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diff --git a/src/docs/rdma.txt b/src/docs/rdma.txt new file mode 100644 index 0000000..2bdd0a5 --- /dev/null +++ b/src/docs/rdma.txt @@ -0,0 +1,420 @@ +(RDMA: Remote Direct Memory Access) +RDMA Live Migration Specification, Version # 1 +============================================== +Wiki: http://wiki.qemu-project.org/Features/RDMALiveMigration +Github: git@github.com:hinesmr/qemu.git, 'rdma' branch + +Copyright (C) 2013 Michael R. Hines <mrhines@us.ibm.com> + +An *exhaustive* paper (2010) shows additional performance details +linked on the QEMU wiki above. + +Contents: +========= +* Introduction +* Before running +* Running +* Performance +* RDMA Migration Protocol Description +* Versioning and Capabilities +* QEMUFileRDMA Interface +* Migration of VM's ram +* Error handling +* TODO + +Introduction: +============= + +RDMA helps make your migration more deterministic under heavy load because +of the significantly lower latency and higher throughput over TCP/IP. This is +because the RDMA I/O architecture reduces the number of interrupts and +data copies by bypassing the host networking stack. In particular, a TCP-based +migration, under certain types of memory-bound workloads, may take a more +unpredicatable amount of time to complete the migration if the amount of +memory tracked during each live migration iteration round cannot keep pace +with the rate of dirty memory produced by the workload. + +RDMA currently comes in two flavors: both Ethernet based (RoCE, or RDMA +over Converged Ethernet) as well as Infiniband-based. This implementation of +migration using RDMA is capable of using both technologies because of +the use of the OpenFabrics OFED software stack that abstracts out the +programming model irrespective of the underlying hardware. + +Refer to openfabrics.org or your respective RDMA hardware vendor for +an understanding on how to verify that you have the OFED software stack +installed in your environment. You should be able to successfully link +against the "librdmacm" and "libibverbs" libraries and development headers +for a working build of QEMU to run successfully using RDMA Migration. + +BEFORE RUNNING: +=============== + +Use of RDMA during migration requires pinning and registering memory +with the hardware. This means that memory must be physically resident +before the hardware can transmit that memory to another machine. +If this is not acceptable for your application or product, then the use +of RDMA migration may in fact be harmful to co-located VMs or other +software on the machine if there is not sufficient memory available to +relocate the entire footprint of the virtual machine. If so, then the +use of RDMA is discouraged and it is recommended to use standard TCP migration. + +Experimental: Next, decide if you want dynamic page registration. +For example, if you have an 8GB RAM virtual machine, but only 1GB +is in active use, then enabling this feature will cause all 8GB to +be pinned and resident in memory. This feature mostly affects the +bulk-phase round of the migration and can be enabled for extremely +high-performance RDMA hardware using the following command: + +QEMU Monitor Command: +$ migrate_set_capability rdma-pin-all on # disabled by default + +Performing this action will cause all 8GB to be pinned, so if that's +not what you want, then please ignore this step altogether. + +On the other hand, this will also significantly speed up the bulk round +of the migration, which can greatly reduce the "total" time of your migration. +Example performance of this using an idle VM in the previous example +can be found in the "Performance" section. + +Note: for very large virtual machines (hundreds of GBs), pinning all +*all* of the memory of your virtual machine in the kernel is very expensive +may extend the initial bulk iteration time by many seconds, +and thus extending the total migration time. However, this will not +affect the determinism or predictability of your migration you will +still gain from the benefits of advanced pinning with RDMA. + +RUNNING: +======== + +First, 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 rdma:host:port + +Finally, perform the actual migration on the source machine: + +QEMU Monitor Command: +$ migrate -d rdma:host:port + +PERFORMANCE +=========== + +Here is a brief summary of total migration time and downtime using RDMA: +Using a 40gbps infiniband link performing a worst-case stress test, +using an 8GB RAM virtual machine: + +Using the following command: +$ apt-get install stress +$ stress --vm-bytes 7500M --vm 1 --vm-keep + +1. Migration throughput: 26 gigabits/second. +2. Downtime (stop time) varies between 15 and 100 milliseconds. + +EFFECTS of memory registration on bulk phase round: + +For example, in the same 8GB RAM example with all 8GB of memory in +active use and the VM itself is completely idle using the same 40 gbps +infiniband link: + +1. rdma-pin-all disabled total time: approximately 7.5 seconds @ 9.5 Gbps +2. rdma-pin-all enabled total time: approximately 4 seconds @ 26 Gbps + +These numbers would of course scale up to whatever size virtual machine +you have to migrate using RDMA. + +Enabling this feature does *not* have any measurable affect on +migration *downtime*. This is because, without this feature, all of the +memory will have already been registered already in advance during +the bulk round and does not need to be re-registered during the successive +iteration rounds. + +RDMA Protocol Description: +========================== + +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 VM's 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) + * 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 12 different command values: + 1. Unused + 2. Error (sent to the source during bad things) + 3. Ready (control-channel is available) + 4. QEMU File (for sending non-live device state) + 5. RAM Blocks request (used right after connection setup) + 6. RAM Blocks result (used right after connection setup) + 7. Compress page (zap zero page and skip registration) + 8. Register request (dynamic chunk registration) + 9. Register result ('rkey' to be used by sender) + 10. Register finished (registration for current iteration finished) + 11. Unregister request (unpin previously registered memory) + 12. Unregister finished (confirmation that unpin completed) + +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, message 5 & 6 are used to exchange ram block +information and optionally pin all the memory if requested by the user. + +After ram block exchange 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 not 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 and Capabilities +=========================== +Current version of the protocol is version #1. + +The same version applies to both for protocol traffic and capabilities +negotiation. (i.e. There is only one version number that is referred to +by all communication). + +librdmacm provides the user with a 'private data' area to be exchanged +at connection-setup time before any infiniband traffic is generated. + +Header: + * Version (protocol version validated before send/recv occurs), + uint32, network byte order + * Flags (bitwise OR of each capability), + uint32, network byte order + +There is no data portion of this header right now, so there is +no length field. The maximum size of the 'private data' section +is only 192 bytes per the Infiniband specification, so it's not +very useful for data anyway. This structure needs to remain small. + +This private data area 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. + +Currently there is only one capability in Version #1: dynamic page registration + +Finally: Negotiation happens with the Flags field: If the primary-VM +sets a flag, but the destination does not support this capability, it +will return a zero-bit for that flag and the primary-VM will understand +that as not being an available capability and will thus disable that +capability on the primary-VM side. + +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 use the protocol +describe above to deliver bytes without changing the upper-level +users of QEMUFile that depend on a bytestream 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 VM's 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, virtual +addresses 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" (hard-coded to 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 is pinned in memory on +both sides using the aforementioned protocol. +After pinning, an RDMA Write is generated and transmitted +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, 'ulimit -l' mlock() limits as well as cgroups swap limits + are not compatible with infinband memory pinning and will result in + an aborted migration (but with the source VM left unaffected). +2. Use of the recent /proc/<pid>/pagemap would likely speed up + the use of KSM and ballooning while using RDMA. +3. Also, some form of balloon-device usage tracking would also + help alleviate some issues. +4. Use LRU to provide more fine-grained direction of UNREGISTER + requests for unpinning memory in an overcommitted environment. +5. Expose UNREGISTER support to the user by way of workload-specific + hints about application behavior. |
