Re: [Qemu-devel] [PULL v4 11/11] rdma: add documentation

[Michael S. Tsirkin]

On Wed, Apr 17, 2013 at 07:07:20PM -0400, address@hidden wrote:
> From: "Michael R. Hines" <address@hidden>
> 
> docs/rdma.txt contains full documentation,
> wiki links, github url and contact information.
> 
> Signed-off-by: Michael R. Hines <address@hidden>

OK that's better. Need to improve the following areas:
- fix half-sentences such as 'faster' (without saying than what)
- document tradeoffs
- better document how to run
- add more examples

comments below.
However, they are not new: I commented on most of these
things already. Please don't ignore review comments.

> ---
>  docs/rdma.txt |  360 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 360 insertions(+)
>  create mode 100644 docs/rdma.txt
> 
> diff --git a/docs/rdma.txt b/docs/rdma.txt
> new file mode 100644
> index 0000000..7f29e79
> --- /dev/null
> +++ b/docs/rdma.txt
> @@ -0,0 +1,360 @@
> +(RDMA: Remote Direct Memory Access)
> +RDMA Live Migration Specification, Version # 1
> +==============================================
> +Wiki: http://wiki.qemu.org/Features/RDMALiveMigration
> +Github: address@hidden:hinesmr/qemu.git, 'rdma' branch
> +
> +Copyright (C) 2013 Michael R. Hines <address@hidden>
> +
> +An *exhaustive* paper (2010) shows additional performance details
> +linked on the QEMU wiki above.
> +
> +Contents:
> +=========
> +* Before running 
> +* Running
> +* RDMA Migration Protocol Description
> +* Versioning and Capabilities
> +* QEMUFileRDMA Interface
> +* Migration of pc.ram
> +* Error handling
> +* TODO
> +* Performance
> +
> +BEFORE RUNNING:
> +===============
> +
> +RDMA helps make your migration more deterministic under heavy load because 
> +of the significantly lower latency and higher throughput

Higher and lower than what? Above is not helpful and subtly wrong. Say instead

'On infiniband networks, RDMA can achieve lower latency and higher
throughput than IP over infiniband based networking by reducing the
amount of interrupts and data copies and bypassing the host networking
stack. Using RDMA for VM migration makes migration more deterministic
under heavy VM load'.

And add an example what 'more deterministic' means.


> provided by infiniband.

Does this works on top of other RDMA transports or just infiniband?
Needs clarification.

> +
> +Use of RDMA during migration requires pinning and registering memory 
> +with the hardware. This means that memory must be resident in memory 
> +before the hardware can transmit that memory to another machine.

Above is too vague to be of real use. Please insert here the
implications on host versus total VMs memory size.
Also add some examples.

> +If this is not acceptable for your application or product, 
> +then the use of RDMA migration is strongly discouraged and you
> +should revert back to standard TCP-based migration.

Above is not helpful and will just lead to more questions.
Remove.

> +
> +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,

This is wrong, isn't it? You only skip zero pages, so any page
that has data, even if it's not in active use, will be pinned.

> then disabling this feature will cause all 8GB to 
> +be pinned and resident in memory.

Add as opposed to the default behaviour which is .... 

> This feature mostly affects the
> +bulk-phase round of the migration and can be disabled for extremely
> +high-performance RDMA hardware

Above is meaningless, it does not help user to know whether her hardware
is "extremely high-performance". Put numbers here please.
Does it help 40G cards but not 20g ones? By how much?

> using the following command:
> +
> +QEMU Monitor Command:
> +$ migrate_set_capability x-chunk-register-destination off # enabled 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.


Please add some example numbers so people know what the tradeoff is.

> +
> +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 x-rdma:host:port
> +
> +Finally, perform the actual migration:
> +
> +QEMU Monitor Command:
> +$ migrate -d x-rdma:host:port
> +

Note users stop reading here, below is info for developers.
So please add here the requirement to do ulimit and with what value.
Also add an example with VM size.

> +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 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)
> +    * 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 emperical
> +observations on the maximum future benefit of simultaneous page 
> registrations.
> +
> +The 'type' field has 9 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        (used right after connection setup)
> +    6. Compress page     (zap zero page and skip registration)
> +    7. Register request  (dynamic chunk registration)
> +    8. Register result   ('rkey' to be used by sender)
> +    9. 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 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 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, 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. 'migrate x-rdma:host:port' and '-incoming x-rdma' options will be
> +   renamed to 'rdma' after the experimental phase of this work has
> +   completed upstream.
> +2. 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).
> +3. Use of the recent /proc/<pid>/pagemap would likely speed up
> +   the use of KSM and ballooning while using RDMA.

For KSM you'll need the _GIFT patch for this I think, maybe note this.

> +4. Also, some form of balloon-device usage tracking would also
> +   help alleviate some 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 26 gpbs
> +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)
> +3. Using chunked registration: approximately 6 gbps.
> +
> +Average downtime (stop time) ranges between 15 and 33 milliseconds.
> -- 
> 1.7.10.4