Based on kernel version 3.19. Page generated on 2015-02-13 21:22 EST.
1 Scaling in the Linux Networking Stack 2 3 4 Introduction 5 ============ 6 7 This document describes a set of complementary techniques in the Linux 8 networking stack to increase parallelism and improve performance for 9 multi-processor systems. 10 11 The following technologies are described: 12 13 RSS: Receive Side Scaling 14 RPS: Receive Packet Steering 15 RFS: Receive Flow Steering 16 Accelerated Receive Flow Steering 17 XPS: Transmit Packet Steering 18 19 20 RSS: Receive Side Scaling 21 ========================= 22 23 Contemporary NICs support multiple receive and transmit descriptor queues 24 (multi-queue). On reception, a NIC can send different packets to different 25 queues to distribute processing among CPUs. The NIC distributes packets by 26 applying a filter to each packet that assigns it to one of a small number 27 of logical flows. Packets for each flow are steered to a separate receive 28 queue, which in turn can be processed by separate CPUs. This mechanism is 29 generally known as “Receive-side Scaling” (RSS). The goal of RSS and 30 the other scaling techniques is to increase performance uniformly. 31 Multi-queue distribution can also be used for traffic prioritization, but 32 that is not the focus of these techniques. 33 34 The filter used in RSS is typically a hash function over the network 35 and/or transport layer headers-- for example, a 4-tuple hash over 36 IP addresses and TCP ports of a packet. The most common hardware 37 implementation of RSS uses a 128-entry indirection table where each entry 38 stores a queue number. The receive queue for a packet is determined 39 by masking out the low order seven bits of the computed hash for the 40 packet (usually a Toeplitz hash), taking this number as a key into the 41 indirection table and reading the corresponding value. 42 43 Some advanced NICs allow steering packets to queues based on 44 programmable filters. For example, webserver bound TCP port 80 packets 45 can be directed to their own receive queue. Such “n-tuple” filters can 46 be configured from ethtool (--config-ntuple). 47 48 ==== RSS Configuration 49 50 The driver for a multi-queue capable NIC typically provides a kernel 51 module parameter for specifying the number of hardware queues to 52 configure. In the bnx2x driver, for instance, this parameter is called 53 num_queues. A typical RSS configuration would be to have one receive queue 54 for each CPU if the device supports enough queues, or otherwise at least 55 one for each memory domain, where a memory domain is a set of CPUs that 56 share a particular memory level (L1, L2, NUMA node, etc.). 57 58 The indirection table of an RSS device, which resolves a queue by masked 59 hash, is usually programmed by the driver at initialization. The 60 default mapping is to distribute the queues evenly in the table, but the 61 indirection table can be retrieved and modified at runtime using ethtool 62 commands (--show-rxfh-indir and --set-rxfh-indir). Modifying the 63 indirection table could be done to give different queues different 64 relative weights. 65 66 == RSS IRQ Configuration 67 68 Each receive queue has a separate IRQ associated with it. The NIC triggers 69 this to notify a CPU when new packets arrive on the given queue. The 70 signaling path for PCIe devices uses message signaled interrupts (MSI-X), 71 that can route each interrupt to a particular CPU. The active mapping 72 of queues to IRQs can be determined from /proc/interrupts. By default, 73 an IRQ may be handled on any CPU. Because a non-negligible part of packet 74 processing takes place in receive interrupt handling, it is advantageous 75 to spread receive interrupts between CPUs. To manually adjust the IRQ 76 affinity of each interrupt see Documentation/IRQ-affinity.txt. Some systems 77 will be running irqbalance, a daemon that dynamically optimizes IRQ 78 assignments and as a result may override any manual settings. 79 80 == Suggested Configuration 81 82 RSS should be enabled when latency is a concern or whenever receive 83 interrupt processing forms a bottleneck. Spreading load between CPUs 84 decreases queue length. For low latency networking, the optimal setting 85 is to allocate as many queues as there are CPUs in the system (or the 86 NIC maximum, if lower). The most efficient high-rate configuration 87 is likely the one with the smallest number of receive queues where no 88 receive queue overflows due to a saturated CPU, because in default 89 mode with interrupt coalescing enabled, the aggregate number of 90 interrupts (and thus work) grows with each additional queue. 91 92 Per-cpu load can be observed using the mpstat utility, but note that on 93 processors with hyperthreading (HT), each hyperthread is represented as 94 a separate CPU. For interrupt handling, HT has shown no benefit in 95 initial tests, so limit the number of queues to the number of CPU cores 96 in the system. 97 98 99 RPS: Receive Packet Steering 100 ============================ 101 102 Receive Packet Steering (RPS) is logically a software implementation of 103 RSS. Being in software, it is necessarily called later in the datapath. 104 Whereas RSS selects the queue and hence CPU that will run the hardware 105 interrupt handler, RPS selects the CPU to perform protocol processing 106 above the interrupt handler. This is accomplished by placing the packet 107 on the desired CPU’s backlog queue and waking up the CPU for processing. 108 RPS has some advantages over RSS: 1) it can be used with any NIC, 109 2) software filters can easily be added to hash over new protocols, 110 3) it does not increase hardware device interrupt rate (although it does 111 introduce inter-processor interrupts (IPIs)). 112 113 RPS is called during bottom half of the receive interrupt handler, when 114 a driver sends a packet up the network stack with netif_rx() or 115 netif_receive_skb(). These call the get_rps_cpu() function, which 116 selects the queue that should process a packet. 117 118 The first step in determining the target CPU for RPS is to calculate a 119 flow hash over the packet’s addresses or ports (2-tuple or 4-tuple hash 120 depending on the protocol). This serves as a consistent hash of the 121 associated flow of the packet. The hash is either provided by hardware 122 or will be computed in the stack. Capable hardware can pass the hash in 123 the receive descriptor for the packet; this would usually be the same 124 hash used for RSS (e.g. computed Toeplitz hash). The hash is saved in 125 skb->rx_hash and can be used elsewhere in the stack as a hash of the 126 packet’s flow. 127 128 Each receive hardware queue has an associated list of CPUs to which 129 RPS may enqueue packets for processing. For each received packet, 130 an index into the list is computed from the flow hash modulo the size 131 of the list. The indexed CPU is the target for processing the packet, 132 and the packet is queued to the tail of that CPU’s backlog queue. At 133 the end of the bottom half routine, IPIs are sent to any CPUs for which 134 packets have been queued to their backlog queue. The IPI wakes backlog 135 processing on the remote CPU, and any queued packets are then processed 136 up the networking stack. 137 138 ==== RPS Configuration 139 140 RPS requires a kernel compiled with the CONFIG_RPS kconfig symbol (on 141 by default for SMP). Even when compiled in, RPS remains disabled until 142 explicitly configured. The list of CPUs to which RPS may forward traffic 143 can be configured for each receive queue using a sysfs file entry: 144 145 /sys/class/net/<dev>/queues/rx-<n>/rps_cpus 146 147 This file implements a bitmap of CPUs. RPS is disabled when it is zero 148 (the default), in which case packets are processed on the interrupting 149 CPU. Documentation/IRQ-affinity.txt explains how CPUs are assigned to 150 the bitmap. 151 152 == Suggested Configuration 153 154 For a single queue device, a typical RPS configuration would be to set 155 the rps_cpus to the CPUs in the same memory domain of the interrupting 156 CPU. If NUMA locality is not an issue, this could also be all CPUs in 157 the system. At high interrupt rate, it might be wise to exclude the 158 interrupting CPU from the map since that already performs much work. 159 160 For a multi-queue system, if RSS is configured so that a hardware 161 receive queue is mapped to each CPU, then RPS is probably redundant 162 and unnecessary. If there are fewer hardware queues than CPUs, then 163 RPS might be beneficial if the rps_cpus for each queue are the ones that 164 share the same memory domain as the interrupting CPU for that queue. 165 166 ==== RPS Flow Limit 167 168 RPS scales kernel receive processing across CPUs without introducing 169 reordering. The trade-off to sending all packets from the same flow 170 to the same CPU is CPU load imbalance if flows vary in packet rate. 171 In the extreme case a single flow dominates traffic. Especially on 172 common server workloads with many concurrent connections, such 173 behavior indicates a problem such as a misconfiguration or spoofed 174 source Denial of Service attack. 175 176 Flow Limit is an optional RPS feature that prioritizes small flows 177 during CPU contention by dropping packets from large flows slightly 178 ahead of those from small flows. It is active only when an RPS or RFS 179 destination CPU approaches saturation. Once a CPU's input packet 180 queue exceeds half the maximum queue length (as set by sysctl 181 net.core.netdev_max_backlog), the kernel starts a per-flow packet 182 count over the last 256 packets. If a flow exceeds a set ratio (by 183 default, half) of these packets when a new packet arrives, then the 184 new packet is dropped. Packets from other flows are still only 185 dropped once the input packet queue reaches netdev_max_backlog. 186 No packets are dropped when the input packet queue length is below 187 the threshold, so flow limit does not sever connections outright: 188 even large flows maintain connectivity. 189 190 == Interface 191 192 Flow limit is compiled in by default (CONFIG_NET_FLOW_LIMIT), but not 193 turned on. It is implemented for each CPU independently (to avoid lock 194 and cache contention) and toggled per CPU by setting the relevant bit 195 in sysctl net.core.flow_limit_cpu_bitmap. It exposes the same CPU 196 bitmap interface as rps_cpus (see above) when called from procfs: 197 198 /proc/sys/net/core/flow_limit_cpu_bitmap 199 200 Per-flow rate is calculated by hashing each packet into a hashtable 201 bucket and incrementing a per-bucket counter. The hash function is 202 the same that selects a CPU in RPS, but as the number of buckets can 203 be much larger than the number of CPUs, flow limit has finer-grained 204 identification of large flows and fewer false positives. The default 205 table has 4096 buckets. This value can be modified through sysctl 206 207 net.core.flow_limit_table_len 208 209 The value is only consulted when a new table is allocated. Modifying 210 it does not update active tables. 211 212 == Suggested Configuration 213 214 Flow limit is useful on systems with many concurrent connections, 215 where a single connection taking up 50% of a CPU indicates a problem. 216 In such environments, enable the feature on all CPUs that handle 217 network rx interrupts (as set in /proc/irq/N/smp_affinity). 218 219 The feature depends on the input packet queue length to exceed 220 the flow limit threshold (50%) + the flow history length (256). 221 Setting net.core.netdev_max_backlog to either 1000 or 10000 222 performed well in experiments. 223 224 225 RFS: Receive Flow Steering 226 ========================== 227 228 While RPS steers packets solely based on hash, and thus generally 229 provides good load distribution, it does not take into account 230 application locality. This is accomplished by Receive Flow Steering 231 (RFS). The goal of RFS is to increase datacache hitrate by steering 232 kernel processing of packets to the CPU where the application thread 233 consuming the packet is running. RFS relies on the same RPS mechanisms 234 to enqueue packets onto the backlog of another CPU and to wake up that 235 CPU. 236 237 In RFS, packets are not forwarded directly by the value of their hash, 238 but the hash is used as index into a flow lookup table. This table maps 239 flows to the CPUs where those flows are being processed. The flow hash 240 (see RPS section above) is used to calculate the index into this table. 241 The CPU recorded in each entry is the one which last processed the flow. 242 If an entry does not hold a valid CPU, then packets mapped to that entry 243 are steered using plain RPS. Multiple table entries may point to the 244 same CPU. Indeed, with many flows and few CPUs, it is very likely that 245 a single application thread handles flows with many different flow hashes. 246 247 rps_sock_flow_table is a global flow table that contains the *desired* CPU 248 for flows: the CPU that is currently processing the flow in userspace. 249 Each table value is a CPU index that is updated during calls to recvmsg 250 and sendmsg (specifically, inet_recvmsg(), inet_sendmsg(), inet_sendpage() 251 and tcp_splice_read()). 252 253 When the scheduler moves a thread to a new CPU while it has outstanding 254 receive packets on the old CPU, packets may arrive out of order. To 255 avoid this, RFS uses a second flow table to track outstanding packets 256 for each flow: rps_dev_flow_table is a table specific to each hardware 257 receive queue of each device. Each table value stores a CPU index and a 258 counter. The CPU index represents the *current* CPU onto which packets 259 for this flow are enqueued for further kernel processing. Ideally, kernel 260 and userspace processing occur on the same CPU, and hence the CPU index 261 in both tables is identical. This is likely false if the scheduler has 262 recently migrated a userspace thread while the kernel still has packets 263 enqueued for kernel processing on the old CPU. 264 265 The counter in rps_dev_flow_table values records the length of the current 266 CPU's backlog when a packet in this flow was last enqueued. Each backlog 267 queue has a head counter that is incremented on dequeue. A tail counter 268 is computed as head counter + queue length. In other words, the counter 269 in rps_dev_flow[i] records the last element in flow i that has 270 been enqueued onto the currently designated CPU for flow i (of course, 271 entry i is actually selected by hash and multiple flows may hash to the 272 same entry i). 273 274 And now the trick for avoiding out of order packets: when selecting the 275 CPU for packet processing (from get_rps_cpu()) the rps_sock_flow table 276 and the rps_dev_flow table of the queue that the packet was received on 277 are compared. If the desired CPU for the flow (found in the 278 rps_sock_flow table) matches the current CPU (found in the rps_dev_flow 279 table), the packet is enqueued onto that CPU’s backlog. If they differ, 280 the current CPU is updated to match the desired CPU if one of the 281 following is true: 282 283 - The current CPU's queue head counter >= the recorded tail counter 284 value in rps_dev_flow[i] 285 - The current CPU is unset (equal to RPS_NO_CPU) 286 - The current CPU is offline 287 288 After this check, the packet is sent to the (possibly updated) current 289 CPU. These rules aim to ensure that a flow only moves to a new CPU when 290 there are no packets outstanding on the old CPU, as the outstanding 291 packets could arrive later than those about to be processed on the new 292 CPU. 293 294 ==== RFS Configuration 295 296 RFS is only available if the kconfig symbol CONFIG_RPS is enabled (on 297 by default for SMP). The functionality remains disabled until explicitly 298 configured. The number of entries in the global flow table is set through: 299 300 /proc/sys/net/core/rps_sock_flow_entries 301 302 The number of entries in the per-queue flow table are set through: 303 304 /sys/class/net/<dev>/queues/rx-<n>/rps_flow_cnt 305 306 == Suggested Configuration 307 308 Both of these need to be set before RFS is enabled for a receive queue. 309 Values for both are rounded up to the nearest power of two. The 310 suggested flow count depends on the expected number of active connections 311 at any given time, which may be significantly less than the number of open 312 connections. We have found that a value of 32768 for rps_sock_flow_entries 313 works fairly well on a moderately loaded server. 314 315 For a single queue device, the rps_flow_cnt value for the single queue 316 would normally be configured to the same value as rps_sock_flow_entries. 317 For a multi-queue device, the rps_flow_cnt for each queue might be 318 configured as rps_sock_flow_entries / N, where N is the number of 319 queues. So for instance, if rps_sock_flow_entries is set to 32768 and there 320 are 16 configured receive queues, rps_flow_cnt for each queue might be 321 configured as 2048. 322 323 324 Accelerated RFS 325 =============== 326 327 Accelerated RFS is to RFS what RSS is to RPS: a hardware-accelerated load 328 balancing mechanism that uses soft state to steer flows based on where 329 the application thread consuming the packets of each flow is running. 330 Accelerated RFS should perform better than RFS since packets are sent 331 directly to a CPU local to the thread consuming the data. The target CPU 332 will either be the same CPU where the application runs, or at least a CPU 333 which is local to the application thread’s CPU in the cache hierarchy. 334 335 To enable accelerated RFS, the networking stack calls the 336 ndo_rx_flow_steer driver function to communicate the desired hardware 337 queue for packets matching a particular flow. The network stack 338 automatically calls this function every time a flow entry in 339 rps_dev_flow_table is updated. The driver in turn uses a device specific 340 method to program the NIC to steer the packets. 341 342 The hardware queue for a flow is derived from the CPU recorded in 343 rps_dev_flow_table. The stack consults a CPU to hardware queue map which 344 is maintained by the NIC driver. This is an auto-generated reverse map of 345 the IRQ affinity table shown by /proc/interrupts. Drivers can use 346 functions in the cpu_rmap (“CPU affinity reverse map”) kernel library 347 to populate the map. For each CPU, the corresponding queue in the map is 348 set to be one whose processing CPU is closest in cache locality. 349 350 ==== Accelerated RFS Configuration 351 352 Accelerated RFS is only available if the kernel is compiled with 353 CONFIG_RFS_ACCEL and support is provided by the NIC device and driver. 354 It also requires that ntuple filtering is enabled via ethtool. The map 355 of CPU to queues is automatically deduced from the IRQ affinities 356 configured for each receive queue by the driver, so no additional 357 configuration should be necessary. 358 359 == Suggested Configuration 360 361 This technique should be enabled whenever one wants to use RFS and the 362 NIC supports hardware acceleration. 363 364 XPS: Transmit Packet Steering 365 ============================= 366 367 Transmit Packet Steering is a mechanism for intelligently selecting 368 which transmit queue to use when transmitting a packet on a multi-queue 369 device. To accomplish this, a mapping from CPU to hardware queue(s) is 370 recorded. The goal of this mapping is usually to assign queues 371 exclusively to a subset of CPUs, where the transmit completions for 372 these queues are processed on a CPU within this set. This choice 373 provides two benefits. First, contention on the device queue lock is 374 significantly reduced since fewer CPUs contend for the same queue 375 (contention can be eliminated completely if each CPU has its own 376 transmit queue). Secondly, cache miss rate on transmit completion is 377 reduced, in particular for data cache lines that hold the sk_buff 378 structures. 379 380 XPS is configured per transmit queue by setting a bitmap of CPUs that 381 may use that queue to transmit. The reverse mapping, from CPUs to 382 transmit queues, is computed and maintained for each network device. 383 When transmitting the first packet in a flow, the function 384 get_xps_queue() is called to select a queue. This function uses the ID 385 of the running CPU as a key into the CPU-to-queue lookup table. If the 386 ID matches a single queue, that is used for transmission. If multiple 387 queues match, one is selected by using the flow hash to compute an index 388 into the set. 389 390 The queue chosen for transmitting a particular flow is saved in the 391 corresponding socket structure for the flow (e.g. a TCP connection). 392 This transmit queue is used for subsequent packets sent on the flow to 393 prevent out of order (ooo) packets. The choice also amortizes the cost 394 of calling get_xps_queues() over all packets in the flow. To avoid 395 ooo packets, the queue for a flow can subsequently only be changed if 396 skb->ooo_okay is set for a packet in the flow. This flag indicates that 397 there are no outstanding packets in the flow, so the transmit queue can 398 change without the risk of generating out of order packets. The 399 transport layer is responsible for setting ooo_okay appropriately. TCP, 400 for instance, sets the flag when all data for a connection has been 401 acknowledged. 402 403 ==== XPS Configuration 404 405 XPS is only available if the kconfig symbol CONFIG_XPS is enabled (on by 406 default for SMP). The functionality remains disabled until explicitly 407 configured. To enable XPS, the bitmap of CPUs that may use a transmit 408 queue is configured using the sysfs file entry: 409 410 /sys/class/net/<dev>/queues/tx-<n>/xps_cpus 411 412 == Suggested Configuration 413 414 For a network device with a single transmission queue, XPS configuration 415 has no effect, since there is no choice in this case. In a multi-queue 416 system, XPS is preferably configured so that each CPU maps onto one queue. 417 If there are as many queues as there are CPUs in the system, then each 418 queue can also map onto one CPU, resulting in exclusive pairings that 419 experience no contention. If there are fewer queues than CPUs, then the 420 best CPUs to share a given queue are probably those that share the cache 421 with the CPU that processes transmit completions for that queue 422 (transmit interrupts). 423 424 425 Further Information 426 =================== 427 RPS and RFS were introduced in kernel 2.6.35. XPS was incorporated into 428 2.6.38. Original patches were submitted by Tom Herbert 429 (firstname.lastname@example.org) 430 431 Accelerated RFS was introduced in 2.6.35. Original patches were 432 submitted by Ben Hutchings (email@example.com) 433 434 Authors: 435 Tom Herbert (firstname.lastname@example.org) 436 Willem de Bruijn (email@example.com)