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TC-HFSC(7) Linux TC-HFSC(7)
tc-hfcs - Hierarchical Fair Service Curve
HFSC (Hierarchical Fair Service Curve) is a network packet
scheduling algorithm that was first presented at SIGCOMM'97.
Developed as a part of ALTQ (ALTernative Queuing) on NetBSD, found
its way quickly to other BSD systems, and then a few years ago
became part of the linux kernel. Still, it's not the most popular
scheduling algorithm - especially if compared to HTB - and it's
not well documented for the enduser. This introduction aims to
explain how HFSC works without using too much math (although some
math it will be inevitable).
In short HFSC aims to:
1) guarantee precise bandwidth and delay allocation for all
leaf classes (realtime criterion)
2) allocate excess bandwidth fairly as specified by class
hierarchy (linkshare & upperlimit criterion)
3) minimize any discrepancy between the service curve and the
actual amount of service provided during linksharing
The main "selling" point of HFSC is feature (1), which is achieved
by using nonlinear service curves (more about what it actually is
later). This is particularly useful in VoIP or games, where not
only a guarantee of consistent bandwidth is important, but also
limiting the initial delay of a data stream. Note that it matters
only for leaf classes (where the actual queues are) - thus class
hierarchy is ignored in the realtime case.
Feature (2) is well, obvious - any algorithm featuring class
hierarchy (such as HTB) strives to achieve that. HFSC does that
well, although you might end with unusual situations, if you
define service curves carelessly - see section CORNER CASES for
examples.
Feature (3) is mentioned due to the nature of the problem. There
may be situations where it's either not possible to guarantee
service of all curves at the same time, and/or it's impossible to
do so fairly. Both will be explained later. Note that this is
mainly related to interior (aka aggregate) classes, as the leafs
are already handled by (1). Still, it's perfectly possible to
create a leaf class without realtime service, and in such a case
the caveats will naturally extend to leaf classes as well.
For the remaining part of the document, we'll use following
shortcuts:
RT - realtime
LS - linkshare
UL - upperlimit
SC - service curve
To understand how HFSC works, we must first introduce a service
curve. Overall, it's a nondecreasing function of some time unit,
returning the amount of service (an allowed or allocated amount of
bandwidth) at some specific point in time. The purpose of it
should be subconsciously obvious: if a class was allowed to
transfer not less than the amount specified by its service curve,
then the service curve is not violated.
Still, we need more elaborate criterion than just the above
(although in the most generic case it can be reduced to it). The
criterion has to take two things into account:
• idling periods
• the ability to "look back", so if during current active
period the service curve is violated, maybe it isn't if we
count excess bandwidth received during earlier active
period(s)
Let's define the criterion as follows:
(1) For each t1, there must exist t0 in set B, so S(t1-t0) <= w(t0,t1)
Here 'w' denotes the amount of service received during some time
period between t0 and t1. B is a set of all times, where a session
becomes active after idling period (further denoted as 'becoming
backlogged'). For a clearer picture, imagine two situations:
a) our session was active during two periods, with a small
time gap between them
b) as in (a), but with a larger gap
Consider (a): if the service received during both periods meets
(1), then all is well. But what if it doesn't do so during the 2nd
period? If the amount of service received during the 1st period is
larger than the service curve, then it might compensate for
smaller service during the 2nd period and the gap - if the gap is
small enough.
If the gap is larger (b) - then it's less likely to happen (unless
the excess bandwidth allocated during the 1st part was really
large). Still, the larger the gap - the less interesting is what
happened in the past (e.g. 10 minutes ago) - what matters is the
current traffic that just started.
From HFSC's perspective, more interesting is answering the
following question: when should we start transferring packets, so
a service curve of a class is not violated. Or rephrasing it: How
much X() amount of service should a session receive by time t, so
the service curve is not violated. Function X() defined as below
is the basic building block of HFSC, used in: eligible, deadline,
virtual-time and fit-time curves. Of course, X() is based on
equation (1) and is defined recursively:
• At the 1st backlogged period beginning function X is
initialized to generic service curve assigned to a class
• At any subsequent backlogged period, X() is:
min(X() from previous period ; w(t0)+S(t-t0) for t>=t0),
... where t0 denotes the beginning of the current
backlogged period.
HFSC uses either linear, or two-piece linear service curves. In
case of linear or two-piece linear convex functions (first slope <
second slope), min() in X's definition reduces to the 2nd
argument. But in case of two-piece concave functions, the 1st
argument might quickly become lesser for some t>=t0. Note, that
for some backlogged period, X() is defined only from that period's
beginning. We also define X^(-1)(w) as smallest t>=t0, for which
X(t) = w. We have to define it this way, as X() is usually not an
injection.
The above generic X() can be one of the following:
E() In realtime criterion, selects packets eligible for
sending. If none are eligible, HFSC will use linkshare
criterion. Eligible time 'et' is calculated with reference
to packets' heads ( et = E^(-1)(w) ). It's based on RT
service curve, but in case of a convex curve, uses its 2nd
slope only.
D() In realtime criterion, selects the most suitable packet
from the ones chosen by E(). Deadline time 'dt'
corresponds to packets' tails (dt = D^(-1)(w+l), where 'l'
is packet's length). Based on RT service curve.
V() In linkshare criterion, arbitrates which packet to send
next. Note that V() is function of a virtual time - see
LINKSHARE CRITERION section for details. Virtual time 'vt'
corresponds to packets' heads (vt = V^(-1)(w)). Based on
LS service curve.
F() An extension to linkshare criterion, used to limit at
which speed linkshare criterion is allowed to dequeue.
Fit-time 'ft' corresponds to packets' heads as well
(ft = F^(-1)(w)). Based on UL service curve.
Be sure to make clean distinction between session's RT, LS and UL
service curves and the above "utility" functions.
RT criterion ignores class hierarchy and guarantees precise
bandwidth and delay allocation. We say that a packet is eligible
for sending, when the current real time is later than the eligible
time of the packet. From all eligible packets, the one most suited
for sending is the one with the shortest deadline time. This
sounds simple, but consider the following example:
Interface 10Mbit, two classes, both with two-piece linear service
curves:
• 1st class - 2Mbit for 100ms, then 7Mbit (convex - 1st
slope < 2nd slope)
• 2nd class - 7Mbit for 100ms, then 2Mbit (concave - 1st
slope > 2nd slope)
Assume for a moment, that we only use D() for both finding
eligible packets, and choosing the most fitting one, thus eligible
time would be computed as D^(-1)(w) and deadline time would be
computed as D^(-1)(w+l). If the 2nd class starts sending packets 1
second after the 1st class, it's of course impossible to guarantee
14Mbit, as the interface capability is only 10Mbit. The only
workaround in this scenario is to allow the 1st class to send the
packets earlier that would normally be allowed. That's where
separate E() comes to help. Putting all the math aside (see HFSC
paper for details), E() for RT concave service curve is just like
D(), but for the RT convex service curve - it's constructed using
only RT service curve's 2nd slope (in our example
7Mbit).
The effect of such E() - packets will be sent earlier, and at the
same time D() will be updated - so the current deadline time
calculated from it will be later. Thus, when the 2nd class starts
sending packets later, both the 1st and the 2nd class will be
eligible, but the 2nd session's deadline time will be smaller and
its packets will be sent first. When the 1st class becomes idle at
some later point, the 2nd class will be able to "buffer" up again
for later active period of the 1st class.
A short remark - in a situation, where the total amount of
bandwidth available on the interface is larger than the allocated
total realtime parts (imagine a 10 Mbit interface, but 1Mbit/2Mbit
and 2Mbit/1Mbit classes), the sole speed of the interface could
suffice to guarantee the times.
Important part of RT criterion is that apart from updating its D()
and E(), also V() used by LS criterion is updated. Generally the
RT criterion is secondary to LS one, and used only if there's a
risk of violating precise realtime requirements. Still, the
"participation" in bandwidth distributed by LS criterion is there,
so V() has to be updated along the way. LS criterion can than
properly compensate for non-ideal fair sharing situation, caused
by RT scheduling. If you use UL service curve its F() will be
updated as well (UL service curve is an extension to LS one - see
UPPERLIMIT CRITERION section).
Anyway - careless specification of LS and RT service curves can
lead to potentially undesired situations (see CORNER CASES for
examples). This wasn't the case in HFSC paper where LS and RT
service curves couldn't be specified separately.
LS criterion's task is to distribute bandwidth according to
specified class hierarchy. Contrary to RT criterion, there're no
comparisons between current real time and virtual time - the
decision is based solely on direct comparison of virtual times of
all active subclasses - the one with the smallest vt wins and gets
scheduled. One immediate conclusion from this fact is that
absolute values don't matter - only ratios between them (so for
example, two children classes with simple linear 1Mbit service
curves will get the same treatment from LS criterion's
perspective, as if they were 5Mbit). The other conclusion is, that
in perfectly fluid system with linear curves, all virtual times
across whole class hierarchy would be equal.
Why is VC defined in term of virtual time (and what is it)?
Imagine an example: class A with two children - A1 and A2, both
with let's say 10Mbit SCs. If A2 is idle, A1 receives all the
bandwidth of A (and update its V() in the process). When A2
becomes active, A1's virtual time is already far later than A2's
one. Considering the type of decision made by LS criterion, A1
would become idle for a long time. We can workaround this
situation by adjusting virtual time of the class becoming active -
we do that by getting such time "up to date". HFSC uses a mean of
the smallest and the biggest virtual time of currently active
children fit for sending. As it's not real time anymore (excluding
trivial case of situation where all classes become active at the
same time, and never become idle), it's called virtual time.
Such approach has its price though. The problem is analogous to
what was presented in previous section and is caused by
non-linearity of service curves:
1) either it's impossible to guarantee service curves and satisfy
fairness during certain time periods:
Recall the example from RT section, slightly modified (with
3Mbit slopes instead of 2Mbit ones):
• 1st class - 3Mbit for 100ms, then 7Mbit (convex - 1st
slope < 2nd slope)
• 2nd class - 7Mbit for 100ms, then 3Mbit (concave - 1st
slope > 2nd slope)
They sum up nicely to 10Mbit - the interface's capacity. But
if we wanted to only use LS for guarantees and fairness - it
simply won't work. In LS context, only V() is used for making
decision which class to schedule. If the 2nd class becomes
active when the 1st one is in its second slope, the fairness
will be preserved - ratio will be 1:1 (7Mbit:7Mbit), but LS
itself is of course unable to guarantee the absolute values
themselves - as it would have to go beyond of what the
interface is capable of.
2) and/or it's impossible to guarantee service curves of all
classes at the same time [fairly or not]:
This is similar to the above case, but a bit more subtle. We
will consider two subtrees, arbitrated by their common (root
here) parent:
R (root) - 10Mbit
A - 7Mbit, then 3Mbit
A1 - 5Mbit, then 2Mbit
A2 - 2Mbit, then 1Mbit
B - 3Mbit, then 7Mbit
R arbitrates between left subtree (A) and right (B). Assume
that A2 and B are constantly backlogged, and at some later
point A1 becomes backlogged (when all other classes are in
their 2nd linear part).
What happens now? B (choice made by R) will always get 7 Mbit
as R is only (obviously) concerned with the ratio between its
direct children. Thus A subtree gets 3Mbit, but its children
would want (at the point when A1 became backlogged) 5Mbit +
1Mbit. That's of course impossible, as they can only get 3Mbit
due to interface limitation.
In the left subtree - we have the same situation as previously
(fair split between A1 and A2, but violated guarantees), but
in the whole tree - there's no fairness (B got 7Mbit, but A1
and A2 have to fit together in 3Mbit) and there's no
guarantees for all classes (only B got what it wanted). Even
if we violated fairness in the A subtree and set A2's service
curve to 0, A1 would still not get the required bandwidth.
UL criterion is an extensions to LS one, that permits sending
packets only if current real time is later than fit-time ('ft').
So the modified LS criterion becomes: choose the smallest virtual
time from all active children, such that fit-time < current real
time also holds. Fit-time is calculated from F(), which is based
on UL service curve. As you can see, its role is kinda similar to
E() used in RT criterion. Also, for obvious reasons - you can't
specify UL service curve without LS one.
The main purpose of the UL service curve is to limit HFSC to
bandwidth available on the upstream router (think adsl home
modem/router, and linux server as NAT/firewall/etc. with 100Mbit+
connection to mentioned modem/router). Typically, it's used to
create a single class directly under root, setting a linear UL
service curve to available bandwidth - and then creating your
class structure from that class downwards. Of course, you're free
to add a UL service curve (linear or not) to any class with LS
criterion.
An important part about the UL service curve is that whenever at
some point in time a class doesn't qualify for linksharing due to
its fit-time, the next time it does qualify it will update its
virtual time to the smallest virtual time of all active children
fit for linksharing. This way, one of the main things the LS
criterion tries to achieve - equality of all virtual times across
whole hierarchy - is preserved (in perfectly fluid system with
only linear curves, all virtual times would be equal).
Without that, 'vt' would lag behind other virtual times, and could
cause problems. Consider an interface with a capacity of 10Mbit,
and the following leaf classes (just in case you're skipping this
text quickly - this example shows behavior that doesn't happen):
A - ls 5.0Mbit
B - ls 2.5Mbit
C - ls 2.5Mbit, ul 2.5Mbit
If B was idle, while A and C were constantly backlogged, A and C
would normally (as far as LS criterion is concerned) divide
bandwidth in 2:1 ratio. But due to UL service curve in place, C
would get at most 2.5Mbit, and A would get the remaining 7.5Mbit.
The longer the backlogged period, the more the virtual times of A
and C would drift apart. If B became backlogged at some later
point in time, its virtual time would be set to
(A's vt + C's vt)/2, thus blocking A from sending any traffic
until B's virtual time catches up with A.
Another difference from the original HFSC paper is that RT and LS
SCs can be specified separately. Moreover, leaf classes are
allowed to have only either RT SC or LS SC. For interior classes,
only LS SCs make sense: any RT SC will be ignored.
Separate service curves for LS and RT criteria can lead to certain
traps that come from "fighting" between ideal linksharing and
enforced realtime guarantees. Those situations didn't exist in
original HFSC paper, where specifying separate LS / RT service
curves was not discussed.
Consider an interface with a 10Mbit capacity, with the following
leaf classes:
A - ls 5.0Mbit, rt 8Mbit
B - ls 2.5Mbit
C - ls 2.5Mbit
Imagine A and C are constantly backlogged. As B is idle, A and C
would divide bandwidth in 2:1 ratio, considering LS service curve
(so in theory - 6.66 and 3.33). Alas RT criterion takes priority,
so A will get 8Mbit and LS will be able to compensate class C for
only 2 Mbit - this will cause discrepancy between virtual times of
A and C.
Assume this situation lasts for a long time with no idle periods,
and suddenly B becomes active. B's virtual time will be updated to
(A's vt + C's vt)/2, effectively landing in the middle between A's
and C's virtual time. The effect - B, having no RT guarantees,
will be punished and will not be allowed to transfer until C's
virtual time catches up.
If the interface had a higher capacity, for example 100Mbit, this
example would behave perfectly fine though.
Let's look a bit closer at the above example - it "cleverly"
invalidates one of the basic things LS criterion tries to achieve
- equality of all virtual times across class hierarchy. Leaf
classes without RT service curves are literally left to their own
fate (governed by messed up virtual times).
Also, it doesn't make much sense. Class A will always be
guaranteed up to 8Mbit, and this is more than any absolute
bandwidth that could happen from its LS criterion (excluding
trivial case of only A being active). If the bandwidth taken by A
is smaller than absolute value from LS criterion, the unused part
will be automatically assigned to other active classes (as A has
idling periods in such case). The only "advantage" is, that even
in case of low bandwidth on average, bursts would be handled at
the speed defined by RT criterion. Still, if extra speed is needed
(e.g. due to latency), non linear service curves should be used in
such case.
In the other words: the LS criterion is meaningless in the above
example.
You can quickly "workaround" it by making sure each leaf class has
RT service curve assigned (thus guaranteeing all of them will get
some bandwidth), but it doesn't make it any more valid.
Keep in mind - if you use nonlinear curves and irregularities
explained above happen only in the first segment, then there's
little wrong with "overusing" RT curve a bit:
A - ls 5.0Mbit, rt 9Mbit/30ms, then 1Mbit
B - ls 2.5Mbit
C - ls 2.5Mbit
Here, the vt of A will "spike" in the initial period, but then A
will never get more than 1Mbit until B & C catch up. Then
everything will be back to normal.
In certain situations, the scheduler can throttle itself and setup
so called watchdog to wakeup dequeue function at some time later.
In case of HFSC it happens when for example no packet is eligible
for scheduling, and UL service curve is used to limit the speed at
which LS criterion is allowed to dequeue packets. It's called
throttling, and accuracy of it is dependent on how the kernel is
compiled.
There're 3 important options in modern kernels, as far as timers'
resolution goes: 'tickless system', 'high resolution timer
support' and 'timer frequency'.
If you have 'tickless system' enabled, then the timer interrupt
will trigger as slowly as possible, but each time a scheduler
throttles itself (or any other part of the kernel needs better
accuracy), the rate will be increased as needed / possible. The
ceiling is either 'timer frequency' if 'high resolution timer
support' is not available or not compiled in, or it's hardware
dependent and can go far beyond the highest 'timer frequency'
setting available.
If 'tickless system' is not enabled, the timer will trigger at a
fixed rate specified by 'timer frequency' - regardless if high
resolution timers are or aren't available.
This is important to keep those settings in mind, as in scenario
like: no tickless, no HR timers, frequency set to 100hz -
throttling accuracy would be at 10ms. It doesn't automatically
mean you would be limited to ~0.8Mbit/s (assuming packets at ~1KB)
- as long as your queues are prepared to cover for timer
inaccuracy. Of course, in case of e.g. locally generated UDP
traffic - appropriate socket size is needed as well. Short example
to make it more understandable (assume hardcore anti-schedule
settings - HZ=100, no HR timers, no tickless):
tc qdisc add dev eth0 root handle 1:0 hfsc default 1
tc class add dev eth0 parent 1:0 classid 1:1 hfsc rt m2 10Mbit
Assuming packet of ~1KB size and HZ=100, that averages to ~0.8Mbit
- anything beyond it (e.g. the above example with specified rate
over 10x larger) will require appropriate queuing and cause bursts
every ~10 ms. As you can imagine, any HFSC's RT guarantees will be
seriously invalidated by that. Aforementioned example is mainly
important if you deal with old hardware - as is particularly
popular for home server chores. Even then, you can easily set
HZ=1000 and have very accurate scheduling for typical adsl speeds.
Anything modern (apic or even hpet msi based timers + 'tickless
system') will provide enough accuracy for superb 1Gbit scheduling.
For example, on one of my cheap dual-core AMD boards I have the
following settings:
tc qdisc add dev eth0 parent root handle 1:0 hfsc default 1
tc class add dev eth0 parent 1:0 classid 1:1 hfsc rt m2 300mbit
And a simple:
nc -u dst.host.com 54321 </dev/zero
nc -l -p 54321 >/dev/null
...will yield the following effects over a period of ~10 seconds
(taken from /proc/interrupts):
319: 42124229 0 HPET_MSI-edge hpet2 (before)
319: 42436214 0 HPET_MSI-edge hpet2 (after 10s.)
That's roughly 31000/s. Now compare it with HZ=1000 setting. The
obvious drawback of it is that cpu load can be rather high with
servicing that many timer interrupts. The example with 300Mbit RT
service curve on 1Gbit link is particularly ugly, as it requires a
lot of throttling with minuscule delays.
Also note that it's just an example showing the capabilities of
current hardware. The above example (essentially a 300Mbit TBF
emulator) is pointless on an internal interface to begin with: you
will pretty much always want a regular LS service curve there, and
in such a scenario HFSC simply doesn't throttle at all.
300Mbit RT service curve (selected columns from mpstat -P ALL 1):
10:56:43 PM CPU %sys %irq %soft %idle
10:56:44 PM all 20.10 6.53 34.67 37.19
10:56:44 PM 0 35.00 0.00 63.00 0.00
10:56:44 PM 1 4.95 12.87 6.93 73.27
So, in the rare case you need those speeds with only a RT service
curve, or with a UL service curve: remember the drawbacks.
For reasons unknown (though well guessed), many examples you can
google love to overuse UL criterion and stuff it in every node
possible. This makes no sense and works against what HFSC tries to
do (and does pretty damn well). Use UL where it makes sense: on
the uppermost node to match upstream router's uplink capacity. Or
in special cases, such as testing (limit certain subtree to some
speed), or customers that must never get more than certain speed.
In the last case you can usually achieve the same by just using a
RT criterion without LS+UL on leaf nodes.
As for the router case - remember it's good to differentiate
between "traffic to router" (remote console, web config, etc.) and
"outgoing traffic", so for example:
tc qdisc add dev eth0 root handle 1:0 hfsc default 0x8002
tc class add dev eth0 parent 1:0 classid 1:999 hfsc rt m2 50Mbit
tc class add dev eth0 parent 1:0 classid 1:1 hfsc ls m2 2Mbit ul m2 2Mbit
... so "internet" tree under 1:1 and "router itself" as 1:999
Please refer to tc-stab(8)
tc(8), tc-hfsc(8), tc-stab(8)
Please direct bugreports and patches to: <netdev@vger.kernel.org>
Manpage created by Michal Soltys (soltys@ziu.info)
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
iproute2 31 October 2011 TC-HFSC(7)
Pages that refer to this page: tc(8), tc-hfsc(8), tc-stab(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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