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diff --git a/Documentation/networking/scaling.txt b/Documentation/networking/scaling.txt new file mode 100644 index 00000000..579994af --- /dev/null +++ b/Documentation/networking/scaling.txt @@ -0,0 +1,378 @@ +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) |