From patchwork Wed Apr 10 04:29:35 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: mrhines@linux.vnet.ibm.com X-Patchwork-Id: 235290 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id BFF302C0091 for ; Wed, 10 Apr 2013 14:41:02 +1000 (EST) Received: from localhost ([::1]:58936 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UPmiJ-0004CW-VC for incoming@patchwork.ozlabs.org; Wed, 10 Apr 2013 00:32:51 -0400 Received: from eggs.gnu.org ([208.118.235.92]:51727) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UPmfo-0000sE-IT for qemu-devel@nongnu.org; Wed, 10 Apr 2013 00:30:20 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UPmfk-0001Fi-Kq for qemu-devel@nongnu.org; Wed, 10 Apr 2013 00:30:16 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:41219) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UPmfk-0001Fe-9q for qemu-devel@nongnu.org; Wed, 10 Apr 2013 00:30:12 -0400 Received: from /spool/local by e37.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Hines" Verbose documentation is included, for both the protocol and interface to QEMU. Additionally, there is a Features/RDMALiveMigration wiki as well as a patch on github.com (hinesmr/qemu.git) Signed-off-by: Michael R. Hines --- docs/rdma.txt | 300 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 300 insertions(+) create mode 100644 docs/rdma.txt diff --git a/docs/rdma.txt b/docs/rdma.txt new file mode 100644 index 0000000..583836e --- /dev/null +++ b/docs/rdma.txt @@ -0,0 +1,300 @@ +Several changes since v5: + +- Only one new file in the patch now! (migration-rdma.c) +- Smaller number of files touched, fewer prototypes +- Merged files as requested (rdma.c and and migration-rdma.c) +- Eliminated header as requested (rdma.h) +- Created new function pointers for hooks in arch_init.c + to be cleaner and removed all explicit RDMA checks + to instead use QEMUFileOps + +Contents: +================================= +* Running +* RDMA Protocol Description +* Versioning +* QEMUFileRDMA Interface +* Migration of pc.ram +* Error handling +* TODO +* Performance + +RUNNING: +=============================== + +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_set_capability 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 + +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 / 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//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. + +PERFORMANCE +=================== + +Using a 40gbps infinband link performing a worst-case stress test: + +RDMA Throughput With $ stress --vm-bytes 1024M --vm 1 --vm-keep +Approximately 30 gpbs (little better than the paper) +1. Average worst-case throughput +TCP Throughput With $ stress --vm-bytes 1024M --vm 1 --vm-keep +2. Approximately 8 gpbs (using IPOIB IP over Infiniband) + +Average downtime (stop time) ranges between 28 and 33 milliseconds. + +An *exhaustive* paper (2010) shows additional performance details +linked on the QEMU wiki: + +http://wiki.qemu.org/Features/RDMALiveMigration