[RC5] OGR-24 suspended

decibel at nodezero.distributed.net decibel at nodezero.distributed.net
Wed Feb 23 08:17:51 EST 2000


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February 22, 2000

Most of you should know by now that for the past week, we have been
working on the OGR-24 project, the search for the 24 mark Optimal Golomb
Ruler. So far, we have completed approximately 24% of the total stubs,
mostly smaller ones. Unfortunately, we have noticed a problem while
analyzing returned data in the master log files. Some clients have been
returning inordinately large node counts for completed stubs. Once we
reran the stubs in question for confirmation, we found that the node
count corruption indicated an endianness problem, where platforms like
SPARC order groups of bytes in memory differently from platforms like
Intel x86.

On Tuesday evening, we were able to pinpoint the missing code and
operational circumstances that caused this problem. There was a missing
ntohl() call in the buffer handling code. Most of the time this caused
no problems, but when sharing buffer files between certain combinations
of platforms, the count of how many nodes had been completed would be
corrupted. Unfortunately, due to the circumstances necessary for this
bug to manifest itself, our testing did not identify it.

The biggest problem with this bug is the difficulty of detecting it. The
clients return the number of nodes done in each stub to the master. If
the number of nodes actually done is greater than 2^32 (about 4
billion), then the bug is detectable due to the way the bytes get
swapped (an impossibly high node count is returned). However, if the
number of nodes is less than 2^32, it is not guaranteed that we will
always be able to identify bad return values.

In light of the above, we have decided to suspend the OGR-24 project.
Clients that are currently working on OGR will revert back to RC5 the
next time they connect to the network; you can force a connection by
shutting the client down, deleting the OGR buffers, and restarting the
client.

We will need to build new clients to address this bug and to allow us to
discard results from bad clients. As a consequence, we will have the
opportunity to improve some other aspects of client operation. In
particular, we plan to add more configurable checkpointing and a better
display of progress.

Having built new clients and added code to discard faulty stubs, we will
restart OGR-24 from the beginning, this time with much smaller 5-stubs
instead of the 4-stubs used in the initial run. All is not lost from
this past run; we will use its results as an additional verification of
future work.

Here are some statistics about the current run to date:

 Total number of 4-stubs to search: 724,782
                    Returned stubs: 173,421
          Known bad stubs returned:     340
      Largest stub returned so far: 2-1-12-6 ... 1,765,158,446,824 nodes
- --
Jim C. Nasby (aka Decibel!)                      decibel at distributed.net
Join the most powerful computer known to man! http://www.distributed.net

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