of
errors, we propose simple changes to several protocols which will
decrease the rate of undetected error. Even so, the highly non-random
distribution of errors strongly suggests some applications should
employ application-level checksums or equivalents."

It may not be a good model for the possiblity of other link-level
errors, but it does make you wonder.

In overclocking tests, I've found that PC's will tolerate significant
memory errors without giving any immediate indication of a problem.
Short of a crash, I don't know how you'd know anything was wrong
without application-level checking.


RM

.

Relevant Pages

  • Re: Is HT faster infiniband?
    ... > Proceedings of the ACM conference on Applications, Technologies, ... > links where link-level CRCs should catch all but 1 in 4 billion errors. ... the rate of checksum failures can be even ... > nearly 500,000 packets which failed the TCP or UDP or IP checksum. ...
    (comp.sys.ibm.pc.hardware.chips)
  • Re: Is HT faster infiniband?
    ... >Proceedings of the ACM conference on Applications, Technologies, ... >links where link-level CRCs should catch all but 1 in 4 billion errors. ... the rate of checksum failures can be even ... >nearly 500,000 packets which failed the TCP or UDP or IP checksum. ...
    (comp.sys.ibm.pc.hardware.chips)
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