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From: Alex Tumanov (atumanov_at_[hidden])
Date: 2007-02-08 18:49:45


Thanks for your insight George.

> 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.

I searched the code base for mca parameter registering and did indeed
discover that latency setting is possible for tcp and tcp alone:
------------------------------------------------------------------------------------------------------
[OMPISRCDIR]# grep -r param_register * |egrep -i "latency|bandwidth"
ompi/mca/btl/openib/btl_openib_component.c:
mca_btl_openib_param_register_int("bandwidth", "Approximate maximum
bandwidth of interconnect",
ompi/mca/btl/tcp/btl_tcp_component.c: btl->super.btl_bandwidth =
mca_btl_tcp_param_register_int(param, 0);
ompi/mca/btl/tcp/btl_tcp_component.c: btl->super.btl_latency =
mca_btl_tcp_param_register_int(param, 0);
ompi/mca/btl/gm/btl_gm_component.c:
mca_btl_gm_param_register_int("bandwidth", 250);
ompi/mca/btl/mvapi/btl_mvapi_component.c:
mca_btl_mvapi_param_register_int("bandwidth", "Approximate maximum
bandwidth of interconnect",
------------------------------------------------------------------------------------------------------
For all others, btl_latency appears to be set to zero when the btl
module gets constructed. Would zero latency prevent message striping?

An interesting side-issue that surfaces as a result of this little
investigation is the inconsistency between the ompi_info output and
the actual mca param availability for tcp_latency:

[OMPISRCDIR]# ompi_info --param all all |egrep -i "latency|bandwidth"
                 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

You also mentioned the exclusivity factor. I looked through the code
for that, and it appears that interconnect btl module developers are
setting exclusivity to various different integer values. In one place,
the comment suggests that exclusivity is what gets used to prioritize
interconnects... So a) I'm not sure what to set exclusivity to, and b)
it's unclear whether its latency or exclusivity that determines the
order. According to btl.h and you - it's the latency, according to the
following - exclusivity has something to do with it as well:

btl/mx/btl_mx_component.c : mca_base_param_reg_int(
(mca_base_component_t*)&mca_btl_mx_component, "exclusivity",
                            "Priority compared with the others devices
(used only when several devices are available",
                            false, false, 50, (int*)
&mca_btl_mx_module.super.btl_exclusivity );

What should exclusivity be set to in order to allow using multiple
interconnects?

Finally,
> 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).
That's precisely how I would've done it and makes perfect sense. Since
it's the relative ratio that matters and not the absolute value, why
then my openib+gm test failed to deliver better bandwidth performance
than just openib? I had bandwidth values set for both of those btls.
The expected behavior in my case would be to send roughly 1/4
(250/1050) across gm and 3/4 (800/1050) across openib? My hunch is
that there's something else preventing message striping other than
incorrect absolute values for the bandwidth here...

Thanks a lot for your feedback on this one. It gave me good pointers
to follow. Please do let me know if you can think of anything else
that I need to check.

Sincerely,
Alex.

> > On 2/8/07, George Bosilca <bosilca_at_[hidden]> 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.