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diff --git a/man/man7/tc-hfsc.7 b/man/man7/tc-hfsc.7 new file mode 100644 index 0000000..72fb5f2 --- /dev/null +++ b/man/man7/tc-hfsc.7 @@ -0,0 +1,563 @@ +.TH "TC\-HFSC" 7 "31 October 2011" iproute2 Linux +.SH "NAME" +tc-hfcs \- Hierarchical Fair Service Curve +. +.SH "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 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: +. +.RS 4 +.IP \fB1)\fR 4 +guarantee precise bandwidth and delay allocation for all leaf classes (realtime +criterion) +.IP \fB2)\fR +allocate excess bandwidth fairly as specified by class hierarchy (linkshare & +upperlimit criterion) +.IP \fB3)\fR +minimize any discrepancy between the service curve and the actual amount of +service provided during linksharing +.RE +.PP +. +The main "selling" point of HFSC is feature \fB(1)\fR, 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 \fB(2)\fR 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 \fB(3)\fR 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 \fB(1)\fR. 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. + +.SH ABBREVIATIONS +For the remaining part of the document, we'll use following shortcuts: +.nf +.RS 4 + +RT \- realtime +LS \- linkshare +UL \- upperlimit +SC \- service curve +.RE +.fi +. +.SH "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 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: +. +.RS 4 +.IP \(bu 4 +idling periods +.IP \(bu +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) +.RE +.PP +Let's define the criterion as follows: +.RS 4 +.nf +.IP "\fB(1)\fR" 4 +For each t1, there must exist t0 in set B, so S(t1\-t0)\~<=\~w(t0,t1) +.fi +.RE +. +.PP +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: +. +.RS 4 +.IP \fBa)\fR 4 +our session was active during two periods, with a small time gap between them +.IP \fBb)\fR +as in (a), but with a larger gap +.RE +. +.PP +Consider \fB(a)\fR: if the service received during both periods meets +\fB(1)\fR, 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 \fIand\fR the gap \- if the gap is small enough. + +If the gap is larger \fB(b)\fR \- 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 +\fB(1)\fR and is defined recursively: + +.RS 4 +.IP \(bu 4 +At the 1st backlogged period beginning function X is initialized to generic +service curve assigned to a class +.IP \(bu +At any subsequent backlogged period, X() is: +.nf +\fBmin(X() from previous period ; w(t0)+S(t\-t0) for t>=t0),\fR +.fi +\&... where t0 denotes the beginning of the current backlogged period. +.RE +. +.PP +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: +. +.RS 4 +.IP "E()" 4 +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, \fIbut in case of a convex curve, uses its 2nd slope only.\fR +.IP "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. +.IP "V()" +In linkshare criterion, arbitrates which packet to send next. Note that V() is +function of a virtual time \- see \fBLINKSHARE CRITERION\fR section for +details. Virtual time \&'vt' corresponds to packets' heads +(vt\~=\~V^(\-1)(w)). Based on LS service curve. +.IP "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. +.RE + +Be sure to make clean distinction between session's RT, LS and UL service +curves and the above "utility" functions. +. +.SH "REALTIME CRITERION" +. +RT criterion \fIignores class hierarchy\fR 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: +.RS 4 +.IP \(bu 4 +1st class \- 2Mbit for 100ms, then 7Mbit (convex \- 1st slope < 2nd slope) +.IP \(bu +2nd class \- 7Mbit for 100ms, then 2Mbit (concave \- 1st slope > 2nd slope) +.RE +.PP +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 \fIonly\fR 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() \fIwill\fR 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 \fIonly\fR 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 \fBUPPERLIMIT CRITERION\fR +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. + +.SH "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 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 +\fIfar\fR 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: +.IP 1) 4 +either it's impossible to guarantee service curves and satisfy fairness +during certain time periods: + +.RS 4 +Recall the example from RT section, slightly modified (with 3Mbit slopes +instead of 2Mbit ones): + +.IP \(bu 4 +1st class \- 3Mbit for 100ms, then 7Mbit (convex \- 1st slope < 2nd slope) +.IP \(bu +2nd class \- 7Mbit for 100ms, then 3Mbit (concave \- 1st slope > 2nd slope) + +.PP +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. +.RE + +.IP 2) 4 +and/or it's impossible to guarantee service curves of all classes at the same +time [fairly or not]: + +.RS 4 + +This is similar to the above case, but a bit more subtle. We will consider two +subtrees, arbitrated by their common (root here) parent: + +.nf +R (root) -\ 10Mbit + +A \- 7Mbit, then 3Mbit +A1 \- 5Mbit, then 2Mbit +A2 \- 2Mbit, then 1Mbit + +B \- 3Mbit, then 7Mbit +.fi + +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 \fIalways\fR 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. +.RE +. +.SH "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. 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 \f(BIdoesn't happen\fR): + +.nf +A \- ls 5.0Mbit +B \- ls 2.5Mbit +C \- ls 2.5Mbit, ul 2.5Mbit +.fi + +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. +. +.SH "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. +. +.SH "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: + +.nf +A \- ls 5.0Mbit, rt 8Mbit +B \- ls 2.5Mbit +C \- ls 2.5Mbit +.fi + +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 \fIonly\fR in the first segment, then there's little wrong with +"overusing" RT curve a bit: + +.nf +A \- ls 5.0Mbit, rt 9Mbit/30ms, then 1Mbit +B \- ls 2.5Mbit +C \- ls 2.5Mbit +.fi + +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. +. +.SH "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: \&'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 \fIfar\fR 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): + +.nf +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 +.fi + +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: + +.nf +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 +.fi + +And a simple: + +.nf +nc \-u dst.host.com 54321 </dev/zero +nc \-l \-p 54321 >/dev/null +.fi + +\&...will yield the following effects over a period of ~10 seconds (taken from +/proc/interrupts): + +.nf +319: 42124229 0 HPET_MSI\-edge hpet2 (before) +319: 42436214 0 HPET_MSI\-edge hpet2 (after 10s.) +.fi + +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): + +.nf +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 +.fi + +So, in the rare case you need those speeds with only a RT service curve, or with a UL +service curve: remember the drawbacks. +. +.SH "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 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: + +.nf +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 +.fi + +\&... so "internet" tree under 1:1 and "router itself" as 1:999 +. +.SH "LAYER2 ADAPTATION" +. +Please refer to \fBtc\-stab\fR(8) +. +.SH "SEE ALSO" +. +\fBtc\fR(8), \fBtc\-hfsc\fR(8), \fBtc\-stab\fR(8) + +Please direct bugreports and patches to: <netdev@vger.kernel.org> +. +.SH "AUTHOR" +. +Manpage created by Michal Soltys (soltys@ziu.info) |