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doc: networking: integrate scaling document into doc tree
Convert scaling document into reStructuredText and add reference to scaling document into main table of contents in network documentation. There are no semantic changes. There are no references to "scaling.txt" file. Whole kernel tree was checked using: $ grep -r "scaling\.txt" Signed-off-by: Otto Sabart <ottosabart@seberm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Otto Sabart
Jonathan Corbet
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e6e37f6368
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@ -34,6 +34,7 @@ Contents:
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snmp_counter
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checksum-offloads
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segmentation-offloads
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scaling
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.. only:: subproject
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@ -1,4 +1,8 @@
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.. SPDX-License-Identifier: GPL-2.0
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=====================================
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Scaling in the Linux Networking Stack
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=====================================
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Introduction
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@ -10,11 +14,11 @@ 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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- 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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@ -45,7 +49,9 @@ 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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RSS Configuration
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-----------------
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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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@ -63,7 +69,9 @@ 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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RSS IRQ Configuration
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~~~~~~~~~~~~~~~~~~~~~
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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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@ -77,7 +85,9 @@ 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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Suggested Configuration
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~~~~~~~~~~~~~~~~~~~~~~~
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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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@ -105,10 +115,12 @@ 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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RPS has some advantages over RSS:
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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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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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@ -135,21 +147,25 @@ 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 Configuration
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-----------------
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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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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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/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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Suggested Configuration
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~~~~~~~~~~~~~~~~~~~~~~~
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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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@ -163,7 +179,9 @@ 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 Flow Limit
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--------------
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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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@ -187,29 +205,33 @@ 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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Interface
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~~~~~~~~~
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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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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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/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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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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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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Suggested Configuration
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~~~~~~~~~~~~~~~~~~~~~~~
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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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@ -280,10 +302,10 @@ 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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- 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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@ -291,19 +313,23 @@ 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 Configuration
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-----------------
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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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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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/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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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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/sys/class/net/<dev>/queues/rx-<n>/rps_flow_cnt
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== Suggested Configuration
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Suggested Configuration
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~~~~~~~~~~~~~~~~~~~~~~~
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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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@ -347,7 +373,9 @@ 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 Configuration
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-----------------------------
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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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@ -356,11 +384,14 @@ 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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Suggested Configuration
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~~~~~~~~~~~~~~~~~~~~~~~
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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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@ -430,20 +461,25 @@ 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.
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==== XPS Configuration
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XPS Configuration
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-----------------
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XPS is only available if the kconfig symbol CONFIG_XPS is enabled (on by
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default for SMP). The functionality remains disabled until explicitly
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configured. To enable XPS, the bitmap of CPUs/receive-queues that may
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use a transmit queue is configured using the sysfs file entry:
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For selection based on CPUs map:
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/sys/class/net/<dev>/queues/tx-<n>/xps_cpus
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For selection based on CPUs map::
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For selection based on receive-queues map:
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/sys/class/net/<dev>/queues/tx-<n>/xps_rxqs
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/sys/class/net/<dev>/queues/tx-<n>/xps_cpus
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== Suggested Configuration
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For selection based on receive-queues map::
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/sys/class/net/<dev>/queues/tx-<n>/xps_rxqs
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Suggested Configuration
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~~~~~~~~~~~~~~~~~~~~~~~
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For a network device with a single transmission queue, XPS configuration
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has no effect, since there is no choice in this case. In a multi-queue
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@ -460,16 +496,18 @@ explicitly configured mapping receive-queue(s) to transmit queue(s). If the
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user configuration for receive-queue map does not apply, then the transmit
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queue is selected based on the CPUs map.
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Per TX Queue rate limitation:
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=============================
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Per TX Queue rate limitation
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============================
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These are rate-limitation mechanisms implemented by HW, where currently
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a max-rate attribute is supported, by setting a Mbps value to
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a max-rate attribute is supported, by setting a Mbps value to::
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/sys/class/net/<dev>/queues/tx-<n>/tx_maxrate
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/sys/class/net/<dev>/queues/tx-<n>/tx_maxrate
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A value of zero means disabled, and this is the default.
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Further Information
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===================
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RPS and RFS were introduced in kernel 2.6.35. XPS was incorporated into
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@ -480,5 +518,6 @@ Accelerated RFS was introduced in 2.6.35. Original patches were
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submitted by Ben Hutchings (bwh@kernel.org)
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Authors:
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Tom Herbert (therbert@google.com)
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Willem de Bruijn (willemb@google.com)
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- Tom Herbert (therbert@google.com)
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- Willem de Bruijn (willemb@google.com)
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