Re: [OMPI users] NetPipe benchmark & spanning multiple interconnects

Thu, 8 Feb 2007 14:02:55 -0500 


On Feb 8, 2007, at 1:12 PM, Alex Tumanov wrote:


George,

Looks like I have some values already set for openib and gm bandwidth:
# ompi_info --param all all |grep -i band
MCA btl: parameter "btl_gm_bandwidth" (current value: "250") MCA btl: parameter "btl_mvapi_bandwidth" (current value: "800") Approximate maximum bandwidth of interconnect 
                 MCA btl: parameter "btl_openib_bandwidth" (current
value: "800")
Approximate maximum bandwidth of interconnect
These are the default values, which are wrong most of the time. The bandwidth is in Mbs, so the default value for GM is set to something ridiculously low. What really matter is the ration between the bandwidth of the components you plan to use.
whereas, ompi_info reports no available parameters dealing with latency: 
# ompi_info --param all all |grep -i laten
<no output>
Strange, the latency is supposed to be there too. Anyway, the latency is only used to determine which one is faster, in order to use it for small messages. 


Also, I'm not entirely sure what value to set the latency to,
especially for tcp. It depends on so many factors and varies. Why does
the latency value have effect on message striping? I can see how
knowing the bandwidth limitations of available interconnects would
allow you to proportionally divide up the message among them, but
latency? Especially for large message sizes the time should be
dominated by the bandwidth limitations.
As I said the bandwidth is in Mbs and the latency is micro-sec. But what really matter for latency is the absolute value, as we will order them starting from the smallest latency. For bandwidth, what really matters is the relative ratio. We sum all bandwidths and they we divide by the device bandwidth to find out how much data we should send over each interconnect (that's really close with what happens there).
How do I compute my latencies and bandwidths ? Well I run NetPIPE over one interconnect at the time and get the latency for the message size 1 and the bandwidth from the message size around 1MB. That should give you a quite accurate values o start from. After you can tweak them, in order to increase the ratio based on the latencies. 


Finally, what are the units for bandwidth and latency mca parameters
and how did you arrive at the values you set in your params file? Is
there a description of the message striping algorithm somewhere (other
than code :) )?
Read the previous paragraph. Unfortunately, except the code and my little explanation there is no documentation about how we do the stripping ... 

  Thanks,
    george.




Thanks,
Alex.


On 2/8/07, George Bosilca <bosi...@cs.utk.edu> wrote:

In order to get any performance improvement from stripping the
messages over multiple interconnects one has to specify the latency
and bandwidth for these interconnects, and to make sure that any of
them don't ask for exclusivity. I'm usually running over multiple TCP
interconnects and here is my mca-params.conf file:
btl_tcp_if_include = eth0,eth1
btl_tcp_max_rdma_size = 524288

btl_tcp_latency_eth0 = 47
btl_tcp_bandwidth_eth0 = 587

btl_tcp_latency_eth1 = 51
btl_tcp_bandwidth_eth1 = 233

Something similar has to be done for openib and gm, in order to allow
us to strip the messages correctly.

   Thanks,
     george.

On Feb 8, 2007, at 12:02 PM, Alex Tumanov wrote:
Hello Jeff. Thanks for pointing out NetPipe to me. I've played around 
with it a little in hope to see clear evidence/effect of message
striping in OpenMPI. Unfortunately, what I saw is that the result of
running NPmpi over several interconnects is identical to running it
over a single fastest one :-( That was not the expected behavior, and 
I'm hoping that I'm doing something wrong. I'm using NetPIPE_3.6.2
over OMPI 1.1.4. NetPipe was compiled by making sure Open MPI's mpicc 
can be found and simply  running 'make mpi' under NetPIPE_3.6.2
directory.

I experimented with 3 interconnects: openib, gm, and gig-e.
Specifically, I found that the times (and, correspondingly, bandwidth) reported for openib+gm is pretty much identical to the times reported 
for just openib. Here are the commands I used to initiate the
benchmark:

#  mpirun -H f0-0,c0-0 --prefix $MPIHOME --mca btl openib,gm,self
~/NPmpi > ~/testdir/ompi/netpipe/ompi_netpipe_openib+gm.log 2>&1
# mpirun -H f0-0,c0-0 --prefix $MPIHOME --mca btl openib,self ~/ NPmpi 
ompi_netpipe_openib.log 2>&1


Similarly, for tcp+gm the reported times were identical to just
running the benchmark over gm alone. The commands were:
#  mpirun -H f0-0,c0-0 --prefix $MPIHOME --mca btl tcp,gm,self --mca
btl_tcp_if_exclude lo,ib0,ib1 ~/NPmpi
#  mpirun -H f0-0,c0-0 --prefix $MPIHOME --mca btl gm,self ~/NPmpi
Orthogonally, I've also observed that trying to use any combination of interconnects that includes openib (except using it exclusively) fails 
as soon as the benchmark reaches trials with 1.5MB message sizes. In
fact the CPU load remained at 100% on the headnode, but no further
output is sent to the log file or the screen (see the tails below).
This behavior is fairly consistent and may be of interest to Open MPI 
development community. If anybody has tried using openib in
combination with other interconnects please let me know what issues
you've encountered and what tips and tricks you could share in this
regard.

Many thanks. Keep up the good work!

Sincerely,
Alex.
Tails (the log file name reflects the combination of interconnects in 
that CL order):
# tail ompi_netpipe_gm+openib.log
101: 786432 bytes 38 times --> 3582.46 Mbps in 1674.83 usec 102: 786435 bytes 39 times --> 3474.50 Mbps in 1726.87 usec 103: 1048573 bytes 19 times --> 3592.47 Mbps in 2226.87 usec 104: 1048576 bytes 22 times --> 3515.15 Mbps in 2275.86 usec 105: 1048579 bytes 21 times --> 3480.22 Mbps in 2298.71 usec 106: 1572861 bytes 21 times --> 4174.76 Mbps in 2874.41 usec 
107: 1572864 bytes     23 times --> mpirun: killing job...

# tail ompi_netpipe_openib+gm.log
100: 786429 bytes 45 times --> 3477.98 Mbps in 1725.13 usec 101: 786432 bytes 38 times --> 3578.94 Mbps in 1676.47 usec 102: 786435 bytes 39 times --> 3480.66 Mbps in 1723.82 usec 103: 1048573 bytes 19 times --> 3594.26 Mbps in 2225.76 usec 104: 1048576 bytes 22 times --> 3517.46 Mbps in 2274.37 usec 105: 1048579 bytes 21 times --> 3482.13 Mbps in 2297.45 usec 
106: 1572861 bytes     21 times --> mpirun: killing job...

# tail ompi_netpipe_openib+tcp+gm.log
100: 786429 bytes 45 times --> 3481.45 Mbps in 1723.41 usec 101: 786432 bytes 38 times --> 3575.83 Mbps in 1677.93 usec 102: 786435 bytes 39 times --> 3479.05 Mbps in 1724.61 usec 103: 1048573 bytes 19 times --> 3589.68 Mbps in 2228.61 usec 104: 1048576 bytes 22 times --> 3517.96 Mbps in 2274.05 usec 105: 1048579 bytes 21 times --> 3484.12 Mbps in 2296.14 usec 
106: 1572861 bytes     21 times --> mpirun: killing job...

# tail -5 ompi_netpipe_openib.log
119: 6291456 bytes 5 times --> 4036.63 Mbps in 11891.10 usec 120: 6291459 bytes 5 times --> 4005.81 Mbps in 11982.61 usec 121: 8388605 bytes 3 times --> 4033.78 Mbps in 15866.00 usec 122: 8388608 bytes 3 times --> 4025.50 Mbps in 15898.66 usec 123: 8388611 bytes 3 times --> 4017.58 Mbps in 15929.98 usec 
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                                  Kahlil Gibran

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