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rdma: update documentation to reflect new unpin support

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As requested, the protocol now includes memory unpinning support.
This has been implemented in a non-optimized manner, in such a way
that one could devise an LRU or other workload-specific information
on top of the basic mechanism to influence the way unpinning happens
during runtime.

The feature is not yet user-facing, and is thus can only be enabled
at compile-time.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
Michael R. Hines 2013-07-22 10:01:51 -04:00 committed by Juan Quintela
parent 3464700f6a
commit a5f56b906e
1 changed files with 30 additions and 21 deletions
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51
docs/rdma.txt
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@ -35,7 +35,7 @@ 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 Convered Ethernet) as well as Infiniband-based. This implementation of
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.
@ -188,9 +188,9 @@ 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)
* 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
@ -202,17 +202,19 @@ 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 10 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)
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
@ -243,7 +245,7 @@ qemu_rdma_exchange_send(header, data, optional response header & data):
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
(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
@ -293,8 +295,10 @@ 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
* 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
@ -313,7 +317,7 @@ 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
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
@ -326,8 +330,8 @@ 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)
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
@ -413,3 +417,8 @@ TODO:
the use of KSM and ballooning while using RDMA.
4. Also, some form of balloon-device usage tracking would also
help alleviate some issues.
5. Move UNREGISTER requests to a separate thread.
6. Use LRU to provide more fine-grained direction of UNREGISTER
requests for unpinning memory in an overcommitted environment.
7. Expose UNREGISTER support to the user by way of workload-specific
hints about application behavior.