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Subject: [OMPI devel] 1.3 PML default choice
From: Bogdan Costescu (Bogdan.Costescu_at_[hidden])
Date: 2008-11-17 12:39:35


Hi!

In testing the 1.3b2, I have encountered a rather strange behaviour.
First the setup:
         dual-CPU dual-core x86_64 with Myrinet 10G card
         self compiled Linux kernel 2.6.22.18, MX 1.2.7(*)
         GCC-4.1.2 (from Debian etch), Torque 2.1.10
         OpenMPI 1.3b2 (tar.gz from download page)
         IMB 3.1

(*) I'm actually tracking a problem together with Myricom people, so
it's not a vanilla 1.2.7, but 1.2.7 with a tiny patch; I believe that
this has no influence

When starting an IMB run with the default settings, in all collective
communication functions I see huge jumps around 32-1024 bytes and flat
results around 1K-16K like:

#----------------------------------------------------------------
# Benchmarking Allgatherv
# #processes = 64
#----------------------------------------------------------------
        #bytes #repetitions t_min[usec] t_max[usec] t_avg[usec]
             0 1000 0.19 0.21 0.19
             1 1000 35.29 35.30 35.29
             2 1000 36.01 36.03 36.02
             4 1000 38.97 38.98 38.98
             8 1000 42.12 42.13 42.13
            16 1000 45.76 45.77 45.77
            32 1000 19991.83 20011.84 20005.29
            64 1000 38561.52 38599.66 38587.74
           128 1000 58263.81 58305.74 58293.48
           256 1000 77382.83 77425.93 77412.49
           512 1000 95981.97 96022.70 96010.73
          1024 1000 480838.00 481214.78 481027.05
          2048 1000 480522.97 480917.02 480727.98
          4096 1000 480762.69 481134.49 480955.03
          8192 1000 481136.70 481505.36 481334.86
         16384 1000 483629.46 483889.28 483759.38
         32768 1000 23809.47 23810.27 23809.62
         65536 640 7085.58 7085.91 7085.69
        131072 320 11928.29 11929.29 11928.72
        262144 160 22174.66 22177.67 22175.94
        524288 80 42270.91 42283.90 42277.55
       1048576 40 82389.85 82461.10 82428.26
       2097152 20 161347.04 161624.54 161485.84
       4194304 10 321467.52 322562.79 322019.24

This happens on various numbers of nodes and is reproducable - I have
repeated the run 5 times on 8 nodes.

I have not seen such results with 1.2.x series with either OB1+BTL or
CM+MTL, timing increases rather smoothly. Trying various options with
1.3b2:

--mca pml cm --mca mtl mx works well
--mca pml cm --mca btl mx,sm,self works well
--mca pml ob1 --mca btl mx,sm,self jumps like above

>From what I know, the 1.2.x series defaulted to OB1+BTL; CM was only
possible with a MTL which internally implemented sm and self, so
second test above would have failed (please correct me if I'm wrong).

The README for 1.3b2 specifies that CM is now chosen if possible; in
my trials, when I specify CM+BTL, it doesn't complain and works well.
However either the default (no options) or OB1+BTL leads to the jumps
mentioned above, which makes me believe that OB1+BTL is still chosen
as default, contrary to what the README specifies.

So there are 2 issues:
- which is right, the README or the runtime behaviour that I see ?
- is it normal for the OB1+BTL to behave so poorly with MX ?

Thanks for any insight into this issues.

-- 
Bogdan Costescu
IWR, University of Heidelberg, INF 368, D-69120 Heidelberg, Germany
Phone: +49 6221 54 8240, Fax: +49 6221 54 8850
E-mail: bogdan.costescu_at_[hidden]