My view is that it depends entirely on the workload, and the systems with which your compute needs to interact.  A few things I’ve experienced before.

 

1. Modern ethernet networks have pretty good latency these days, and so MPI codes can run over them.   Whether IB is worth the money is a cost/benefit calculation for the codes you want to run.  The ethernet network we put in at Sanger in 2016 or so we measured as having similar latency, in practice, as FDR infiniband, if I remember correctly.  So it wasn’t as good as state-of-the-art IB at the time, but not bad.  Certainly good enough for our purposes, and we gained a lot of flexibility through software-defined networking, important if you have workloads which require better security boundaries than just a big shared network.
2. If your workload is predominantly single node, embarrassingly parallel, you might do better to go with ethernet and invest the saved money in more compute nodes.
3. If you only have ethernet, your cluster will be simpler, and require less specialised expertise to run
4. If your parallel filesystem is Lustre, IB seems to be the more well-worn path than ethernet.  We encountered a few Lustre bugs early on because of that.
5. On the other hand, if you need to talk to Weka, ethernet is the well-worn path.  Weka’s IB implementation requires the dedication of some cores on every client node, so you lose some compute capacity, which you don’t need to do if you’re using ethernet.

 

So, as any lawyer would say “it depends”.  Most of my career has been in genomics, where IB definitely wasn’t necessary.  Now that I’m in pharma, there’s more MPI code, so there’s more of a case for it.

 

Ultimately, I think you need to run the real benchmarks with real code, and as Jason says, work out whether the additional complexity and cost of the IB network is worth it for your particular workload.  I don’t think the mantra “It’s HPC so it has to be Infiniband” is a given.

 

Tim

 

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AstraZeneca

 

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From: Jason Simms via slurm-users <slurm-users@lists.schedmd.com>
Date: Monday, 26 February 2024 at 01:13
To: Dan Healy <daniel.t.healy@gmail.com>
Cc: slurm-users@lists.schedmd.com <slurm-users@lists.schedmd.com>
Subject: [slurm-users] Re: Question about IB and Ethernet networks

Hello Daniel,

 

In my experience, if you have a high-speed interconnect such as IB, you would do IPoIB. You would likely still have a "regular" Ethernet connection for management purposes, and yes that means both an IB switch and an Ethernet switch, but that switch doesn't have to be anything special. Any "real" traffic is routed over IB, everything is mounted via IB, etc. That's how the last two clusters I've worked with have been configured, and the next one will be the same (but will use Omnipath rather than IB). We likewise use BeeGFS.

 

These next comments are perhaps more likely to encounter differences of opinion, but I would say that sufficiently fast Ethernet is often "good enough" for most workloads (e.g., MPI). I'd wager that for all but the most demanding of workloads, it's entirely acceptable. You'll also save a bit of money, of course. HOWEVER, I do think there is, shall we say, an expectation from many researchers that any cluster worth its salt will have some kind of fast interconnect, even if at the scale of most on-prem work, you might be hard-pressed in real-world conditions to notice much of a difference. If you're running jobs that take weeks and hundreds of nodes, the time (and other) savings may add up, but if we're talking the difference between a job running on 5 nodes taking 48 hours vs. slightly less, then?? Your mileage may vary, as they say...

 

Warmest regards,

Jason

 

On Sun, Feb 25, 2024 at 3:13 PM Dan Healy via slurm-users <slurm-users@lists.schedmd.com> wrote:

Hi Fellow Slurm Users,

 

This question is not slurm-specific, but it might develop into that. 

 

My question relates to understanding how typical HPCs are designed in terms of networking. To start, is it typical for there to be a high speed Ethernet and Infiniband networks (meaning separate switches, NICs)? I know you can easily setup IP over IB, but is IB usually fully reserved for MPI messages? I’m tempted to spec all new HPCs with only a high speed (200Gbps) IB network, and use IPoIB for all slurm comms with compute nodes. I plan on using BeeGFS for the file system with RDMA. 

 

Just looking for some feedback, please. Is this OK? Is there a better way? If yes, please share why it’s better. 

Thanks,

Daniel Healy

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