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Omni-Path TID RDMA Feature
Intel Omni-Path (OPA) TID RDMA support is a feature that accelerates
data movement between two OPA nodes through the IB Verbs interface. It
improves RDMA READ/WRITE performance by delivering the data payload to a
user buffer directly without any software copying.
Architecture
=============
The TID RDMA protocol is implemented on the hfi1 driver level and is
therefore transparent to the ULPs. It is designed to facilitate the data
transactions for two specific RDMA requests:
- RDMA READ;
- RDMA WRITE.
Previously, when a verbs data packet is received at the destination
(requester side for RDMA READ and responder side for RDMA WRITE), the
data payload is copied to the user buffer by software, which slows down
the performance significantly for large requests.
Internally, hfi1 converts qualified RDMA READ/WRITE requests into TID
RDMA READ/WRITE requests when the requests are post sent to the hfi1
driver. Non-qualified RDMA requests are handled by normal RDMA protocol.
For TID RDMA requests, hardware resources (hardware flow and TID entries)
are allocated on the destination side (the requester side for TID RDMA
READ and the responder side for TID RDMA WRITE). The information for
these resources is conveyed to the data source side (the responder side
for TID RDMA READ and the requester side for TID RDMA WRITE) and embedded
in data packets. When data packets are received by the destination,
hardware will deliver the data payload to the destination buffer without
involving software and therefore improve the performance.
Details
=======
RDMA READ/WRITE requests are qualified by the following:
- Total data length >= 256k;
- Totoal data length is a multiple of 4K pages.
Additional qualifications are enforced for the destination buffers:
For RDMA RAED:
- Each destination sge buffer is 4K aligned;
- Each destination sge buffer is a multiple of 4K pages.
For RDMA WRITE:
- The destination number is 4K aligned.
In addition, in an OPA fabric, some nodes may support TID RDMA while
others may not. As such, it is important for two transaction nodes to
exchange the information about the features they support. This discovery
mechanism is called OPA Feature Negotion (OPFN) and is described in
details in the patch series. Through OPFN, two nodes can find whether
they both support TID RDMA and subsequently convert RDMA requests into
TID RDMA requests.
* hfi1-tid: (46 commits)
IB/hfi1: Prioritize the sending of ACK packets
IB/hfi1: Add static trace for TID RDMA WRITE protocol
IB/hfi1: Enable TID RDMA WRITE protocol
IB/hfi1: Add interlock between TID RDMA WRITE and other requests
IB/hfi1: Add TID RDMA WRITE functionality into RDMA verbs
IB/hfi1: Add the dual leg code
IB/hfi1: Add the TID second leg ACK packet builder
IB/hfi1: Add the TID second leg send packet builder
IB/hfi1: Resend the TID RDMA WRITE DATA packets
IB/hfi1: Add a function to receive TID RDMA RESYNC packet
IB/hfi1: Add a function to build TID RDMA RESYNC packet
IB/hfi1: Add TID RDMA retry timer
IB/hfi1: Add a function to receive TID RDMA ACK packet
IB/hfi1: Add a function to build TID RDMA ACK packet
IB/hfi1: Add a function to receive TID RDMA WRITE DATA packet
IB/hfi1: Add a function to build TID RDMA WRITE DATA packet
IB/hfi1: Add a function to receive TID RDMA WRITE response
IB/hfi1: Add TID resource timer
IB/hfi1: Add a function to build TID RDMA WRITE response
IB/hfi1: Add functions to receive TID RDMA WRITE request
...
Signed-off-by: Doug Ledford <dledford@redhat.com>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.