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docs: networking: mention that RSS table should be 4x the queue count

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Spell out the recommendation that the RSS table should be
4x the queue count to avoid traffic imbalance. Include minor
rephrasing and removal of the explicit 128 entry example
since a 128 entry table is inadequate on modern machines.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260131225454.1225151-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Kicinski 2026-01-31 14:54:54 -08:00
parent 9e3d4dae98
commit ba9c5611f0
1 changed files with 8 additions and 4 deletions
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12
Documentation/networking/scaling.rst
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@ -38,11 +38,15 @@ that is not the focus of these techniques.
The filter used in RSS is typically a hash function over the network
and/or transport layer headers-- for example, a 4-tuple hash over
IP addresses and TCP ports of a packet. The most common hardware
implementation of RSS uses a 128-entry indirection table where each entry
implementation of RSS uses an indirection table where each entry
stores a queue number. The receive queue for a packet is determined
by masking out the low order seven bits of the computed hash for the
packet (usually a Toeplitz hash), taking this number as a key into the
indirection table and reading the corresponding value.
by indexing the indirection table with the low order bits of the
computed hash for the packet (usually a Toeplitz hash).
The indirection table helps even out the traffic distribution when queue
count is not a power of two. NICs should provide an indirection table
at least 4 times larger than the queue count. 4x table results in ~16%
imbalance between the queues, which is acceptable for most applications.
Some NICs support symmetric RSS hashing where, if the IP (source address,
destination address) and TCP/UDP (source port, destination port) tuples