tc-hfcs(7) — Linux manual page


TC-HFSC(7)                          Linux                         TC-HFSC(7)

NAME         top

       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 or CBQ) 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

       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

           RT - realtime
           LS - linkshare
           UL - upperlimit
           SC - service curve

BASICS OF HFSC         top

       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

       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

       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

           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

           ·   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

       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

       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)

           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

           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.

SEPARATE LS / RT SCs         top

       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.

CORNER CASES         top

       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

       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

       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 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)

SEE ALSO         top

       tc(8), tc-hfsc(8), tc-stab(8)

       Please direct bugreports and patches to: <>

AUTHOR         top

       Manpage created by Michal Soltys (

COLOPHON         top

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iproute2                       31 October 2011                    TC-HFSC(7)