Re: [Qemu-devel] [RFC PATCH RDMA support v6: 2/7] documentation (docs/rdma.txt)

[Michael S. Tsirkin]

On Wed, Apr 10, 2013 at 12:29:35AM -0400, address@hidden wrote:
> From: "Michael R. Hines" <address@hidden>
> 
> 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 <address@hidden>


Ouch, I responded to v5. but this file seems unchanged, right?

> ---
>  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/<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.
> +
> +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
> -- 
> 1.7.10.4