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Based on kernel version 3.9. Page generated on 2013-05-02 23:11 EST.

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

• [ networking ]
• 00-INDEX
• 3c505.txt
• 3c509.txt
• 6pack.txt
• alias.txt
• arcnet-hardware.txt
• arcnet.txt
• atm.txt
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• batman-adv.txt
• baycom.txt
• bonding.txt
• bridge.txt
• [ caif ]
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• cops.txt
• cs89x0.txt
• cxacru-cf.py
• cxacru.txt
• cxgb.txt
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• decnet.txt
• dl2k.txt
• dm9000.txt
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• dns_resolver.txt
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• e100.txt
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• e1000e.txt
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• filter.txt
• fore200e.txt
• framerelay.txt
• gen_stats.txt
• generic-hdlc.txt
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• gianfar.txt
• ieee802154.txt
• ifenslave.c
• igb.txt
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• ip-sysctl.txt
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• ipddp.txt
• iphase.txt
• ipv6.txt
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• l2tp.txt
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• LICENSE.qla3xxx
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• operstates.txt
• packet_mmap.txt
• phonet.txt
• phy.txt
• pktgen.txt
• PLIP.txt
• policy-routing.txt
• ppp_generic.txt
• proc_net_tcp.txt
• radiotap-headers.txt
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• README.ipw2100
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• stmmac.txt
• tc-actions-env-rules.txt
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• [ timestamping ]
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• xfrm_sync.txt
• xfrm_sysctl.txt
• z8530drv.txt
•