Documentation/networking/scaling.rst

1 .. SPDX-License-Identifier: GPL-2.0
13 multi-processor systems.
17 - RSS: Receive Side Scaling
18 - RPS: Receive Packet Steering
19 - RFS: Receive Flow Steering
20 - Accelerated Receive Flow Steering
21 - XPS: Transmit Packet Steering
28 (multi-queue). On reception, a NIC can send different packets to different
33 generally known as “Receive-side Scaling” (RSS). The goal of RSS and
35 Multi-queue distribution can also be used for traffic prioritization, but
39 and/or transport layer headers-- for example, a 4-tuple hash over
41 implementation of RSS uses a 128-entry indirection table where each entry
48 destination address) and TCP/UDP (source port, destination port) tuples
51 both directions of the flow to land on the same Rx queue (and CPU). The
52 "Symmetric-XOR" is a type of RSS algorithms that achieves this hash
63 programmable filters. For example, webserver bound TCP port 80 packets
64 can be directed to their own receive queue. Such “n-tuple” filters can
65 be configured from ethtool (--config-ntuple).
69 -----------------
71 The driver for a multi-queue capable NIC typically provides a kernel
81 default mapping is to distribute the queues evenly in the table, but the
83 commands (--show-rxfh-indir and --set-rxfh-indir). Modifying the
93 signaling path for PCIe devices uses message signaled interrupts (MSI-X),
94 that can route each interrupt to a particular CPU. The active mapping
96 an IRQ may be handled on any CPU. Because a non-negligible part of packet
99 affinity of each interrupt see Documentation/core-api/irq/irq-affinity.rst. Some systems
111 NIC maximum, if lower). The most efficient high-rate configuration
117 Per-cpu load can be observed using the mpstat utility, but note that on
126 Modern NICs support creating multiple co-existing RSS configurations
129 traffic for e.g. a particular destination port or IP address.
130 The example below shows how to direct all traffic to TCP port 22
135   # ethtool -X eth0 hfunc toeplitz context new
142   # ethtool -x eth0 context 1
143   RX flow hash indirection table for eth0 with 13 RX ring(s):
147   # ethtool -X eth0 equal 2 context 1
148   # ethtool -x eth0 context 1
149   RX flow hash indirection table for eth0 with 13 RX ring(s):
154 To make use of the new context direct traffic to it using an n-tuple
157   # ethtool -N eth0 flow-type tcp6 dst-port 22 context 1
162   # ethtool -N eth0 delete 1023
163   # ethtool -X eth0 context 1 delete
180    introduce inter-processor interrupts (IPIs))
188 flow hash over the packet’s addresses or ports (2-tuple or 4-tuple hash
194 skb->hash and can be used elsewhere in the stack as a hash of the
209 -----------------
216   /sys/class/net/<dev>/queues/rx-<n>/rps_cpus
220 CPU. Documentation/core-api/irq/irq-affinity.rst explains how CPUs are assigned to
233 For a multi-queue system, if RSS is configured so that a hardware
241 --------------
244 reordering. The trade-off to sending all packets from the same flow
256 net.core.netdev_max_backlog), the kernel starts a per-flow packet
277 Per-flow rate is calculated by hashing each packet into a hashtable
278 bucket and incrementing a per-bucket counter. The hash function is
280 be much larger than the number of CPUs, flow limit has finer-grained
296 network rx interrupts (as set in /proc/irq/N/smp_affinity).
362   - The current CPU's queue head counter >= the recorded tail counter
364   - The current CPU is unset (>= nr_cpu_ids)
365   - The current CPU is offline
375 -----------------
383 The number of entries in the per-queue flow table are set through::
385   /sys/class/net/<dev>/queues/rx-<n>/rps_flow_cnt
400 For a multi-queue device, the rps_flow_cnt for each queue might be
410 Accelerated RFS is to RFS what RSS is to RPS: a hardware-accelerated load
427 is maintained by the NIC driver. This is an auto-generated reverse map of
435 -----------------------------
456 which transmit queue to use when transmitting a packet on a multi-queue
458 a mapping of CPU to hardware queue(s) or a mapping of receive queue(s)
463 The goal of this mapping is usually to assign queues
475 This mapping is used to pick transmit queue based on the receive
478 the common use case is a 1:1 mapping. This will enable sending packets
480 busy polling multi-threaded workloads where there are challenges in
487 the same queue-association that a given application is polling on. This
494 CPUs/receive-queues that may use that queue to transmit. The reverse
495 mapping, from CPUs to transmit queues or from receive-queues to transmit
499 for the socket connection for a match in the receive queue-to-transmit queue
501 running CPU as a key into the CPU-to-queue lookup table. If the
514 skb->ooo_okay is set for a packet in the flow. This flag indicates that
522 -----------------
526 how, XPS is configured at device init. The mapping of CPUs/receive-queues
531   /sys/class/net/<dev>/queues/tx-<n>/xps_cpus
533 For selection based on receive-queues map::
535   /sys/class/net/<dev>/queues/tx-<n>/xps_rxqs
542 has no effect, since there is no choice in this case. In a multi-queue
552 explicitly configured mapping receive-queue(s) to transmit queue(s). If the
553 user configuration for receive-queue map does not apply, then the transmit
560 These are rate-limitation mechanisms implemented by HW, where currently
561 a max-rate attribute is supported, by setting a Mbps value to::
563   /sys/class/net/<dev>/queues/tx-<n>/tx_maxrate
579 - Tom Herbert (therbert@google.com)
580 - Willem de Bruijn (willemb@google.com)