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



NAME
       tc-hfcs - Hierarchical Fair Service Curve

HISTORY & INTRODUCTION
       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  Net-
       BSD,  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 ser-
               vice provided during linksharing

       The main "selling" point of HFSC is feature (1), which is achieved by using nonlinear ser-
       vice 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  exam-
       ples.

       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.


ABBREVIATIONS
       For the remaining part of the document, we'll use following shortcuts:

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

BASICS OF HFSC
       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 speci-
       fied 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:

           o   idling periods

           o   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 ser-
       vice 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  cur-
       rent 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  rephras-
       ing  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:


           o   At the 1st backlogged period beginning function X is initialized to  generic  ser-
               vice curve assigned to a class

           o   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 crite-
               rion  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.

REALTIME CRITERION
       RT  criterion  ignores  class hierarchy and guarantees precise bandwidth and delay alloca-
       tion. 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:

           o   1st class - 2Mbit for 100ms, then 7Mbit (convex - 1st slope < 2nd slope)

           o   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 "buf-
       fer" 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 "participa-
       tion" 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 unde-
       sired 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.


LINKSHARING CRITERION
       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  conclu-
       sion  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 conclu-
       sion 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 pre-
       vious 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):


           o   1st class - 3Mbit for 100ms, then 7Mbit (convex - 1st slope < 2nd slope)

           o   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  sub-
           trees, 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 con-
           stantly 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.

UPPERLIMIT CRITERION
       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. Con-
       sider 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
       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
       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  hierar-
       chy. 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.

LINUX AND TIMER RESOLUTION
       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: 'tick-
       less 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 com-
       piled 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 automat-
       ically 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/inter-
       rupts):

       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 ser-
       vice curve: remember the drawbacks.

CAVEAT: RANDOM ONLINE EXAMPLES
       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 upper-
       most 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

LAYER2 ADAPTATION
       Please refer to tc-stab(8)

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

       Please direct bugreports and patches to: <netdev AT vger.org>

AUTHOR
       Manpage created by Michal Soltys (soltys AT ziu.info)



iproute2                                 31 October 2011                               TC-HFSC(7)

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