103 lines
4.1 KiB
Plaintext
103 lines
4.1 KiB
Plaintext
IP OVER INFINIBAND
|
|
|
|
The ib_ipoib driver is an implementation of the IP over InfiniBand
|
|
protocol as specified by RFC 4391 and 4392, issued by the IETF ipoib
|
|
working group. It is a "native" implementation in the sense of
|
|
setting the interface type to ARPHRD_INFINIBAND and the hardware
|
|
address length to 20 (earlier proprietary implementations
|
|
masqueraded to the kernel as ethernet interfaces).
|
|
|
|
Partitions and P_Keys
|
|
|
|
When the IPoIB driver is loaded, it creates one interface for each
|
|
port using the P_Key at index 0. To create an interface with a
|
|
different P_Key, write the desired P_Key into the main interface's
|
|
/sys/class/net/<intf name>/create_child file. For example:
|
|
|
|
echo 0x8001 > /sys/class/net/ib0/create_child
|
|
|
|
This will create an interface named ib0.8001 with P_Key 0x8001. To
|
|
remove a subinterface, use the "delete_child" file:
|
|
|
|
echo 0x8001 > /sys/class/net/ib0/delete_child
|
|
|
|
The P_Key for any interface is given by the "pkey" file, and the
|
|
main interface for a subinterface is in "parent."
|
|
|
|
Datagram vs Connected modes
|
|
|
|
The IPoIB driver supports two modes of operation: datagram and
|
|
connected. The mode is set and read through an interface's
|
|
/sys/class/net/<intf name>/mode file.
|
|
|
|
In datagram mode, the IB UD (Unreliable Datagram) transport is used
|
|
and so the interface MTU has is equal to the IB L2 MTU minus the
|
|
IPoIB encapsulation header (4 bytes). For example, in a typical IB
|
|
fabric with a 2K MTU, the IPoIB MTU will be 2048 - 4 = 2044 bytes.
|
|
|
|
In connected mode, the IB RC (Reliable Connected) transport is used.
|
|
Connected mode takes advantage of the connected nature of the IB
|
|
transport and allows an MTU up to the maximal IP packet size of 64K,
|
|
which reduces the number of IP packets needed for handling large UDP
|
|
datagrams, TCP segments, etc and increases the performance for large
|
|
messages.
|
|
|
|
In connected mode, the interface's UD QP is still used for multicast
|
|
and communication with peers that don't support connected mode. In
|
|
this case, RX emulation of ICMP PMTU packets is used to cause the
|
|
networking stack to use the smaller UD MTU for these neighbours.
|
|
|
|
Stateless offloads
|
|
|
|
If the IB HW supports IPoIB stateless offloads, IPoIB advertises
|
|
TCP/IP checksum and/or Large Send (LSO) offloading capability to the
|
|
network stack.
|
|
|
|
Large Receive (LRO) offloading is also implemented and may be turned
|
|
on/off using ethtool calls. Currently LRO is supported only for
|
|
checksum offload capable devices.
|
|
|
|
Stateless offloads are supported only in datagram mode.
|
|
|
|
Interrupt moderation
|
|
|
|
If the underlying IB device supports CQ event moderation, one can
|
|
use ethtool to set interrupt mitigation parameters and thus reduce
|
|
the overhead incurred by handling interrupts. The main code path of
|
|
IPoIB doesn't use events for TX completion signaling so only RX
|
|
moderation is supported.
|
|
|
|
Debugging Information
|
|
|
|
By compiling the IPoIB driver with CONFIG_INFINIBAND_IPOIB_DEBUG set
|
|
to 'y', tracing messages are compiled into the driver. They are
|
|
turned on by setting the module parameters debug_level and
|
|
mcast_debug_level to 1. These parameters can be controlled at
|
|
runtime through files in /sys/module/ib_ipoib/.
|
|
|
|
CONFIG_INFINIBAND_IPOIB_DEBUG also enables files in the debugfs
|
|
virtual filesystem. By mounting this filesystem, for example with
|
|
|
|
mount -t debugfs none /sys/kernel/debug
|
|
|
|
it is possible to get statistics about multicast groups from the
|
|
files /sys/kernel/debug/ipoib/ib0_mcg and so on.
|
|
|
|
The performance impact of this option is negligible, so it
|
|
is safe to enable this option with debug_level set to 0 for normal
|
|
operation.
|
|
|
|
CONFIG_INFINIBAND_IPOIB_DEBUG_DATA enables even more debug output in
|
|
the data path when data_debug_level is set to 1. However, even with
|
|
the output disabled, enabling this configuration option will affect
|
|
performance, because it adds tests to the fast path.
|
|
|
|
References
|
|
|
|
Transmission of IP over InfiniBand (IPoIB) (RFC 4391)
|
|
http://ietf.org/rfc/rfc4391.txt
|
|
IP over InfiniBand (IPoIB) Architecture (RFC 4392)
|
|
http://ietf.org/rfc/rfc4392.txt
|
|
IP over InfiniBand: Connected Mode (RFC 4755)
|
|
http://ietf.org/rfc/rfc4755.txt
|