<span style=" font-size:10pt;font-family:sans-serif">Jonathan:</span><br><br><span style=" font-size:10pt;font-family:sans-serif">You posed a reasonable
question, which was "when is RDMA worth the hassle?" I
agree with part of your premises, which is that it only matters when the
bottleneck isn't somewhere else. With a parallel file system, like Scale/GPFS,
the absolute performance bottleneck is not the throughput of a single drive.
In a majority of Scale/GPFS clusters the network data path is the performance
limitation. If they deploy HDR or 100/200/400Gbps Ethernet... At
that point, the buffer copy time inside the server matters. </span><br><br><span style=" font-size:10pt;font-family:sans-serif">When the device
is an accelerator, like a GPU, the benefit of RDMA (GDS) is easily demonstrated
because it eliminates the bounce copy through the system memory. In our
NVIDIA DGX A100 server testing testing we were able to get around 2x the
per system throughput by using RDMA direct to GPU (GUP Direct Storage).
(Tested on 2 DGX system with 4x HDR links per storage node.) </span><br><br><span style=" font-size:10pt;font-family:sans-serif">However, your
question remains. Synthetic benchmarks are good indicators of technical
benefit, but do your users and applications need that extra performance?
</span><br><br><span style=" font-size:10pt;font-family:sans-serif">These are probably
only a handful of codes in organizations that need this. However, they
are high-value use cases. We have client applications that either read
a lot of data semi-randomly and not-cached - think mini-Epics for scaling
ML training. Or, demand lowest response time, like production inference
on voice recognition and NLP. </span><br><br><span style=" font-size:10pt;font-family:sans-serif">If anyone has
use cases for GPU accelerated codes with truly demanding data needs, please
reach out directly. We are looking for more use cases to characterize the
benefit for a new paper. f you can provide some code examples, we can help
test if RDMA direct to GPU (GPUdirect Storage) is a benefit. </span><br><br><span style=" font-size:10pt;font-family:sans-serif">Thanks,</span><br><br><span style=" font-size:10pt;font-family:sans-serif">doug</span><br><br><span style=" font-size:10pt;font-family:sans-serif">Douglas O'Flaherty<br>douglasof@us.ibm.com</span><br><br><br><br><tt><span style=" font-size:10pt"><br><br></span></tt><span style=" font-size:10pt;color:#800080;font-family:sans-serif"><br>----- Message from Jonathan Buzzard <jonathan.buzzard@strath.ac.uk>
on Fri, 10 Dec 2021 00:27:23 +0000 -----</span><table width=513 style="border-collapse:collapse;"><tr height=8><td width=55 style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:1px 1px;"><div align=right><span style=" font-size:12pt"><b>To:</b></span></div><td width=454 style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:1px 1px;"><span style=" font-size:12pt">gpfsug-discuss@spectrumscale.org</span><tr height=8><td width=55 style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:1px 1px;"><div align=right><span style=" font-size:12pt"><b>Subject:</b></span></div><td width=454 style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:1px 1px;"><span style=" font-size:12pt">Re:
[gpfsug-discuss] </span></table><br><tt><span style=" font-size:10pt">On 09/12/2021 16:04, Douglas O'flaherty
wrote:<br>> <br>> Though not directly about your design, our work with NVIDIA on GPUdirect
<br>> Storage and SuperPOD has shown how sensitive RDMA (IB & RoCE)
to both <br>> MOFED and Firmware version compatibility can be.<br>> <br>> I would suggest anyone debugging RDMA issues should look at those
closely.<br>> <br>May I ask what are the alleged benefits of using RDMA in GPFS?<br><br>I can see there would be lower latency over a plain IP Ethernet or IPoIB
<br>solution but surely disk latency is going to swamp that?<br><br>I guess SSD drives might change that calculation but I have never seen
<br>proper benchmarks comparing the two, or even better yet all four <br>connection options.<br><br>Just seems a lot of complexity and fragility for very little gain to me.<br><br><br>JAB.<br><br>-- <br>Jonathan A. Buzzard
Tel: +44141-5483420<br>HPC System Administrator, ARCHIE-WeSt.<br>University of Strathclyde, John Anderson Building, Glasgow. G4 0NG<br><br></span></tt><br><span style=" font-size:10pt;font-family:Arial"> </span><br><span style=" font-size:10pt;font-family:Arial"> </span><br><span style=" font-size:10pt;font-family:Arial">----- Original message
-----<br>From: "Jonathan Buzzard" <jonathan.buzzard@strath.ac.uk><br>Sent by: gpfsug-discuss-bounces@spectrumscale.org<br>To: gpfsug-discuss@spectrumscale.org<br>Cc:<br>Subject: [EXTERNAL] Re: [gpfsug-discuss] alternate path between ESS Servers
for Datamigration<br>Date: Fri, Dec 10, 2021 10:27<br> </span><br><tt><span style=" font-size:10pt">On 09/12/2021 16:04, Douglas O'flaherty
wrote:<br>><br>> Though not directly about your design, our work with NVIDIA on GPUdirect<br>> Storage and SuperPOD has shown how sensitive RDMA (IB & RoCE)
to both<br>> MOFED and Firmware version compatibility can be.<br>><br>> I would suggest anyone debugging RDMA issues should look at those
closely.<br>><br>May I ask what are the alleged benefits of using RDMA in GPFS?<br><br>I can see there would be lower latency over a plain IP Ethernet or IPoIB<br>solution but surely disk latency is going to swamp that?<br><br>I guess SSD drives might change that calculation but I have never seen<br>proper benchmarks comparing the two, or even better yet all four<br>connection options.<br><br>Just seems a lot of complexity and fragility for very little gain to me.<br><br><br>JAB.<br><br>--<br>Jonathan A. Buzzard
Tel: +44141-5483420<br>HPC System Administrator, ARCHIE-WeSt.<br>University of Strathclyde, John Anderson Building, Glasgow. G4 0NG<br>_______________________________________________<br>gpfsug-discuss mailing list<br>gpfsug-discuss at spectrumscale.org</span></tt><tt><span style=" font-size:10pt;color:blue"><u><br></u></span></tt><a href="http://gpfsug.org/mailman/listinfo/gpfsug-discuss" target="_blank"><tt><span style=" font-size:10pt;color:blue"><u>http://gpfsug.org/mailman/listinfo/gpfsug-discuss</u></span></tt></a><tt><span style=" font-size:10pt"></span></tt><br><span style=" font-size:10pt;font-family:Arial"> </span><br><br><br><BR>
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