Roland’s Blog

I saw that the InfiniBand Trade Association announced the “RDMA over Converged Ethernet (RoCE)” specification today.  I’ve already discussed my thoughts on the underlying technology (although I have a bit more to say), so for now I just want to say that I really, truly hate the name they chose.  There are at least two things that suck about the name:

1. Calling the technology “RDMA over” instead of “InfiniBand over” is overly vague and intentionally deceptive.  We already have “RDMA over Ethernet” — except we’ve been calling it iWARP.  Choosing “RoCE” is somewhat like talking about “Storage over Ethernet” instead of “Fibre Channel over Ethernet.”  Sure, FCoE is storage over ethernet, but so is iSCSI.  As for the intentionally deceptive part: I’ve been told that “InfiniBand” was left out of the name because the InfiniBand Trade Association felt that InfiniBand is viewed negatively in some of the markets they’re going after.  What does that say about your marketing when you are running away from your own main trademark?
2. The term “Converged Ethernet” is also pretty meaningless.  The actual technology has nothing to do with “converged” ethernet (whatever that is, exactly); the annex that was just release simply describes how to stick InfiniBand packets inside a MAC header and Ethernet FCS, so simply “Ethernet” would be more accurate.  At least the “CE” part is an improvement over the previous try, “Converged Enhanced Ethernet” or “CEE”; not only does the technology have nothing to do with CEE either, “CEE” was an IBM-specific marketing term for what eventually became Data Center Bridging or “DCB.”  (At Cisco we used to use the term “Data Center Ethernet” or “DCE”)

So both the “R” and the “CE” of “RoCE” aren’t very good choices.  It would be a lot clearer and more intellectually honest if we could just call InfiniBand over Ethernet by its proper name: IBoE.  And explaining the technology would be a bit simpler too, since the analogy with FCoE becomes a lot more explicit.

I found this article in “Network Computing” pretty interesting, although not exactly for the content.   Just the framing of the whole article, with Microsoft is touting the fact that they’ve managed to achieve performance parity with Linux on some HPC benchmarks as an achievement (and putting up a graph that shows they are still at least a few percent behind), shows how dominant Linux is in HPC.  Also, the article says:

The beta also reportedly includes optimizations for new processors and can deploy and manage up to 1,000 nodes.

So in other words Microsoft is stuck at the low end of the HPC market, only usable on small clusters.

I recently read Andy Grover’s post about converged fabrics, and since I particupated in the OpenFabrics panel in Sonoma that he alluded to, I thought it might be worth sharing my (somewhat different) thoughts.

The question that Andy is dealing with is how to run RDMA on “Converged Ethernet.” I’ve already explained what RDMA is, so I won’t go into that here, but it’s probably worth talking about Ethernet, since I think the latest developments are not that familiar to many people.  The IEEE has been developing a few standards they collectively refer to as “Data Center Bridging” (DCB) and that are also sometimes referred to as “Converged Enhanced Ethernet” (CEE).  This refers to high speed Ethernet (currently 10 Gb/sec, with a clear path to 40 Gb/sec and 100 Gb/sec), plus new features.  The main new features are:

• Priority-Based Flow Control (802.1Qbb), sometimes called “per-priority pause”
• Enhanced Transmission Selection (802.1Qaz)
• Congestion Notification (802.1Qau)

The first two features let an Ethernet link be split into multiple “virtual links” that operate pretty independently — bandwidth can be reserved for a given virtual link so that it can’t be starved, and by having per-virtual-link flow control, we can make sure certain traffic classes don’t overrun their buffers and avoid dropping packets.  Then congestion notification means that we can tell senders to slow down to avoid congestion spreading caused by that flow control.

The main use case that DCB was developed for was Fibre Channel over Ethernet (FCoE).  FC requires a very reliable network — it simply doesn’t work if packets are dropped because of congestion — and so DCB provides the ability to segregate FCoE traffic onto a “no drop” virtual link.  However, I think Andy misjudges the real motivation for FCoE; the TCP/IP overhead of iSCSI was not really an issue (and indeed there are many people running iSCSI with very high performance on 10 Gb/sec Ethernet).

The real motivation for FCoE is to give a way for users to continue using all the FC storage they already have, while not requiring every server that wants to talk to the storage to have both a NIC and an FC HBA.  With a gateway that’s easy to build an scale, legacy FC storage can be connected to an FCoE fabric, and now servers with a “converged network adapter” that functions as both an Ethernet NIC and an FCoE HBA can talk to network and storage over one (Ethernet) wire.

Now, of course for servers that want to do RDMA, it makes sense that they want a triple-threat converged adapter that does Ethernet NIC, FCoE HBA, and RDMA.  The way that people are running RDMA over Ethernet today is via iWARP, which runs an RDMA protocol layered on top of TCP.  The idea that Andy and several other people in Sonoma are pushing is to do something analogous to FCoE instead, that is, take the InfiniBand transport layer and stick it into Ethernet somehow.  I see a number of problems with this idea.

First, one of the big reasons given for wanting to use InfiniBand on Ethernet instead of iWARP is that it’s the fastest path forward.  The argument is, “we just scribble down a spec, and everyone can ship it easily.”  That ignores the fact that iWARP adapters are already shipping from multiple vendors (although, to be fair, none with support for the proposed IEEE DCB standards yet; but DCB support should soon be ubiquitous in all 10 gigE NICs, iWARP and non-iWARP alike).  And the idea that an IBoE spec is going to be quick or easy to write flies in the face of the experience with FCoE; FCoE sounded dead simple in theory (just stick an Ethernet header on FC frames, what more could there be?) it turns out that the standards work has taken at least 3 years, and a final spec is still not done.  I believe that IBoE would be more complicated to specify, and fewer resources are available for the job, so a realistic view is that a true standard is very far away.

Andy points at a TOE page to say why running TCP on an iWARP NIC sucks.  But when I look at that page, pretty much all the issues are still there with running the IB transport on a NIC.  Just to take the first few on that page (without quibbling about the fact that many of the issues are just wrong even about TCP offload):

• Security updates: yup, still there for IB
• Point-in-time solution: yup, same for IB
• Different network behavior: a hundred times worse if you’re running IB instead of TCP
• Performance: yup
• Hardware-specific limits: yup

And so on…

Certainly, given infinite resources, one could design an RDMA protocol that was cleaner than iWARP and took advantage of all the spiffy DCB features.  But worse is better and iWARP mostly works well right now; fixing the worst warts of iWARP has a much better chance of success than trying to shoehorn IB onto Ethernet and ending up with a whole bunch of unforseen problems to solve.

I’ve been trying to get a udev rule added to Ubuntu so that /dev/infiniband/rdma_cm is owned by group “rdma” instead of by root, so that unprivileged user applications can be given permission to use it by adding the user to the group rdma.  This matches the practice in the Debian udev rules and is a simple way to allow unprivileged use of RDMA while still giving the administrator some control over who exactly uses it.

I created a patch to the Ubuntu librdmacm package containing the appropriate rule and opened a Launchpad bug report requesting that it be applied.  After two months of waiting, I got a response that basically said, “no, we don’t want to do that.”  After another month of asking, I finally found out what solution Ubuntu would rather have:

Access to system devices is provided through the HAL or DeviceKit interface. Permission to access is managed through the PolicyKit layer, where the D-Bus system bus service providing the device access negotiates privilege with the application requesting it.

Because of course, rather than having an application simply open a special device node, mediated by standard Unix permissions, we’d rather have to run a daemon (bonus points for using DBus activation, I guess) and have applications ask that daemon to open the node for them.  More work to implement, harder to administer, less reliable for users — everyone wins!

Sigh….

At long last, after several requests, I’ve posted the slides, notes, and client and server examples from the tutorial I gave at LinuxConf.eu 2007 in Cambridge back in September.  Hyper-observant readers will notice that the client program I posted does not match the listing in the notes I handed out; this is because I fixed a race condition in how completions are collected.

I’m not sure how useful all this is without me talking about it, but I guess every little bit helps.  And of course, if you have questions about RDMA or InfiniBand programming, come on over to the mailing list and fire away.

With yesterday’s release of  kernel 2.6.23, I thought it might be a good time to look back at what significant changes are in 2.6.23, and what we have queued up for 2.6.24..

So first I looked at the kernel git log from the v2.6.22 tag to the v2.6.23 tag, and I was surprised to find that nothing really stood out.  We merged something like 158 patches that touched 123 files, but I couldn’t really find any headline-worthy new features in there.  There were just tons of fixes and cleanups all over, although mostly in the low-level hardware drivers.  For some reason, 2.6.23 was a pretty calm development cycle for InfiniBand and RDMA, which means that at least that part of 2.6.23 should be rock solid.

2.6.24 promises to be a somewhat more exciting release for us.  In my for-2.6.24 branch, in addition to the usual pile of fixes and cleanups, I have a couple of interesting changes queued up to merge as soon as Linus starts pulling things in:

• Sean Hefty’s quality-of-service support.  These changes allow administrators to configure the network to give different QoS parameters to different types of traffic (eg IPoIB, SRP, and so on).
• A patch from me (based on Sean Hefty’s work) to handle multiple P_Keys for userspace management applications.  This is one of the last pieces to make the InfiniBand stack support IB partitions fully.

Also, bonding support for IP-over-InfiniBand looks set to go in through Jeff Garzik’s tree.  This is something that I’ve been wanting to see for years now; the patches allow the standard bonding module to enslave IPoIB interfaces, which means that multiple IB ports can finally be used for IPoIB high-availability failover.  Moni Shoua and others did a lot of work and stuck with this for a long time, and the final set of patches turned out to be very clean and nice, so I’m really pleased to see this get merged.

My tutorial Writing RDMA applications on Linux has been accepted at LinuxConf Europe 2007. I’ll try to give practical introduction to writing native RDMA applications on Linux — “native” meaning directly to RDMA verbs as opposed to using an additional library layer such as MPI or uDAPL.  I’m aiming to make it accessible to people who know nothing about RDMA, so if you read my blog you’re certainly qualified.  Start planning your trip now!

My presentation is on the morning of Monday, September 3, and I’m flying to England across 7 time zones on Sunday, September 2, so I hope I’m able to remain upright and somewhat coherent for the whole three hours I’m supposed to be speaking….

…computing icon Andy Bechtolsheim is a top-poster?

Sun just introduced their Constellation supercomputer at ISC Dresden. They’ve managed to get a lot of hype out of this, including mentions in places like the New York Times. But the most interesting part to me is the 3,456-port “Magnum” InfiniBand switch. I haven’t seen many details about it and I couldn’t find anything about it on Sun’s Constellation web site.

However I’ve managed to piece together some info about the switch from the new stories as well as the pictures in this blog entry. Physically, this thing is huge–it looks like it’s about half a rack high and two racks wide. The number 3,456 gives a big clue as to the internal architecture: 3456 = 288 * 12. Current InfiniBand switch chips have 24 ports, and the biggest non-blocking switch one can build with two levels (spine and leaf) is 24 * 12 = 288 ports: 24 leaf switches each of which have 12 ports to the outside and 12 ports to the spines (one port to each of the 12 spine switches).

Then, using 12 288-port switches as spines, one can take 288 24-port leaf switches that each have 12 ports to the outside and end up with 288 * 12 = 3456 ports, just like Sun’s Magnum switch. From the pictures of the chassis, it looks like Magnum has the spine switches on cards on one side of the midplane and the leaf switches on the other side, using the cute trick of having one set of cards be vertical and one set horizontal to get all-to-all connections between spines and leaves without having too-long midplane traces.

All of this sounds quite reasonable until you start to consider putting all of this in one box. Each 288 port switch (which is on one card in this design!) has 36 switch chips on it. At about 30 Watts per switch chip, each of this cards is over 1 kilowatt, and there are 12 of these in a system. In fact, with 720 switch chips in the box, the total system is well over 20 kW!

It also seems that the switch is using proprietary high-density connectors that bring three IB ports out of each connector, which reduces the number of external connectors on the switch down to a mere 1152.

One other thing I noticed is that the Sun press stuff is billing the Constellation as running Solaris, while the actual TACC page about the Ranger system says the cluster will be running Linux. I’m inclined to believe TACC, since running Solaris for an InfiniBand cluster seems a little silly, given how far behind Solaris’s InfiniBand support is when compared to Linux, whose InfiniBand stack is lovingly maintained by yours truly.

Here’s a short summary of what I plan to merge for 2.6.19. I sent this out via email to all the relevant lists, but I figured it can’t hurt to blog it too. Some of this is already in infiniband.git (git://git.kernel.org/pub/scm/linux/kernel/git/roland/infiniband.git), while some still needs to be merged up. Highlights:

• iWARP core support. This updates drivers/infiniband to work with devices that do RDMA over IP/ethernet in addition to InfiniBand devices. As a first user of this support, I also plan to merge the amso1100 driver for Ammasso RNICs.I will post this for review one more time after I pull it into my git tree for last minute cleanups. But if you feel this iWARP support should not be merged, please let me know why now.
• IBM eHCA driver, which supports IBM pSeries-specific InfiniBand hardware. This is in the ehca branch of infiniband.git, and I will post it for review one more time. My feeling is that more cleanups are certainly possible, but this driver is “good enough to merge” now and has languished out of tree for long enough. I’m certainly happy to merge cleanup patches, though.
• mmap()ed userspace work queues for ipath. This is a performance enhancement for QLogic/PathScale HCAs but it does touch core stuff in minor ways. Should not be controversial.
• I also have the following minor changes queued in the for-2.6.19 branch of infiniband.git:

       Ishai Rabinovitz:
             IB/srp: Add port/device attributes

       James Lentini:
             IB/mthca: Include the header we really want

       Michael S. Tsirkin:
             IB/mthca: Don't use privileged UAR for kernel access
             IB/ipoib: Fix flush/start xmit race (from code review)

       Roland Dreier:
             IB/uverbs: Use idr_read_cq() where appropriate
             IB/uverbs: Fix lockdep warning when QP is created with 2 CQs