Scaling in the Linux Networking Stack
  
  
  Introduction
  ============
  
  This document describes a set of complementary techniques in the Linux
  networking stack to increase parallelism and improve performance for
  multi-processor systems.
  
  The following technologies are described:
  
    RSS: Receive Side Scaling
    RPS: Receive Packet Steering
    RFS: Receive Flow Steering
    Accelerated Receive Flow Steering
    XPS: Transmit Packet Steering
  
  
  RSS: Receive Side Scaling
  =========================
  
  Contemporary NICs support multiple receive and transmit descriptor queues
  (multi-queue). On reception, a NIC can send different packets to different
  queues to distribute processing among CPUs. The NIC distributes packets by
  applying a filter to each packet that assigns it to one of a small number
  of logical flows. Packets for each flow are steered to a separate receive
  queue, which in turn can be processed by separate CPUs. This mechanism is
  generally known as “Receive-side Scaling” (RSS). The goal of RSS and

  the other scaling techniques is to increase performance uniformly.

  Multi-queue distribution can also be used for traffic prioritization, but
  that is not the focus of these techniques.
  
  The filter used in RSS is typically a hash function over the network
  and/or transport layer headers-- for example, a 4-tuple hash over
  IP addresses and TCP ports of a packet. The most common hardware
  implementation of RSS uses a 128-entry indirection table where each entry
  stores a queue number. The receive queue for a packet is determined
  by masking out the low order seven bits of the computed hash for the
  packet (usually a Toeplitz hash), taking this number as a key into the
  indirection table and reading the corresponding value.
  
  Some advanced NICs allow steering packets to queues based on
  programmable filters. For example, webserver bound TCP port 80 packets
  can be directed to their own receive queue. Such “n-tuple” filters can
  be configured from ethtool (--config-ntuple).
  
  ==== RSS Configuration
  
  The driver for a multi-queue capable NIC typically provides a kernel
  module parameter for specifying the number of hardware queues to
  configure. In the bnx2x driver, for instance, this parameter is called
  num_queues. A typical RSS configuration would be to have one receive queue
  for each CPU if the device supports enough queues, or otherwise at least

  one for each memory domain, where a memory domain is a set of CPUs that
  share a particular memory level (L1, L2, NUMA node, etc.).

  
  The indirection table of an RSS device, which resolves a queue by masked
  hash, is usually programmed by the driver at initialization. The
  default mapping is to distribute the queues evenly in the table, but the
  indirection table can be retrieved and modified at runtime using ethtool
  commands (--show-rxfh-indir and --set-rxfh-indir). Modifying the
  indirection table could be done to give different queues different
  relative weights.
  
  == RSS IRQ Configuration
  
  Each receive queue has a separate IRQ associated with it. The NIC triggers
  this to notify a CPU when new packets arrive on the given queue. The
  signaling path for PCIe devices uses message signaled interrupts (MSI-X),
  that can route each interrupt to a particular CPU. The active mapping
  of queues to IRQs can be determined from /proc/interrupts. By default,
  an IRQ may be handled on any CPU. Because a non-negligible part of packet
  processing takes place in receive interrupt handling, it is advantageous
  to spread receive interrupts between CPUs. To manually adjust the IRQ

  affinity of each interrupt see Documentation/IRQ-affinity.txt. Some systems

  will be running irqbalance, a daemon that dynamically optimizes IRQ
  assignments and as a result may override any manual settings.
  
  == Suggested Configuration
  
  RSS should be enabled when latency is a concern or whenever receive
  interrupt processing forms a bottleneck. Spreading load between CPUs
  decreases queue length. For low latency networking, the optimal setting
  is to allocate as many queues as there are CPUs in the system (or the

  NIC maximum, if lower). The most efficient high-rate configuration

  is likely the one with the smallest number of receive queues where no

  receive queue overflows due to a saturated CPU, because in default
  mode with interrupt coalescing enabled, the aggregate number of
  interrupts (and thus work) grows with each additional queue.
  
  Per-cpu load can be observed using the mpstat utility, but note that on
  processors with hyperthreading (HT), each hyperthread is represented as
  a separate CPU. For interrupt handling, HT has shown no benefit in
  initial tests, so limit the number of queues to the number of CPU cores
  in the system.

  
  
  RPS: Receive Packet Steering
  ============================
  
  Receive Packet Steering (RPS) is logically a software implementation of
  RSS. Being in software, it is necessarily called later in the datapath.
  Whereas RSS selects the queue and hence CPU that will run the hardware
  interrupt handler, RPS selects the CPU to perform protocol processing
  above the interrupt handler. This is accomplished by placing the packet
  on the desired CPU’s backlog queue and waking up the CPU for processing.
  RPS has some advantages over RSS: 1) it can be used with any NIC,
  2) software filters can easily be added to hash over new protocols,
  3) it does not increase hardware device interrupt rate (although it does
  introduce inter-processor interrupts (IPIs)).
  
  RPS is called during bottom half of the receive interrupt handler, when
  a driver sends a packet up the network stack with netif_rx() or
  netif_receive_skb(). These call the get_rps_cpu() function, which
  selects the queue that should process a packet.
  
  The first step in determining the target CPU for RPS is to calculate a
  flow hash over the packet’s addresses or ports (2-tuple or 4-tuple hash
  depending on the protocol). This serves as a consistent hash of the
  associated flow of the packet. The hash is either provided by hardware
  or will be computed in the stack. Capable hardware can pass the hash in
  the receive descriptor for the packet; this would usually be the same
  hash used for RSS (e.g. computed Toeplitz hash). The hash is saved in
  skb->rx_hash and can be used elsewhere in the stack as a hash of the
  packet’s flow.
  
  Each receive hardware queue has an associated list of CPUs to which
  RPS may enqueue packets for processing. For each received packet,
  an index into the list is computed from the flow hash modulo the size
  of the list. The indexed CPU is the target for processing the packet,
  and the packet is queued to the tail of that CPU’s backlog queue. At
  the end of the bottom half routine, IPIs are sent to any CPUs for which
  packets have been queued to their backlog queue. The IPI wakes backlog
  processing on the remote CPU, and any queued packets are then processed
  up the networking stack.
  
  ==== RPS Configuration
  
  RPS requires a kernel compiled with the CONFIG_RPS kconfig symbol (on
  by default for SMP). Even when compiled in, RPS remains disabled until
  explicitly configured. The list of CPUs to which RPS may forward traffic
  can be configured for each receive queue using a sysfs file entry:
  
   /sys/class/net/<dev>/queues/rx-<n>/rps_cpus
  
  This file implements a bitmap of CPUs. RPS is disabled when it is zero
  (the default), in which case packets are processed on the interrupting
  CPU. Documentation/IRQ-affinity.txt explains how CPUs are assigned to
  the bitmap.
  
  == Suggested Configuration
  
  For a single queue device, a typical RPS configuration would be to set

  the rps_cpus to the CPUs in the same memory domain of the interrupting

  CPU. If NUMA locality is not an issue, this could also be all CPUs in
  the system. At high interrupt rate, it might be wise to exclude the
  interrupting CPU from the map since that already performs much work.
  
  For a multi-queue system, if RSS is configured so that a hardware
  receive queue is mapped to each CPU, then RPS is probably redundant
  and unnecessary. If there are fewer hardware queues than CPUs, then
  RPS might be beneficial if the rps_cpus for each queue are the ones that

  share the same memory domain as the interrupting CPU for that queue.

  ==== RPS Flow Limit
  
  RPS scales kernel receive processing across CPUs without introducing
  reordering. The trade-off to sending all packets from the same flow
  to the same CPU is CPU load imbalance if flows vary in packet rate.
  In the extreme case a single flow dominates traffic. Especially on
  common server workloads with many concurrent connections, such
  behavior indicates a problem such as a misconfiguration or spoofed
  source Denial of Service attack.
  
  Flow Limit is an optional RPS feature that prioritizes small flows
  during CPU contention by dropping packets from large flows slightly
  ahead of those from small flows. It is active only when an RPS or RFS
  destination CPU approaches saturation.  Once a CPU's input packet
  queue exceeds half the maximum queue length (as set by sysctl
  net.core.netdev_max_backlog), the kernel starts a per-flow packet
  count over the last 256 packets. If a flow exceeds a set ratio (by
  default, half) of these packets when a new packet arrives, then the
  new packet is dropped. Packets from other flows are still only
  dropped once the input packet queue reaches netdev_max_backlog.
  No packets are dropped when the input packet queue length is below
  the threshold, so flow limit does not sever connections outright:
  even large flows maintain connectivity.
  
  == Interface
  
  Flow limit is compiled in by default (CONFIG_NET_FLOW_LIMIT), but not
  turned on. It is implemented for each CPU independently (to avoid lock
  and cache contention) and toggled per CPU by setting the relevant bit
  in sysctl net.core.flow_limit_cpu_bitmap. It exposes the same CPU
  bitmap interface as rps_cpus (see above) when called from procfs:
  
   /proc/sys/net/core/flow_limit_cpu_bitmap
  
  Per-flow rate is calculated by hashing each packet into a hashtable
  bucket and incrementing a per-bucket counter. The hash function is
  the same that selects a CPU in RPS, but as the number of buckets can
  be much larger than the number of CPUs, flow limit has finer-grained
  identification of large flows and fewer false positives. The default
  table has 4096 buckets. This value can be modified through sysctl
  
   net.core.flow_limit_table_len
  
  The value is only consulted when a new table is allocated. Modifying
  it does not update active tables.
  
  == Suggested Configuration
  
  Flow limit is useful on systems with many concurrent connections,
  where a single connection taking up 50% of a CPU indicates a problem.
  In such environments, enable the feature on all CPUs that handle
  network rx interrupts (as set in /proc/irq/N/smp_affinity).
  
  The feature depends on the input packet queue length to exceed
  the flow limit threshold (50%) + the flow history length (256).
  Setting net.core.netdev_max_backlog to either 1000 or 10000
  performed well in experiments.

  
  RFS: Receive Flow Steering
  ==========================
  
  While RPS steers packets solely based on hash, and thus generally
  provides good load distribution, it does not take into account
  application locality. This is accomplished by Receive Flow Steering
  (RFS). The goal of RFS is to increase datacache hitrate by steering
  kernel processing of packets to the CPU where the application thread
  consuming the packet is running. RFS relies on the same RPS mechanisms
  to enqueue packets onto the backlog of another CPU and to wake up that
  CPU.
  
  In RFS, packets are not forwarded directly by the value of their hash,
  but the hash is used as index into a flow lookup table. This table maps
  flows to the CPUs where those flows are being processed. The flow hash
  (see RPS section above) is used to calculate the index into this table.
  The CPU recorded in each entry is the one which last processed the flow.
  If an entry does not hold a valid CPU, then packets mapped to that entry
  are steered using plain RPS. Multiple table entries may point to the
  same CPU. Indeed, with many flows and few CPUs, it is very likely that
  a single application thread handles flows with many different flow hashes.

  rps_sock_flow_table is a global flow table that contains the *desired* CPU
  for flows: the CPU that is currently processing the flow in userspace.
  Each table value is a CPU index that is updated during calls to recvmsg
  and sendmsg (specifically, inet_recvmsg(), inet_sendmsg(), inet_sendpage()

  and tcp_splice_read()).
  
  When the scheduler moves a thread to a new CPU while it has outstanding
  receive packets on the old CPU, packets may arrive out of order. To
  avoid this, RFS uses a second flow table to track outstanding packets
  for each flow: rps_dev_flow_table is a table specific to each hardware
  receive queue of each device. Each table value stores a CPU index and a
  counter. The CPU index represents the *current* CPU onto which packets
  for this flow are enqueued for further kernel processing. Ideally, kernel
  and userspace processing occur on the same CPU, and hence the CPU index
  in both tables is identical. This is likely false if the scheduler has
  recently migrated a userspace thread while the kernel still has packets
  enqueued for kernel processing on the old CPU.
  
  The counter in rps_dev_flow_table values records the length of the current
  CPU's backlog when a packet in this flow was last enqueued. Each backlog
  queue has a head counter that is incremented on dequeue. A tail counter
  is computed as head counter + queue length. In other words, the counter

  in rps_dev_flow[i] records the last element in flow i that has

  been enqueued onto the currently designated CPU for flow i (of course,
  entry i is actually selected by hash and multiple flows may hash to the
  same entry i).
  
  And now the trick for avoiding out of order packets: when selecting the
  CPU for packet processing (from get_rps_cpu()) the rps_sock_flow table
  and the rps_dev_flow table of the queue that the packet was received on
  are compared. If the desired CPU for the flow (found in the
  rps_sock_flow table) matches the current CPU (found in the rps_dev_flow
  table), the packet is enqueued onto that CPU’s backlog. If they differ,
  the current CPU is updated to match the desired CPU if one of the
  following is true:
  
  - The current CPU's queue head counter >= the recorded tail counter
    value in rps_dev_flow[i]

  - The current CPU is unset (equal to RPS_NO_CPU)

  - The current CPU is offline
  
  After this check, the packet is sent to the (possibly updated) current
  CPU. These rules aim to ensure that a flow only moves to a new CPU when
  there are no packets outstanding on the old CPU, as the outstanding
  packets could arrive later than those about to be processed on the new
  CPU.
  
  ==== RFS Configuration

  RFS is only available if the kconfig symbol CONFIG_RPS is enabled (on

  by default for SMP). The functionality remains disabled until explicitly
  configured. The number of entries in the global flow table is set through:
  
   /proc/sys/net/core/rps_sock_flow_entries
  
  The number of entries in the per-queue flow table are set through:

   /sys/class/net/<dev>/queues/rx-<n>/rps_flow_cnt

  
  == Suggested Configuration
  
  Both of these need to be set before RFS is enabled for a receive queue.
  Values for both are rounded up to the nearest power of two. The
  suggested flow count depends on the expected number of active connections
  at any given time, which may be significantly less than the number of open
  connections. We have found that a value of 32768 for rps_sock_flow_entries
  works fairly well on a moderately loaded server.
  
  For a single queue device, the rps_flow_cnt value for the single queue
  would normally be configured to the same value as rps_sock_flow_entries.
  For a multi-queue device, the rps_flow_cnt for each queue might be
  configured as rps_sock_flow_entries / N, where N is the number of

  queues. So for instance, if rps_sock_flow_entries is set to 32768 and there

  are 16 configured receive queues, rps_flow_cnt for each queue might be
  configured as 2048.
  
  
  Accelerated RFS
  ===============
  
  Accelerated RFS is to RFS what RSS is to RPS: a hardware-accelerated load
  balancing mechanism that uses soft state to steer flows based on where
  the application thread consuming the packets of each flow is running.
  Accelerated RFS should perform better than RFS since packets are sent
  directly to a CPU local to the thread consuming the data. The target CPU
  will either be the same CPU where the application runs, or at least a CPU
  which is local to the application thread’s CPU in the cache hierarchy.
  
  To enable accelerated RFS, the networking stack calls the
  ndo_rx_flow_steer driver function to communicate the desired hardware
  queue for packets matching a particular flow. The network stack
  automatically calls this function every time a flow entry in
  rps_dev_flow_table is updated. The driver in turn uses a device specific
  method to program the NIC to steer the packets.
  
  The hardware queue for a flow is derived from the CPU recorded in
  rps_dev_flow_table. The stack consults a CPU to hardware queue map which
  is maintained by the NIC driver. This is an auto-generated reverse map of
  the IRQ affinity table shown by /proc/interrupts. Drivers can use
  functions in the cpu_rmap (“CPU affinity reverse map”) kernel library
  to populate the map. For each CPU, the corresponding queue in the map is
  set to be one whose processing CPU is closest in cache locality.
  
  ==== Accelerated RFS Configuration
  
  Accelerated RFS is only available if the kernel is compiled with
  CONFIG_RFS_ACCEL and support is provided by the NIC device and driver.
  It also requires that ntuple filtering is enabled via ethtool. The map
  of CPU to queues is automatically deduced from the IRQ affinities
  configured for each receive queue by the driver, so no additional
  configuration should be necessary.
  
  == Suggested Configuration
  
  This technique should be enabled whenever one wants to use RFS and the
  NIC supports hardware acceleration.
  
  XPS: Transmit Packet Steering
  =============================
  
  Transmit Packet Steering is a mechanism for intelligently selecting
  which transmit queue to use when transmitting a packet on a multi-queue
  device. To accomplish this, a mapping from CPU to hardware queue(s) is
  recorded. The goal of this mapping is usually to assign queues
  exclusively to a subset of CPUs, where the transmit completions for
  these queues are processed on a CPU within this set. This choice
  provides two benefits. First, contention on the device queue lock is
  significantly reduced since fewer CPUs contend for the same queue
  (contention can be eliminated completely if each CPU has its own
  transmit queue). Secondly, cache miss rate on transmit completion is
  reduced, in particular for data cache lines that hold the sk_buff
  structures.
  
  XPS is configured per transmit queue by setting a bitmap of CPUs that
  may use that queue to transmit. The reverse mapping, from CPUs to
  transmit queues, is computed and maintained for each network device.
  When transmitting the first packet in a flow, the function
  get_xps_queue() is called to select a queue. This function uses the ID
  of the running CPU as a key into the CPU-to-queue lookup table. If the
  ID matches a single queue, that is used for transmission. If multiple
  queues match, one is selected by using the flow hash to compute an index
  into the set.
  
  The queue chosen for transmitting a particular flow is saved in the
  corresponding socket structure for the flow (e.g. a TCP connection).
  This transmit queue is used for subsequent packets sent on the flow to
  prevent out of order (ooo) packets. The choice also amortizes the cost

  of calling get_xps_queues() over all packets in the flow. To avoid

  ooo packets, the queue for a flow can subsequently only be changed if
  skb->ooo_okay is set for a packet in the flow. This flag indicates that
  there are no outstanding packets in the flow, so the transmit queue can
  change without the risk of generating out of order packets. The
  transport layer is responsible for setting ooo_okay appropriately. TCP,
  for instance, sets the flag when all data for a connection has been
  acknowledged.
  
  ==== XPS Configuration
  
  XPS is only available if the kconfig symbol CONFIG_XPS is enabled (on by
  default for SMP). The functionality remains disabled until explicitly
  configured. To enable XPS, the bitmap of CPUs that may use a transmit
  queue is configured using the sysfs file entry:
  
  /sys/class/net/<dev>/queues/tx-<n>/xps_cpus
  
  == Suggested Configuration
  
  For a network device with a single transmission queue, XPS configuration
  has no effect, since there is no choice in this case. In a multi-queue
  system, XPS is preferably configured so that each CPU maps onto one queue.
  If there are as many queues as there are CPUs in the system, then each
  queue can also map onto one CPU, resulting in exclusive pairings that
  experience no contention. If there are fewer queues than CPUs, then the
  best CPUs to share a given queue are probably those that share the cache
  with the CPU that processes transmit completions for that queue
  (transmit interrupts).
  
  
  Further Information
  ===================
  RPS and RFS were introduced in kernel 2.6.35. XPS was incorporated into
  2.6.38. Original patches were submitted by Tom Herbert
  (therbert@google.com)
  
  Accelerated RFS was introduced in 2.6.35. Original patches were

  submitted by Ben Hutchings (bwh@kernel.org)

  
  Authors:
  Tom Herbert (therbert@google.com)
  Willem de Bruijn (willemb@google.com)