What is this thing called RDMA?

A good way to kick this blog off is probably to explain what this RDMA stuff that I work on really is.
RDMA stands for Remote Direct Memory Access, but the term “RDMA” is usually used to refer to networking technologies that have a software interface with three features:

  • Remote direct memory access (Remote DMA)
  • Asynchronous work queues
  • Kernel bypass

InfiniBand host channel adapters (HCAs) are an example of network adapters that offer such an interface, but RDMA over IP (iWARP) adapters are starting to appear as well.

Anyway, let’s take a look at what these three features really mean.

Remote DMA

Remote DMA is pretty much what it sounds like: DMA on a remote system. The adapter on system 1 can send a message to the adapter on system 2 that causes the adapter on system 2 to DMA data to or from system 2’s memory. The messages come in two main types:

  • RDMA Write: includes an address and data to put at that address, and causes the adapter that receives it to put the supplied data at the specified address
  • RDMA Read: includes an address and a length, and causes the adapter that receives it to generate a reply that sends back the data at the address requested.

These messages are “one-sided” in the sense that they will be processed by the adapter that receive them without involving the CPU on the system that receives the messages.

Letting a remote system DMA into your memory sounds pretty scary, but RDMA adapters give fine-grained control over what remote systems are allowed to do. Going into the details now will make this entry way too long, so for now just trust me that things like protection domains and memory keys allow you to control connection-by-connection and byte-by-byte with separate read and write permissions.

To see why RDMA is useful, you can think of RDMA operations as “direct placement” operations: data comes along with information about where it’s supposed to go. For example, there is a spec for NFS/RDMA, and it’s pretty easy to see why RDMA is nice for NFS. The NFS/RDMA server can service requests in whatever order it wants and return responses via RDMA as they become available; by using direct placement, the responses can go right into the buffers where the client wants them, without requiring the NFS client to do any copying of data.

(There are actually some more complicated RDMA operations that are supported on InfiniBand, namely atomic fetch & add, and atomic compare & swap, but those aren’t quite as common so you can ignore them for now)

Asynchronous work queues

Software talks to RDMA adapters via an aynchronous interface. This doesn’t really have all that much to do with remote DMA, but when we talk about RDMA adapters, we expect this type of interface (which is called a “verbs” interface for some obscure historical reason).

Basically, to use an RDMA adapter, you create objects called queue pairs (or QPs), which as the name suggests are a pair of work queues: a send queue and a receive queue, and completion queues (or CQs). When you want to do something, you tell the adapter to post an operation to one of your work queues. The operation executes asynchronously, and when it’s done, the adapter adds work completion information onto the end of your CQ. When you’re ready, you can go retrieve completion information from the CQ to see which requests have completed.

Operating asynchronously like this makes it easier to overlap computation and communication.

(Incidentally, as the existence of receive queues might make you think, RDMA adapters support plain old “two-sided’ send/receive operations, in addition to one-sided RDMA operations. You can post a receive request to your local receive queue, and then next send message that comes in will be received into the buffer you provided. RDMA operations and send operations can be mixed on the same send queue, too)

Kernel bypass

The last feature which is common to RDMA adapters also has nothing to do with remote DMA per se. But RDMA adapters allow userspace processes to do fast-path operations (posting work requests and retreiving work completions) directly with the hardware without involving the kernel at all. This is nice because these adapters are typically used for high-performance, latency-sensitive applications, and saving the system call overhead is a big win when you’re counting nanoseconds.

OK, that’s it for today’s edition of “RDMA 101.” Now we have some background to talk about some of the more interesting features of RDMA networking.

One Response to “What is this thing called RDMA?”

  1. [...] 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 [...]