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Subject: Re: [OMPI devel] mpirun Produces Extraneous Output
From: Ralph Castain (rhc_at_[hidden])
Date: 2014-07-02 23:50:17


Rats - this is one tough race condition. I've been cycling it since your note, without hitting one problem.

I'm running Centos6 on a dual processor, 6-core Xeon, with a dedicated disk installed on the node. Apparently, that seems to be adequate to mitigate the problem.

I'll keep trying to find a way to replicate this, and will audit the code and see if I can visually spot something. If I do and send you a patch, would you be willing/able to test it?

On Jul 2, 2014, at 11:50 AM, Greg Thomsen <gthomsen_at_[hidden]> wrote:

> Ralph,
>
> I finally got some time to look into this again. I can reproduce the issue when running against the latest nightlies for 1.8.2 (r32119) and 1.9 (r32113).
>
> While verifying that, I noticed that testing on a different system required a much larger number of iterations (~2000), while on the original system the iterations were unaffected (~3). Both are RHEL5 systems and which differ by the kernel revision booted, and the underlying hardware. The harder to reproduce system is a quad processor, 16-core Opteron 6276 running 2.6.18-274.el5xen while the original system is a quad processor, 4-core Xeon X5450 running 2.6.18-308.el5.
>
> The only option supplied during configuration was the prefix to install. Attached is the log used for the latest trunk nightly (r32113).
>
> Thanks for looking into this. Let me know if you need anything else.
>
> Greg
>
> On 6/13/14 10:38 PM, Ralph Castain wrote:
>> Hi Greg
>>
>> I've been running your script over and over again with the current 1.8.2 and svn developer's trunk, and I cannot get a failure. It just merrily runs.
>>
>> Could you tell me how you configured OMPI to get this behavior?
>>
>> Thanks
>> Ralph
>>
>> On Jun 10, 2014, at 11:59 AM, Ralph Castain <rhc_at_[hidden]> wrote:
>>
>>> Ouch - I'll try to chase it down.I'm unaware of anyone passing a significant amount of data via stdin before, so it's quite possible this has been around for awhile. Normally one avoids that practice.
>>>
>>>
>>> On Jun 10, 2014, at 11:36 AM, Greg Thomsen <gthomsen_at_[hidden]> wrote:
>>>
>>>> All,
>>>>
>>>> I believe I've found a bug in the I/O forwarding portion of OpenMPI which occasionally causes mpirun to generate additional data on standard output that was not produced by the application being run.
>>>>
>>>> The application in question reads from standard input and writes to standard output only on the rank 0 process. All non-rank 0 processes only participate in computation and do not produce data on standard output. The application is used in standard Unix-like pipelines like so:
>>>>
>>>> A | mpirun -np 4 application | B
>>>>
>>>> Since B is looking for structured input, it is sensitive to additional data being generated.
>>>>
>>>> While chasing down the source of this problem, I've observed the following:
>>>>
>>>> * The problem is sensitive to timing. Using strace to figure out where
>>>> the problem lies can easily hide it. Either of the following would
>>>> change how the issue was expressed:
>>>>
>>>> A | mpirun -np 4 strace -o output.txt -e read,write application | B
>>>> A | strace -ff -o output.txt -e read,write mpirun -np 4 application
>>>> | B
>>>>
>>>> While harder to state definitively, redirecting in from file and out
>>>> to file, rather than through pipes, also appears to hide the problem.
>>>> Since the workflow in question has large volumes of data, using
>>>> file-based I/O isn't feasible and wasn't thoroughly explored during
>>>> testing.
>>>>
>>>> * It appears to be correlated to a short I/O operation. A short read
>>>> from application's standard output maps to the first byte of the
>>>> extraneous output sent to B. Looking at hex dumps indicates that the
>>>> contents of a recent buffer are inadvertently written to B.
>>>>
>>>> The attached text case can show this.
>>>>
>>>> * This also is an issue for forwarding standard error from the rank 0
>>>> process. Modifying application so that it only writes to standard
>>>> error, and then redirecting standard error to standard output in the
>>>> shell, will still cause the problem:
>>>>
>>>> A | mpirun -np 4 application 2>&1 | B
>>>>
>>>> * This seems to only occur at the end of the data stream. The pipeline
>>>> in question works through records and if it occurred earlier than the
>>>> last, it would be noticed. The conditions where it was seen
>>>> regularly always pointed to the end of the data stream.
>>>>
>>>> The attached shell script reproduces the problem in every version of OpenMPI tested (1.5.0, 1.6.3, 1.7.4, and 1.8.1). Without any arguments, it reads a fixed amount of data from /dev/zero and then compares the size of the output from the above pipeline. For versions exhibiting the bug, and the above conditions, the problem should be seen within the first ~20 attempts. For other conditions I've seen the script run for a week without problem.
>>>>
>>>> With an input path, it reads the first 1,000,000 bytes from the path supplied. With a fixed pattern in the data (see the compressed test input), it is easy to see that the extra data generated is a copy from earlier in the data stream.
>>>>
>>>> Hopefully this gets someone in the right section of the code. Let me know if additional information is needed.
>>>>
>>>> Thanks!
>>>>
>>>> Greg
>>>> <mpi_test_input.bz2><mpi-test.sh>_______________________________________________
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>>>
>>
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