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
52 "Symmetric-XOR" and "Symmetric-OR-XOR" are types of RSS algorithms that
57 Specifically, the "Symmetric-XOR" algorithm XORs the input
62 The "Symmetric-OR-XOR" algorithm, on the other hand, transforms the input as
71 can be directed to their own receive queue. Such “n-tuple” filters can
72 be configured from ethtool (--config-ntuple).
76 -----------------
78 The driver for a multi-queue capable NIC typically provides a kernel
90 commands (--show-rxfh-indir and --set-rxfh-indir). Modifying the
100 signaling path for PCIe devices uses message signaled interrupts (MSI-X),
103 an IRQ may be handled on any CPU. Because a non-negligible part of packet
106 affinity of each interrupt see Documentation/core-api/irq/irq-affinity.rst. Some systems
118 NIC maximum, if lower). The most efficient high-rate configuration
124 Per-cpu load can be observed using the mpstat utility, but note that on
133 Modern NICs support creating multiple co-existing RSS configurations
142   # ethtool -X eth0 hfunc toeplitz context new
149   # ethtool -x eth0 context 1
154   # ethtool -X eth0 equal 2 context 1
155   # ethtool -x eth0 context 1
161 To make use of the new context direct traffic to it using an n-tuple
164   # ethtool -N eth0 flow-type tcp6 dst-port 22 context 1
169   # ethtool -N eth0 delete 1023
170   # ethtool -X eth0 context 1 delete
187    introduce inter-processor interrupts (IPIs))
195 flow hash over the packet’s addresses or ports (2-tuple or 4-tuple hash
201 skb->hash and can be used elsewhere in the stack as a hash of the
216 -----------------
223   /sys/class/net/<dev>/queues/rx-<n>/rps_cpus
227 CPU. Documentation/core-api/irq/irq-affinity.rst explains how CPUs are assigned to
236 CPU. If NUMA locality is not an issue, this could also be all CPUs in
240 For a multi-queue system, if RSS is configured so that a hardware
248 --------------
251 reordering. The trade-off to sending all packets from the same flow
263 net.core.netdev_max_backlog), the kernel starts a per-flow packet
284 Per-flow rate is calculated by hashing each packet into a hashtable
285 bucket and incrementing a per-bucket counter. The hash function is
287 be much larger than the number of CPUs, flow limit has finer-grained
316 application locality. This is accomplished by Receive Flow Steering
342 for each flow: rps_dev_flow_table is a table specific to each hardware
369   - The current CPU's queue head counter >= the recorded tail counter
371   - The current CPU is unset (>= nr_cpu_ids)
372   - The current CPU is offline
382 -----------------
390 The number of entries in the per-queue flow table are set through::
392   /sys/class/net/<dev>/queues/rx-<n>/rps_flow_cnt
407 For a multi-queue device, the rps_flow_cnt for each queue might be
417 Accelerated RFS is to RFS what RSS is to RPS: a hardware-accelerated load
429 rps_dev_flow_table is updated. The driver in turn uses a device specific
434 is maintained by the NIC driver. This is an auto-generated reverse map of
440 closest in cache locality.
444 -----------------------------
465 which transmit queue to use when transmitting a packet on a multi-queue
489 busy polling multi-threaded workloads where there are challenges in
496 the same queue-association that a given application is polling on. This
503 CPUs/receive-queues that may use that queue to transmit. The reverse
504 mapping, from CPUs to transmit queues or from receive-queues to transmit
508 for the socket connection for a match in the receive queue-to-transmit queue
510 running CPU as a key into the CPU-to-queue lookup table. If the
523 skb->ooo_okay is set for a packet in the flow. This flag indicates that
531 -----------------
535 how, XPS is configured at device init. The mapping of CPUs/receive-queues
540   /sys/class/net/<dev>/queues/tx-<n>/xps_cpus
542 For selection based on receive-queues map::
544   /sys/class/net/<dev>/queues/tx-<n>/xps_rxqs
551 has no effect, since there is no choice in this case. In a multi-queue
561 explicitly configured mapping receive-queue(s) to transmit queue(s). If the
562 user configuration for receive-queue map does not apply, then the transmit
569 These are rate-limitation mechanisms implemented by HW, where currently
570 a max-rate attribute is supported, by setting a Mbps value to::
572   /sys/class/net/<dev>/queues/tx-<n>/tx_maxrate
588 - Tom Herbert (therbert@google.com)
589 - Willem de Bruijn (willemb@google.com)