Features/RDMALiveMigration

Summary

Live migration using RDMA instead of TCP.

Contact

• Name: Michael Hines
• Email: mrhines@us.ibm.com

Description

Uses standard OFED software stack, which supports both RoCE and Infiniband.

Usage

First, decide if you want dynamic page registration on the server-side. The only reason to change this setting is if you have a super-fast RDMA link which could be fully utlized during the bulk-phase round of migration. NOTE: Disabling this will pin all of the memory on the destination, so if that is not what you want, then don't do it!

QEMU Monitor Command:

$ migrate_set_capability chunk_register_destination off # enabled by default

Next, set the migration speed to match your hardware's capabilities:

QEMU Monitor Command:

$ migrate_set_speed 40g # or whatever is the MAX of your RDMA device

Next, on the destination machine, add the following to the QEMU command line:

qemu ..... -incoming x-rdma:host:port

Finally, perform the actual migration:

QEMU Monitor Command:

$ migrate -d x-rdma:host:port

Performance

Protocol Design

Migration with RDMA is separated into two parts:

1. The transmission of the pages using RDMA
2. Everything else (a control channel is introduced)

"Everything else" is transmitted using a formal protocol now, consisting of infiniband SEND messages.

An infiniband SEND message is the standard ibverbs message used by applications of infiniband hardware. The only difference between a SEND message and an RDMA message is that SEND messages cause notifications to be posted to the completion queue (CQ) on the infiniband receiver side, whereas RDMA messages (used for pc.ram) do not (to behave like an actual DMA).

Messages in infiniband require two things:

1. registration of the memory that will be transmitted
2. (SEND only) work requests to be posted on both sides of the network before the actual transmission can occur.

RDMA messages are much easier to deal with. Once the memory on the receiver side is registered and pinned, we're basically done. All that is required is for the sender side to start dumping bytes onto the link.

(Memory is not released from pinning until the migration completes, given that RDMA migrations are very fast.)

SEND messages require more coordination because the receiver must have reserved space (using a receive work request) on the receive queue (RQ) before QEMUFileRDMA can start using them to carry all the bytes as a control transport for migration of device state.

To begin the migration, the initial connection setup is as follows (migration-rdma.c):

1. Receiver and Sender are started (command line or libvirt):
2. Both sides post two RQ work requests
3. Receiver does listen()
4. Sender does connect()
5. Receiver accept()
6. Check versioning and capabilities (described later)

At this point, we define a control channel on top of SEND messages which is described by a formal protocol. Each SEND message has a header portion and a data portion (but together are transmitted as a single SEND message).

Header:

   * Length  (of the data portion, uint32, network byte order)
   * Type    (what command to perform, uint32, network byte order)
   * Version (protocol version validated before send/recv occurs, uint32, network byte order
   * Repeat  (Number of commands in data portion, same type only)

The 'Repeat' field is here to support future multiple page registrations in a single message without any need to change the protocol itself so that the protocol is compatible against multiple versions of QEMU. Version #1 requires that all server implementations of the protocol must check this field and register all requests found in the array of commands located in the data portion and return an equal number of results in the response. The maximum number of repeats is hard-coded to 4096. This is a conservative limit based on the maximum size of a SEND message along with empirical observations on the maximum future benefit of simultaneous page registrations.

The 'type' field has 7 different command values:

   1. Unused
   2. Ready             (control-channel is available)
   3. QEMU File         (for sending non-live device state)
   4. RAM Blocks        (used right after connection setup)
   5. Compress page     (zap zero page and skip registration)
   6. Register request  (dynamic chunk registration)
   7. Register result   ('rkey' to be used by sender)
   8. Register finished (registration for current iteration finished)

A single control message, as hinted above, can contain within the data portion an array of many commands of the same type. If there is more than one command, then the 'repeat' field will be greater than 1.

After connection setup is completed, we have two protocol-level functions, responsible for communicating control-channel commands using the above list of values:

Logically:

qemu_rdma_exchange_recv(header, expected command type)

1. We transmit a READY command to let the sender know that we are *ready* to receive some data bytes on the control channel.
2. Before attempting to receive the expected command, we post another RQ work request to replace the one we just used up.
3. Block on a CQ event channel and wait for the SEND to arrive.
4. When the send arrives, librdmacm will unblock us.
5. Verify that the command-type and version received matches the one we expected.

qemu_rdma_exchange_send(header, data, optional response header & data):

1. Block on the CQ event channel waiting for a READY command from the receiver to tell us that the receiver is *ready* for us to transmit some new bytes.
2. Optionally: if we are expecting a response from the command (that we have no yet transmitted), let's post an RQ work request to receive that data a few moments later.
3. When the READY arrives, librdmacm will unblock us and we immediately post a RQ work request to replace the one we just used up.
4. Now, we can actually post the work request to SEND the requested command type of the header we were asked for.
5. Optionally, if we are expecting a response (as before), we block again and wait for that response using the additional work request we previously posted. (This is used to carry 'Register result' commands #6 back to the sender which hold the rkey need to perform RDMA. Note that the virtual address corresponding to this rkey was already exchanged at the beginning of the connection (described below).

All of the remaining command types (not including 'ready') described above all use the aformentioned two functions to do the hard work:

1. After connection setup, RAMBlock information is exchanged using this protocol before the actual migration begins. This information includes a description of each RAMBlock on the server side as well as the virtual addresses and lengths of each RAMBlock. This is used by the client to determine the start and stop locations of chunks and how to register them dynamically before performing the RDMA operations.
2. During runtime, once a 'chunk' becomes full of pages ready to be sent with RDMA, the registration commands are used to ask the
  other side to register the memory for this chunk and respond with the result (rkey) of the registration.
3. Also, the QEMUFile interfaces also call these functions (described below) when transmitting non-live state, such as devices or to send its own protocol information during the migration process.
4. Finally, zero pages are only checked if a page has not yet been registered using chunk registration (or not checked at all and unconditionally written if chunk registration is disabled. This is accomplished using the "Compress" command listed above. If the page *has* been registered then we check the entire chunk for zero. Only if the entire chunk is zero, then we send a compress command to zap the page on the other side.

Versioning

librdmacm provides the user with a 'private data' area to be exchanged at connection-setup time before any infiniband traffic is generated.

This is a convenient place to check for protocol versioning because the user does not need to register memory to transmit a few bytes of version information.

This is also a convenient place to negotiate capabilities (like dynamic page registration).

If the version is invalid, we throw an error.

If the version is new, we only negotiate the capabilities that the requested version is able to perform and ignore the rest.

QEMUFileRDMA Interface

QEMUFileRDMA introduces a couple of new functions:

1. qemu_rdma_get_buffer()  (QEMUFileOps rdma_read_ops)
2. qemu_rdma_put_buffer()  (QEMUFileOps rdma_write_ops)

These two functions are very short and simply used the protocol describe above to deliver bytes without changing the upper-level users of QEMUFile that depend on a bytstream abstraction.

Finally, how do we handoff the actual bytes to get_buffer()?

Again, because we're trying to "fake" a bytestream abstraction using an analogy not unlike individual UDP frames, we have to hold on to the bytes received from control-channel's SEND messages in memory.

Each time we receive a complete "QEMU File" control-channel message, the bytes from SEND are copied into a small local holding area.

Then, we return the number of bytes requested by get_buffer() and leave the remaining bytes in the holding area until get_buffer() comes around for another pass.

If the buffer is empty, then we follow the same steps listed above and issue another "QEMU File" protocol command, asking for a new SEND message to re-fill the buffer.

Migration of pc.ram

At the beginning of the migration, (migration-rdma.c), the sender and the receiver populate the list of RAMBlocks to be registered with each other into a structure. Then, using the aforementioned protocol, they exchange a description of these blocks with each other, to be used later during the iteration of main memory. This description includes a list of all the RAMBlocks, their offsets and lengths and possibly includes pre-registered RDMA keys in case dynamic page registration was disabled on the server-side, otherwise not.

Main memory is not migrated with the aforementioned protocol, but is instead migrated with normal RDMA Write operations.

Pages are migrated in "chunks" (about 1 Megabyte right now). Chunk size is not dynamic, but it could be in a future implementation. There's nothing to indicate that this is useful right now.

When a chunk is full (or a flush() occurs), the memory backed by the chunk is registered with librdmacm and pinned in memory on both sides using the aforementioned protocol.

After pinning, an RDMA Write is generated and tramsmitted for the entire chunk.

Chunks are also transmitted in batches: This means that we do not request that the hardware signal the completion queue for the completion of *every* chunk. The current batch size is about 64 chunks (corresponding to 64 MB of memory). Only the last chunk in a batch must be signaled. This helps keep everything as asynchronous as possible and helps keep the hardware busy performing RDMA operations.

Error-handling

Infiniband has what is called a "Reliable, Connected" link (one of 4 choices). This is the mode in which we use for RDMA migration.

If a *single* message fails, the decision is to abort the migration entirely and cleanup all the RDMA descriptors and unregister all the memory.

After cleanup, the Virtual Machine is returned to normal operation the same way that would happen if the TCP socket is broken during a non-RDMA based migration.

TODO

1. Currently, cgroups swap limits for *both* TCP and RDMA on the sender-side is broken. This is more poignant for RDMA because RDMA requires memory registration. Fixing this requires infiniband page registrations to be zero-page aware, and this does not yet work properly. 
2. Currently overcommit for the the *receiver* side of TCP works, but not for RDMA. While dynamic page registration *does* work, it is only useful if the is_zero_page() capability is remained enabled (which it is by default). However, leaving this capability turned on *significantly* slows down the RDMA throughput, particularly on hardware capable of transmitting faster than 10 gbps (such as 40gbps links).
3. Use of the recent /dev/<pid>/pagemap would likely solve some of these problems. 
4. Also, some form of balloon-device usage tracking would also help aleviate some of these issues.

Links

• Original RDMA Live Migration Paper