приоритет qdisc (Priority qdisc)
Имя (Name)
PRIO - Priority qdisc
Синопсис (Synopsis)
tc qdisc ... dev dev ( parent classid | root) [ handle major: ]
prio [ bands bands ] [ priomap band band band... ] [ estimator
interval timeconstant ]
Описание (Description)
The PRIO qdisc is a simple classful queueing discipline that
contains an arbitrary number of classes of differing priority.
The classes are dequeued in numerical descending order of
priority. PRIO is a scheduler and never delays packets - it is a
work-conserving qdisc, though the qdiscs contained in the classes
may not be.
Very useful for lowering latency when there is no need for
slowing down traffic.
Алгоритм (Algorithm)
On creation with 'tc qdisc add', a fixed number of bands is
created. Each band is a class, although is not possible to add
classes with 'tc qdisc add', the number of bands to be created
must instead be specified on the command line attaching PRIO to
its root.
When dequeueing, band 0 is tried first and only if it did not
deliver a packet does PRIO try band 1, and so onwards. Maximum
reliability packets should therefore go to band 0, minimum delay
to band 1 and the rest to band 2.
As the PRIO qdisc itself will have minor number 0, band 0 is
actually major:1, band 1 is major:2, etc. For major, substitute
the major number assigned to the qdisc on 'tc qdisc add' with the
handle parameter.
Классификация (Classification)
Three methods are available to PRIO to determine in which band a
packet will be enqueued.
From userspace
A process with sufficient privileges can encode the
destination class directly with SO_PRIORITY, see
socket(7).
with a tc filter
A tc filter attached to the root qdisc can point traffic
directly to a class
with the priomap
Based on the packet priority, which in turn is derived
from the Type of Service assigned to the packet.
Only the priomap is specific to this qdisc.
Параметры QDISC (QDISC parameters)
bands Number of bands. If changed from the default of 3, priomap
must be updated as well.
priomap
The priomap maps the priority of a packet to a class. The
priority can either be set directly from userspace, or be
derived from the Type of Service of the packet.
Determines how packet priorities, as assigned by the
kernel, map to bands. Mapping occurs based on the TOS
octet of the packet, which looks like this:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| | | |
|PRECEDENCE | TOS |MBZ|
| | | |
+---+---+---+---+---+---+---+---+
The four TOS bits (the 'TOS field') are defined as:
Binary Decimal Meaning
-----------------------------------------
1000 8 Minimize delay (md)
0100 4 Maximize throughput (mt)
0010 2 Maximize reliability (mr)
0001 1 Minimize monetary cost (mmc)
0000 0 Normal Service
As there is 1 bit to the right of these four bits, the
actual value of the TOS field is double the value of the
TOS bits. Tcpdump -v -v shows you the value of the entire
TOS field, not just the four bits. It is the value you see
in the first column of this table:
TOS Bits Means Linux Priority Band
------------------------------------------------------------
0x0 0 Normal Service 0 Best Effort 1
0x2 1 Minimize Monetary Cost 0 Best Effort 1
0x4 2 Maximize Reliability 0 Best Effort 1
0x6 3 mmc+mr 0 Best Effort 1
0x8 4 Maximize Throughput 2 Bulk 2
0xa 5 mmc+mt 2 Bulk 2
0xc 6 mr+mt 2 Bulk 2
0xe 7 mmc+mr+mt 2 Bulk 2
0x10 8 Minimize Delay 6 Interactive 0
0x12 9 mmc+md 6 Interactive 0
0x14 10 mr+md 6 Interactive 0
0x16 11 mmc+mr+md 6 Interactive 0
0x18 12 mt+md 4 Int. Bulk 1
0x1a 13 mmc+mt+md 4 Int. Bulk 1
0x1c 14 mr+mt+md 4 Int. Bulk 1
0x1e 15 mmc+mr+mt+md 4 Int. Bulk 1
The second column contains the value of the relevant four
TOS bits, followed by their translated meaning. For
example, 15 stands for a packet wanting Minimal Monetary
Cost, Maximum Reliability, Maximum Throughput AND Minimum
Delay.
The fourth column lists the way the Linux kernel
interprets the TOS bits, by showing to which Priority they
are mapped.
The last column shows the result of the default priomap.
On the command line, the default priomap looks like this:
1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
This means that priority 4, for example, gets mapped to
band number 1. The priomap also allows you to list higher
priorities (> 7) which do not correspond to TOS mappings,
but which are set by other means.
This table from RFC 1349 (read it for more details)
explains how applications might very well set their TOS
bits:
TELNET 1000 (minimize delay)
FTP
Control 1000 (minimize delay)
Data 0100 (maximize throughput)
TFTP 1000 (minimize delay)
SMTP
Command phase 1000 (minimize delay)
DATA phase 0100 (maximize throughput)
Domain Name Service
UDP Query 1000 (minimize delay)
TCP Query 0000
Zone Transfer 0100 (maximize throughput)
NNTP 0001 (minimize monetary cost)
ICMP
Errors 0000
Requests 0000 (mostly)
Responses <same as request> (mostly)
Классы (Classes)
PRIO classes cannot be configured further - they are
automatically created when the PRIO qdisc is attached. Each class
however can contain yet a further qdisc.
Ошибки (баги) (Bugs)
Large amounts of traffic in the lower bands can cause starvation
of higher bands. Can be prevented by attaching a shaper (for
example, tc-tbf(8) to these bands to make sure they cannot
dominate the link.
