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
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:
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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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.commailto: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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I’m very appreciative for each person who’s provided some feedback, especially the lengthy replies.
Sounds like RoCE capable Ethernet backbone may be the default way to go *unless* the end users have some specific requirements that might need IB. At this point, we wouldn’t be interested in anything slower than 200Gbps. So perhaps Eth and IB are equivalent in terms of latency and RDMA capabilities, except one is an open standard.
Thanks,
Daniel Healy
On Mon, Feb 26, 2024 at 3:40 AM Cutts, Tim tim.cutts@astrazeneca.com wrote:
I concur with what folks have written so far, it really depends on your use case. For instance if you are looking at a cluster with GPU's and intend to do some serious computing there you are going to need RDMA of some sort. But it all depends on what you end up needing for your workflows.
For us we put most of our network traffic over the IB using IPoIB combined with aliasing all the nodes to their IB address. Thus all the internode network traffic spans the IB fabric rather than the ethernet. We then have 1GbE for our ethernet backend which we mainly use for management purposes. So we haven't heavily invested in a high speed ethernet backbone but instead invested in IB.
To invest in both seems to me to be overkill, you should focus on one or the other unless you have the cash to spend and a good use case.
-Paul Edmon-
On 2/26/24 7:07 AM, Dan Healy via slurm-users wrote:
Just looking for some feedback, please. Is this OK? Is there a better
way?
I’m tempted to spec all new HPCs with only a high speed (200Gbps) IB
network,
Well you need Ethernet for OOB management (bmc/ipmi/ilo/whatever) anyway.. or?
cheers
josef
On 25. 02. 24 21:12, Dan Healy via slurm-users wrote:
This question is not slurm-specific, but it might develop into that.
Hi Josef,
on a cluster using pxe boot and automatic (re) installation of nodes, I do not think you can do this with IPoIB on an infiniband interface.
On my cluster nodes I have: - 1Gb ethernet network for OOB - 10 or 25Gb ethernet for session, automatic deployment and management - IB HDR100 for MPI
Today data storage is reached via IPoIB on one cluster and via ethernet for the second one because it is not located in the same building (old IB QDR setup). I'm working in deploying an additional ceph storage cluster and it will also require ethernet as there are no IB on these new storage nodes (eth 25Gb only)
These clusters are small (300-400 cores each). The HDR100 IB network is shared by a third cluster in the laboratory (shared purchase for the switch).
So different technologies can be required together. This represents an investment but with costs amortization over a decade or more (my QDR setup is from 2012 and still in production).
Patrick
Le 26/02/2024 à 08:59, Josef Dvoracek via slurm-users a écrit :
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Cutts, Tim -
Dan Healy -
Jason Simms -
Josef Dvoracek -
Patrick Begou -
Paul Edmon