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tc: man: update sfq man page 

Document 'divisor' option and mention that external classifiers can be used.
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Florian Westphal 2011-08-01 11:23:37 +00:00 committed by Stephen Hemminger
parent b1978178fa
commit 610b22a30f
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31
man/man8/tc-sfq.8
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@ -2,7 +2,12 @@
.SH NAME
sfq \- Stochastic Fairness Queueing
.SH SYNOPSIS
.B tc qdisc ... perturb
.B tc qdisc ...
.B divisor
hashtablesize
.B limit
packets
.B perturb
seconds
.B quantum
bytes
@ -22,7 +27,13 @@ This may in fact have some effect in mitigating a Denial of Service attempt.
SFQ is work-conserving and therefore always delivers a packet if it has one available.
.SH ALGORITHM
On enqueueing, each packet is assigned to a hash bucket, based on
On enqueueing, each packet is assigned to a hash bucket, based on the packets hash value.
This hash value is either obtained from an external flow classifier (use
.B
tc filter
to set them), or a default internal classifier if no external classifier has been configured.
When the internal classifier is used, sfq uses
.TP
(i)
Source address
@ -39,7 +50,7 @@ destination and the socket they belong to. A flow corresponds mostly to a TCP/IP
connection.
Each of these buckets should represent a unique flow. Because multiple flows may
get hashed to the same bucket, the hashing algorithm is perturbed at configurable
get hashed to the same bucket, sfqs internal hashing algorithm may be perturbed at configurable
intervals so that the unfairness lasts only for a short while. Perturbation may
however cause some inadvertent packet reordering to occur.
@ -50,6 +61,11 @@ at most 128 buckets of 1024 available. In case of overflow, tail-drop is perform
on the fullest bucket, thus maintaining fairness.
.SH PARAMETERS
.TP
divisor
Can be used to set a different hash table size, available from kernel 2.6.39 onwards.
The specified divisor must be a power of two and cannot be larger than 65536.
Default value: 1024.
.TP
limit
Upper limit of the SFQ. Can be used to reduce the default length of 128 packets.
@ -58,6 +74,7 @@ perturb
Interval in seconds for queue algorithm perturbation. Defaults to 0, which means that
no perturbation occurs. Do not set too low for each perturbation may cause some packet
reordering. Advised value: 10
This value has no effect when external flow classification is used.
.TP
quantum
Amount of bytes a flow is allowed to dequeue during a round of the round robin process.
@ -83,6 +100,14 @@ available for scheduling.
.P
Embed SFQ in a classful qdisc to make sure it owns the queue.
It is possible to use external classifiers with sfq, for example to hash traffic based only
on source/destination ip addresses:
.P
# tc filter add ... flow hash keys src,dst perturb 30 divisor 1024
Note that the given divisor should match the one used by sfq. If you have
changed the sfq default of 1024, use the same value for the flow hash filter, too.
.SH SOURCE
.TP
o