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From: Galen Shipman (gshipman_at_[hidden])
Date: 2007-02-13 10:23:45


For OpenIB + GM you are probably going to be limited by the memory bus.
Take the InfiniBand Nic, it peaks at say 900 MBytes/Sec, the Myrinet
2-G will peak at say 250 MBytes/Sec.
Unless you are doing direct DMAs from pre-registered host memory than
you will not see 900 + 250 MBytes/Sec bandwidth.
The reason is that either you copy into registered memory, so the
copy is the bottleneck, or you register/unregister memory on demand,
which would be the bottleneck.

So the "solution" for micro-benchmarks is to register the memory and
leave it registered. Probably the best way to do this is to use
MPI_ALLOC_MEM when allocating memory, this allows us to register the
memory with all the available NICs.

For applications it is bit more difficult to say if there is benefit.
You may be able to decrease congestion for some systems. Some
applications that use pack buffers and are bandwidth limited may also
benefit, the pack buffer could be obtained via MPI_ALLOC_MEM.

I would also say that this is a very uncommon mode of operation, our
architecture allows it, but certainly isn't optimized for this case.

- Galen

On Feb 12, 2007, at 6:48 PM, Alex Tumanov wrote:

> Anyone else who may provide some feedback/comments on this issue? How
> typical/widespread is the use of multiple interconnects in the HPC
> community? Judging from the feedback I'm getting in this thread, it
> appears that this is fairly uncommon?
> Thanks for your attention to this thread.
> Alex.
> On 2/8/07, Alex Tumanov <atumanov_at_[hidden]> wrote:
>> 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.
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