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Signed-off-by: Amritha Nambiar <amritha.nambiar@intel.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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485 lines
23 KiB
Plaintext
Scaling in the Linux Networking Stack
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Introduction
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============
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This document describes a set of complementary techniques in the Linux
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networking stack to increase parallelism and improve performance for
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multi-processor systems.
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The following technologies are described:
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RSS: Receive Side Scaling
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RPS: Receive Packet Steering
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RFS: Receive Flow Steering
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Accelerated Receive Flow Steering
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XPS: Transmit Packet Steering
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RSS: Receive Side Scaling
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=========================
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Contemporary NICs support multiple receive and transmit descriptor queues
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(multi-queue). On reception, a NIC can send different packets to different
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queues to distribute processing among CPUs. The NIC distributes packets by
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applying a filter to each packet that assigns it to one of a small number
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of logical flows. Packets for each flow are steered to a separate receive
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queue, which in turn can be processed by separate CPUs. This mechanism is
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generally known as “Receive-side Scaling” (RSS). The goal of RSS and
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the other scaling techniques is to increase performance uniformly.
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Multi-queue distribution can also be used for traffic prioritization, but
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that is not the focus of these techniques.
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The filter used in RSS is typically a hash function over the network
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and/or transport layer headers-- for example, a 4-tuple hash over
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IP addresses and TCP ports of a packet. The most common hardware
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implementation of RSS uses a 128-entry indirection table where each entry
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stores a queue number. The receive queue for a packet is determined
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by masking out the low order seven bits of the computed hash for the
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packet (usually a Toeplitz hash), taking this number as a key into the
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indirection table and reading the corresponding value.
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Some advanced NICs allow steering packets to queues based on
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programmable filters. For example, webserver bound TCP port 80 packets
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can be directed to their own receive queue. Such “n-tuple” filters can
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be configured from ethtool (--config-ntuple).
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==== RSS Configuration
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The driver for a multi-queue capable NIC typically provides a kernel
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module parameter for specifying the number of hardware queues to
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configure. In the bnx2x driver, for instance, this parameter is called
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num_queues. A typical RSS configuration would be to have one receive queue
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for each CPU if the device supports enough queues, or otherwise at least
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one for each memory domain, where a memory domain is a set of CPUs that
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share a particular memory level (L1, L2, NUMA node, etc.).
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The indirection table of an RSS device, which resolves a queue by masked
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hash, is usually programmed by the driver at initialization. The
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default mapping is to distribute the queues evenly in the table, but the
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indirection table can be retrieved and modified at runtime using ethtool
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commands (--show-rxfh-indir and --set-rxfh-indir). Modifying the
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indirection table could be done to give different queues different
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relative weights.
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== RSS IRQ Configuration
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Each receive queue has a separate IRQ associated with it. The NIC triggers
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this to notify a CPU when new packets arrive on the given queue. The
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signaling path for PCIe devices uses message signaled interrupts (MSI-X),
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that can route each interrupt to a particular CPU. The active mapping
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of queues to IRQs can be determined from /proc/interrupts. By default,
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an IRQ may be handled on any CPU. Because a non-negligible part of packet
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processing takes place in receive interrupt handling, it is advantageous
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to spread receive interrupts between CPUs. To manually adjust the IRQ
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affinity of each interrupt see Documentation/IRQ-affinity.txt. Some systems
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will be running irqbalance, a daemon that dynamically optimizes IRQ
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assignments and as a result may override any manual settings.
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== Suggested Configuration
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RSS should be enabled when latency is a concern or whenever receive
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interrupt processing forms a bottleneck. Spreading load between CPUs
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decreases queue length. For low latency networking, the optimal setting
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is to allocate as many queues as there are CPUs in the system (or the
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NIC maximum, if lower). The most efficient high-rate configuration
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is likely the one with the smallest number of receive queues where no
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receive queue overflows due to a saturated CPU, because in default
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mode with interrupt coalescing enabled, the aggregate number of
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interrupts (and thus work) grows with each additional queue.
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Per-cpu load can be observed using the mpstat utility, but note that on
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processors with hyperthreading (HT), each hyperthread is represented as
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a separate CPU. For interrupt handling, HT has shown no benefit in
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initial tests, so limit the number of queues to the number of CPU cores
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in the system.
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RPS: Receive Packet Steering
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============================
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Receive Packet Steering (RPS) is logically a software implementation of
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RSS. Being in software, it is necessarily called later in the datapath.
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Whereas RSS selects the queue and hence CPU that will run the hardware
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interrupt handler, RPS selects the CPU to perform protocol processing
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above the interrupt handler. This is accomplished by placing the packet
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on the desired CPU’s backlog queue and waking up the CPU for processing.
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RPS has some advantages over RSS: 1) it can be used with any NIC,
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2) software filters can easily be added to hash over new protocols,
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3) it does not increase hardware device interrupt rate (although it does
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introduce inter-processor interrupts (IPIs)).
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RPS is called during bottom half of the receive interrupt handler, when
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a driver sends a packet up the network stack with netif_rx() or
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netif_receive_skb(). These call the get_rps_cpu() function, which
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selects the queue that should process a packet.
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The first step in determining the target CPU for RPS is to calculate a
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flow hash over the packet’s addresses or ports (2-tuple or 4-tuple hash
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depending on the protocol). This serves as a consistent hash of the
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associated flow of the packet. The hash is either provided by hardware
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or will be computed in the stack. Capable hardware can pass the hash in
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the receive descriptor for the packet; this would usually be the same
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hash used for RSS (e.g. computed Toeplitz hash). The hash is saved in
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skb->hash and can be used elsewhere in the stack as a hash of the
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packet’s flow.
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Each receive hardware queue has an associated list of CPUs to which
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RPS may enqueue packets for processing. For each received packet,
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an index into the list is computed from the flow hash modulo the size
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of the list. The indexed CPU is the target for processing the packet,
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and the packet is queued to the tail of that CPU’s backlog queue. At
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the end of the bottom half routine, IPIs are sent to any CPUs for which
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packets have been queued to their backlog queue. The IPI wakes backlog
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processing on the remote CPU, and any queued packets are then processed
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up the networking stack.
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==== RPS Configuration
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RPS requires a kernel compiled with the CONFIG_RPS kconfig symbol (on
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by default for SMP). Even when compiled in, RPS remains disabled until
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explicitly configured. The list of CPUs to which RPS may forward traffic
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can be configured for each receive queue using a sysfs file entry:
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/sys/class/net/<dev>/queues/rx-<n>/rps_cpus
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This file implements a bitmap of CPUs. RPS is disabled when it is zero
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(the default), in which case packets are processed on the interrupting
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CPU. Documentation/IRQ-affinity.txt explains how CPUs are assigned to
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the bitmap.
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== Suggested Configuration
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For a single queue device, a typical RPS configuration would be to set
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the rps_cpus to the CPUs in the same memory domain of the interrupting
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CPU. If NUMA locality is not an issue, this could also be all CPUs in
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the system. At high interrupt rate, it might be wise to exclude the
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interrupting CPU from the map since that already performs much work.
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For a multi-queue system, if RSS is configured so that a hardware
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receive queue is mapped to each CPU, then RPS is probably redundant
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and unnecessary. If there are fewer hardware queues than CPUs, then
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RPS might be beneficial if the rps_cpus for each queue are the ones that
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share the same memory domain as the interrupting CPU for that queue.
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==== RPS Flow Limit
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RPS scales kernel receive processing across CPUs without introducing
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reordering. The trade-off to sending all packets from the same flow
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to the same CPU is CPU load imbalance if flows vary in packet rate.
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In the extreme case a single flow dominates traffic. Especially on
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common server workloads with many concurrent connections, such
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behavior indicates a problem such as a misconfiguration or spoofed
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source Denial of Service attack.
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Flow Limit is an optional RPS feature that prioritizes small flows
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during CPU contention by dropping packets from large flows slightly
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ahead of those from small flows. It is active only when an RPS or RFS
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destination CPU approaches saturation. Once a CPU's input packet
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queue exceeds half the maximum queue length (as set by sysctl
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net.core.netdev_max_backlog), the kernel starts a per-flow packet
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count over the last 256 packets. If a flow exceeds a set ratio (by
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default, half) of these packets when a new packet arrives, then the
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new packet is dropped. Packets from other flows are still only
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dropped once the input packet queue reaches netdev_max_backlog.
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No packets are dropped when the input packet queue length is below
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the threshold, so flow limit does not sever connections outright:
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even large flows maintain connectivity.
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== Interface
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Flow limit is compiled in by default (CONFIG_NET_FLOW_LIMIT), but not
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turned on. It is implemented for each CPU independently (to avoid lock
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and cache contention) and toggled per CPU by setting the relevant bit
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in sysctl net.core.flow_limit_cpu_bitmap. It exposes the same CPU
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bitmap interface as rps_cpus (see above) when called from procfs:
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/proc/sys/net/core/flow_limit_cpu_bitmap
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Per-flow rate is calculated by hashing each packet into a hashtable
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bucket and incrementing a per-bucket counter. The hash function is
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the same that selects a CPU in RPS, but as the number of buckets can
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be much larger than the number of CPUs, flow limit has finer-grained
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identification of large flows and fewer false positives. The default
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table has 4096 buckets. This value can be modified through sysctl
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net.core.flow_limit_table_len
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The value is only consulted when a new table is allocated. Modifying
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it does not update active tables.
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== Suggested Configuration
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Flow limit is useful on systems with many concurrent connections,
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where a single connection taking up 50% of a CPU indicates a problem.
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In such environments, enable the feature on all CPUs that handle
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network rx interrupts (as set in /proc/irq/N/smp_affinity).
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The feature depends on the input packet queue length to exceed
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the flow limit threshold (50%) + the flow history length (256).
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Setting net.core.netdev_max_backlog to either 1000 or 10000
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performed well in experiments.
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RFS: Receive Flow Steering
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==========================
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While RPS steers packets solely based on hash, and thus generally
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provides good load distribution, it does not take into account
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application locality. This is accomplished by Receive Flow Steering
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(RFS). The goal of RFS is to increase datacache hitrate by steering
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kernel processing of packets to the CPU where the application thread
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consuming the packet is running. RFS relies on the same RPS mechanisms
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to enqueue packets onto the backlog of another CPU and to wake up that
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CPU.
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In RFS, packets are not forwarded directly by the value of their hash,
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but the hash is used as index into a flow lookup table. This table maps
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flows to the CPUs where those flows are being processed. The flow hash
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(see RPS section above) is used to calculate the index into this table.
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The CPU recorded in each entry is the one which last processed the flow.
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If an entry does not hold a valid CPU, then packets mapped to that entry
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are steered using plain RPS. Multiple table entries may point to the
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same CPU. Indeed, with many flows and few CPUs, it is very likely that
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a single application thread handles flows with many different flow hashes.
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rps_sock_flow_table is a global flow table that contains the *desired* CPU
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for flows: the CPU that is currently processing the flow in userspace.
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Each table value is a CPU index that is updated during calls to recvmsg
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and sendmsg (specifically, inet_recvmsg(), inet_sendmsg(), inet_sendpage()
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and tcp_splice_read()).
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When the scheduler moves a thread to a new CPU while it has outstanding
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receive packets on the old CPU, packets may arrive out of order. To
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avoid this, RFS uses a second flow table to track outstanding packets
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for each flow: rps_dev_flow_table is a table specific to each hardware
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receive queue of each device. Each table value stores a CPU index and a
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counter. The CPU index represents the *current* CPU onto which packets
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for this flow are enqueued for further kernel processing. Ideally, kernel
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and userspace processing occur on the same CPU, and hence the CPU index
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in both tables is identical. This is likely false if the scheduler has
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recently migrated a userspace thread while the kernel still has packets
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enqueued for kernel processing on the old CPU.
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The counter in rps_dev_flow_table values records the length of the current
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CPU's backlog when a packet in this flow was last enqueued. Each backlog
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queue has a head counter that is incremented on dequeue. A tail counter
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is computed as head counter + queue length. In other words, the counter
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in rps_dev_flow[i] records the last element in flow i that has
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been enqueued onto the currently designated CPU for flow i (of course,
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entry i is actually selected by hash and multiple flows may hash to the
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same entry i).
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And now the trick for avoiding out of order packets: when selecting the
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CPU for packet processing (from get_rps_cpu()) the rps_sock_flow table
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and the rps_dev_flow table of the queue that the packet was received on
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are compared. If the desired CPU for the flow (found in the
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rps_sock_flow table) matches the current CPU (found in the rps_dev_flow
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table), the packet is enqueued onto that CPU’s backlog. If they differ,
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the current CPU is updated to match the desired CPU if one of the
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following is true:
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- The current CPU's queue head counter >= the recorded tail counter
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value in rps_dev_flow[i]
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- The current CPU is unset (>= nr_cpu_ids)
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- The current CPU is offline
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After this check, the packet is sent to the (possibly updated) current
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CPU. These rules aim to ensure that a flow only moves to a new CPU when
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there are no packets outstanding on the old CPU, as the outstanding
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packets could arrive later than those about to be processed on the new
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CPU.
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==== RFS Configuration
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RFS is only available if the kconfig symbol CONFIG_RPS is enabled (on
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by default for SMP). The functionality remains disabled until explicitly
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configured. The number of entries in the global flow table is set through:
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/proc/sys/net/core/rps_sock_flow_entries
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The number of entries in the per-queue flow table are set through:
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/sys/class/net/<dev>/queues/rx-<n>/rps_flow_cnt
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== Suggested Configuration
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Both of these need to be set before RFS is enabled for a receive queue.
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Values for both are rounded up to the nearest power of two. The
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suggested flow count depends on the expected number of active connections
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at any given time, which may be significantly less than the number of open
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connections. We have found that a value of 32768 for rps_sock_flow_entries
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works fairly well on a moderately loaded server.
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For a single queue device, the rps_flow_cnt value for the single queue
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would normally be configured to the same value as rps_sock_flow_entries.
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For a multi-queue device, the rps_flow_cnt for each queue might be
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configured as rps_sock_flow_entries / N, where N is the number of
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queues. So for instance, if rps_sock_flow_entries is set to 32768 and there
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are 16 configured receive queues, rps_flow_cnt for each queue might be
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configured as 2048.
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Accelerated RFS
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===============
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Accelerated RFS is to RFS what RSS is to RPS: a hardware-accelerated load
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balancing mechanism that uses soft state to steer flows based on where
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the application thread consuming the packets of each flow is running.
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Accelerated RFS should perform better than RFS since packets are sent
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directly to a CPU local to the thread consuming the data. The target CPU
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will either be the same CPU where the application runs, or at least a CPU
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which is local to the application thread’s CPU in the cache hierarchy.
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To enable accelerated RFS, the networking stack calls the
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ndo_rx_flow_steer driver function to communicate the desired hardware
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queue for packets matching a particular flow. The network stack
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automatically calls this function every time a flow entry in
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rps_dev_flow_table is updated. The driver in turn uses a device specific
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method to program the NIC to steer the packets.
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The hardware queue for a flow is derived from the CPU recorded in
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rps_dev_flow_table. The stack consults a CPU to hardware queue map which
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is maintained by the NIC driver. This is an auto-generated reverse map of
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the IRQ affinity table shown by /proc/interrupts. Drivers can use
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functions in the cpu_rmap (“CPU affinity reverse map”) kernel library
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to populate the map. For each CPU, the corresponding queue in the map is
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set to be one whose processing CPU is closest in cache locality.
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==== Accelerated RFS Configuration
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Accelerated RFS is only available if the kernel is compiled with
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CONFIG_RFS_ACCEL and support is provided by the NIC device and driver.
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It also requires that ntuple filtering is enabled via ethtool. The map
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of CPU to queues is automatically deduced from the IRQ affinities
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configured for each receive queue by the driver, so no additional
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configuration should be necessary.
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== Suggested Configuration
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This technique should be enabled whenever one wants to use RFS and the
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NIC supports hardware acceleration.
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XPS: Transmit Packet Steering
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=============================
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Transmit Packet Steering is a mechanism for intelligently selecting
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which transmit queue to use when transmitting a packet on a multi-queue
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device. This can be accomplished by recording two kinds of maps, either
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a mapping of CPU to hardware queue(s) or a mapping of receive queue(s)
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to hardware transmit queue(s).
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1. XPS using CPUs map
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The goal of this mapping is usually to assign queues
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exclusively to a subset of CPUs, where the transmit completions for
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these queues are processed on a CPU within this set. This choice
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provides two benefits. First, contention on the device queue lock is
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significantly reduced since fewer CPUs contend for the same queue
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(contention can be eliminated completely if each CPU has its own
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transmit queue). Secondly, cache miss rate on transmit completion is
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reduced, in particular for data cache lines that hold the sk_buff
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structures.
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2. XPS using receive queues map
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This mapping is used to pick transmit queue based on the receive
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queue(s) map configuration set by the administrator. A set of receive
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queues can be mapped to a set of transmit queues (many:many), although
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the common use case is a 1:1 mapping. This will enable sending packets
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on the same queue associations for transmit and receive. This is useful for
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busy polling multi-threaded workloads where there are challenges in
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associating a given CPU to a given application thread. The application
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threads are not pinned to CPUs and each thread handles packets
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received on a single queue. The receive queue number is cached in the
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socket for the connection. In this model, sending the packets on the same
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transmit queue corresponding to the associated receive queue has benefits
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in keeping the CPU overhead low. Transmit completion work is locked into
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the same queue-association that a given application is polling on. This
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avoids the overhead of triggering an interrupt on another CPU. When the
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application cleans up the packets during the busy poll, transmit completion
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may be processed along with it in the same thread context and so result in
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reduced latency.
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XPS is configured per transmit queue by setting a bitmap of
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CPUs/receive-queues that may use that queue to transmit. The reverse
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mapping, from CPUs to transmit queues or from receive-queues to transmit
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queues, is computed and maintained for each network device. When
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transmitting the first packet in a flow, the function get_xps_queue() is
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called to select a queue. This function uses the ID of the receive queue
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for the socket connection for a match in the receive queue-to-transmit queue
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lookup table. Alternatively, this function can also use the ID of the
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running CPU as a key into the CPU-to-queue lookup table. If the
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ID matches a single queue, that is used for transmission. If multiple
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queues match, one is selected by using the flow hash to compute an index
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into the set. When selecting the transmit queue based on receive queue(s)
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map, the transmit device is not validated against the receive device as it
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requires expensive lookup operation in the datapath.
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The queue chosen for transmitting a particular flow is saved in the
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corresponding socket structure for the flow (e.g. a TCP connection).
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This transmit queue is used for subsequent packets sent on the flow to
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prevent out of order (ooo) packets. The choice also amortizes the cost
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of calling get_xps_queues() over all packets in the flow. To avoid
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ooo packets, the queue for a flow can subsequently only be changed if
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skb->ooo_okay is set for a packet in the flow. This flag indicates that
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there are no outstanding packets in the flow, so the transmit queue can
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change without the risk of generating out of order packets. The
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transport layer is responsible for setting ooo_okay appropriately. TCP,
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for instance, sets the flag when all data for a connection has been
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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/receive-queues that may
|
||
use a transmit queue is configured using the sysfs file entry:
|
||
|
||
For selection based on CPUs map:
|
||
/sys/class/net/<dev>/queues/tx-<n>/xps_cpus
|
||
|
||
For selection based on receive-queues map:
|
||
/sys/class/net/<dev>/queues/tx-<n>/xps_rxqs
|
||
|
||
== 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).
|
||
|
||
For transmit queue selection based on receive queue(s), XPS has to be
|
||
explicitly configured mapping receive-queue(s) to transmit queue(s). If the
|
||
user configuration for receive-queue map does not apply, then the transmit
|
||
queue is selected based on the CPUs map.
|
||
|
||
Per TX Queue rate limitation:
|
||
=============================
|
||
|
||
These are rate-limitation mechanisms implemented by HW, where currently
|
||
a max-rate attribute is supported, by setting a Mbps value to
|
||
|
||
/sys/class/net/<dev>/queues/tx-<n>/tx_maxrate
|
||
|
||
A value of zero means disabled, and this is the default.
|
||
|
||
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)
|