[Beowulf] Two problems related to slowness and TASK_UNINTERRUPTABLE process

Tahir Malas tmalas at ee.bilkent.edu.tr
Wed Jun 13 08:37:08 EDT 2007


> -----Original Message-----
> From: Mark Hahn [mailto:hahn at mcmaster.ca]
> Sent: Tuesday, June 12, 2007 6:15 PM
> To: Tahir Malas
> Cc: mvapich-discuss at cse.ohio-state.edu; beowulf at beowulf.org;
> teoman.terzi at gmail.com; 'Ozgur Ergul'
> Subject: Re: [Beowulf] Two problems related to slowness and
> TASK_UNINTERRUPTABLE process
> 
> > For 32 processes (4 process per node), the arrays with 512-Byte size are
> > communicated slower than the 4096-Byte size arrays. For both of them, we
> 
> do you mean that this is not the case in other configurations?
> an interconnect _should_ have some steep rise in effective bandwidth
> as packet size is increased.  it's a useful metric to know the packet
> size at which half-peak bandwidth is achieved, since this offers some
> "sense of scale" to programmers judging whether their own packet sizes
> are appropriate.



> 
> > this abnormal case is persistent. More specifically, communication of
> > 4k-Byte packages are 2 times faster than the communication of 512-Byte
> > packages.
> 
> perhaps I'm dense this morning, but what's unexpected about that?
Considering the latency and bw measures, my expectation for the
communication times in us:
512: 5.48 + 512/592.34 = 6.34
4096: 11.02 + 4096/906.04 = 15.54
Our test:
512: 29.434
4096: 16.209

So, somehow, isn't communication time for 512 bytes is unexpectedly slow?

> >
> > 2. SOMETIMES, after the test with overall 32 processes, one of the four
> > processes at node3 hangs in TASK_UNINTERRUPTABLE "D" state. Hence, the
> test
> > program shows a "done." and waits for sometime. We can neither kill the
> > process nor soft reboot the node. We have to wait for that process to
> > terminate, which can last long.
> 
> does /proc/$pid/wchan (on the 'D' state process) tell you anything?
> do all the ranks return from MPI_Finalize?
> 

The file tells "__lock_buffer". Yes, all ranks return; but I think, this
problematic process (i.e. one of the processes on node3) returns always the
latest. 

Thanks, and regards,
Tahir.



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