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. This always happens on the primary-VM side, but is optional on the server. Doing this allows you to support overcommit (such as cgroups or ballooning) with a smaller footprint on the server-side without having to register the entire VM memory footprint.
NOTE: This significantly slows down RDMA throughput (about 30% slower).
$ virsh qemu-monitor-command --hmp --cmd "migrate_set_capability chunk_register_destination on" # disabled by default
Next, if you decided *not* to use chunked registration on the server, it is recommended to also disable zero page detection. While this is not strictly necessary, zero page detection also significantly slows down performance on higher-throughput links (by about 50%), like 40 gbps infiniband cards:
$ virsh qemu-monitor-command --hmp --cmd "migrate_check_for_zero off" # always enabled by default
Finally, set the migration speed to match your hardware's capabilities:
$ virsh qemu-monitor-command --hmp --cmd "migrate_set_speed 40g" # or whatever is the MAX of your RDMA device
Finally, perform the actual migration:
$ virsh migrate domain rdma:xx.xx.xx.xx: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 / RECV 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 message cause completion 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/RECV only) work requests to be posted on both sides of the network before the actual transmission can occur.
RDMA messages 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.
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 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) * Type (what command to perform, described below) * Version (protocol version validated before send/recv occurs)
The 'type' field has 7 different command values:
1. None
2. Ready (control-channel is available)
3. QEMU File (for sending non-live device state)
4. RAM Blocks (used right after connection setup)
5. Register request (dynamic chunk registration)
6. Register result ('rkey' to be used by sender)
7. Register finished (registration for current iteration finished)
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. 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. 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.
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.