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7.1 KiB
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142 lines
7.1 KiB
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=======================================
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Performance
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=======================================
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Receive-side scaling
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--------------------------------------------------
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**Introduction**
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=====================================================================================================================
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RSS is used to distribute packets over CPU cores using a hashing function – either with support in the hardware which offloads the hashing for you, or in software.
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The idea is to take as input the TCP 4-tuple (source address, source port, destination address, destination port) of a packet, hash this input using
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an in-kernel defined key, and selecting the resulting values’ LSB as an index into a user-configurable indirection table.
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The indirection table is loaded into the hardware during boot and is used by the NIC to decide which CPU to interrupt with a given packet.
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All of this allows packets of the same origin/destination (a.k.a. flows) to be queued consistently on the same CPU.
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.. Note::
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**By default, RSS will be disabled since it’s impact is quite far reaching.**
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Only enable this feature if you’re interested in testing it and seeing if it will increase your
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throughput under high load – such as when using IDS/IPS.
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Since we do not have every type of hardware available – nor the time to test all of them,
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no guarantee is given that a NIC driver will properly handle the kernel
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implementation or is even capable of using it.
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The implementation of RSS is coupled with `PCBGROUP <https://www.freebsd.org/cgi/man.cgi?query=PCBGROUP>`__
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– an implementation which introduces notions of CPU affinity for connections.
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While the latter will be of lesser importance for OPNsense, since it specifically applies to connections
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built up in userland using sockets (which is relevant to servers, not middleboxes),
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the idea of distributing work on a lower level with hardware support provides a myriad of benefits.
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**Network card / driver**
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=====================================================================================================================
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Assuming you are using a modern NIC which supports multiple hardware queues and RSS, the configuration of a NIC will decide how and on which queue packets
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arrive on your system. This is also hardware dependent and will not be the same on every NIC. Should your driver support the option to enable/disable RSS,
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a sysctl tunable will be available.
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It is possible for a NIC to perform RSS without being able to configure it. Should you wish to know if it can be enabled/disabled:
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.. code-block::
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sysctl -a | grep rss
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should show any drivers exposing the option via a tunable.
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It is also possible that a driver does not expose this ability to the user, in which case you’d want to look up whether the NIC/driver supports RSS at all using online
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datasheets or a simple google search. For example, igb enables RSS by default, but does not reflect this in any configuration parameter. However, since it uses multiple queues:
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.. code-block::
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# dmesg | grep vectors
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igb0: Using MSI-X interrupts with 5 vectors
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igb1: Using MSI-X interrupts with 5 vectors
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igb2: Using MSI-X interrupts with 5 vectors
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igb3: Using MSI-X interrupts with 5 vectors
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It will most likely have some form of packet filtering to distribute packets over the hardware queues. In fact, igb does RSS by default.
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For most NICs, RSS is the primary method of deciding which CPU to interrupt with a packet. NICs that do not implement any other type of filter and whose RSS feature
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is missing or turned off, will most likely interrupt only CPU 0 at all times – which will reduce potential throughput due to cache line migrations and lock contention.
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Please keep system-wide RSS disabled if this is the case.
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The last but not least thing to consider is the fact that driver support with the in-kernel implementation of RSS is a must. Proper driver support will ensure the correct key
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and indirection table being set in hardware. Drivers which support RSS according to the source code (but mostly untested):
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* em
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* igb -> tested & working
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* axgbe -> tested & working
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* netvsc
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* ixgbe
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* ixl
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* cxgbe
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* lio
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* mlx5
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* sfxge
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**Kernel support**
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=====================================================================================================================
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Internally, FreeBSD uses `netisr <https://www.freebsd.org/cgi/man.cgi?format=html&query=netisr(9)>`__
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as an abstraction layer for dispatching packets to the upper protocols.
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Within the implementation, the default setting is to restrict packet processing to one thread only.
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Since RSS now provides a way to keep flows local to a CPU, the following sysctls should be set in :menuselection:`System->Settings->Tunables`:
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* net.isr.bindthreads = 1
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* net.isr.maxthreads = -1
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The first option causes threads to be bound to a CPU, the next one assigns a workstream to each core available.
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Furthermore, the RSS implementation also provides a few necessary sysctls:
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* net.inet.rss.enabled = 1
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Makes sure RSS is enabled. This is disabled by default to prevent regressions on NICs that do not properly implement the RSS interface.
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* net.inet.rss.bits = X
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This one is dependent on the amount of cores you have.
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By default the amount of bits here represent the amount of cores x 2 in binary.
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This is done on purpose to provide load-balancing, though there is no current implementation for this so we
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recommend setting this value to the amount of bits representing the number of CPU cores.
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This means we use the following values:
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- for 4-core systems, use ‘2’
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- for 8-core systems, use ‘3’
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- for 16-core systems, use ‘4’
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- Etc.
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.. Note::
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Remember to reboot the machine after changing :code:`net.inet.rss.enabled` as this option can only be set during boot.
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**Validate settings**
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=====================================================================================================================
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If RSS is enabled with the 'enabled' sysctl, the packet dispatching policy will move from ‘direct’ to ‘hybrid’. This will directly dispatch a packet on the current context when allowed,
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otherwise it will queue the packet on the bound CPU on which it came in on. Please note that this will increase the interrupt load as seen in ‘top -P’.
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This simply means that packets are being processed with the highest priority in the CPU scheduler - it does not mean the CPU is under more load than normal.
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The correct working of netisr can be verified by running:
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.. code-block::
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netstat -Q
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**Note regarding IPS**
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=====================================================================================================================
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When Suricata is running in IPS mode, Netmap is utilized to fetch packets off the line for inspection. By default, OPNsense has configured Suricata in such a way that the packet which
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has passed inspection will be re-injected into the host networking stack for routing/firewalling purposes. The current Suricata/Netmap implementation limits this re-injection to one thread only.
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Work is underway to address this issue since the new Netmap API (V14+) is now capable of increasing this thread count. Until then, no benefit is gained from RSS when using IPS.
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