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[libvirt] [PATCH v1 11/11] bandwidth: Add documentation



This approach implemented in previous patches is not trivial
and deserves small description.
---
 src/util/virnetdevbandwidth.c |   68 +++++++++++++++++++++++++++++++++++++---
 1 files changed, 62 insertions(+), 6 deletions(-)

diff --git a/src/util/virnetdevbandwidth.c b/src/util/virnetdevbandwidth.c
index b4ffc29..d32c7db 100644
--- a/src/util/virnetdevbandwidth.c
+++ b/src/util/virnetdevbandwidth.c
@@ -92,13 +92,69 @@ virNetDevBandwidthSet(const char *ifname,
         if (virCommandRun(cmd, NULL) < 0)
             goto cleanup;
 
+        /* If we are creating hierarchical class, all non guaranteed traffic
+         * goes to 1:2 class which will adjust 'rate' dynamically as NICs with
+         * guaranteed throughput are plugged and unplugged. Class 1:1 is there
+         * so we don't exceed the maximum limit for network. For each NIC with
+         * guaranteed throughput a separate classid will be created.
+         * NB '1:' is just a shorter notation of '1:0'.
+         *
+         * To get a picture how this works:
+         *
+         * +-----+     +---------+     +-----------+      +-----------+     +-----+
+         * |     |     |  qdisc  |     | class 1:1 |      | class 1:2 |     |     |
+         * | NIC |     | def 1:2 |     |   rate    |      |   rate    |     | sfq |
+         * |     | --> |         | --> |   peak    | -+-> |   peak    | --> |     |
+         * +-----+     +---------+     +-----------+  |   +-----------+     +-----+
+         *                                            |
+         *                                            |   +-----------+     +-----+
+         *                                            |   | class 1:3 |     |     |
+         *                                            |   |   rate    |     | sfq |
+         *                                            +-> |   peak    | --> |     |
+         *                                            |   +-----------+     +-----+
+         *                                           ...
+         *                                            |   +-----------+     +-----+
+         *                                            |   | class 1:n |     |     |
+         *                                            |   |   rate    |     | sfq |
+         *                                            +-> |   peak    | --> |     |
+         *                                                +-----------+     +-----+
+         *
+         * After the routing decision, when is it clear a packet is to be send
+         * via NIC, it is sent to root qdisc (queueing discipline). In this case
+         * HTB (Hierarchical Token Bucket). It has only one direct child class
+         * (with id 1:1) which shapes the overall rate that is sent through NIC.
+         * This class have at least one child (1:2). This is meant for whole
+         * non-privileged (non guaranteed) traffic from all domains. Then, for
+         * each interface with guaranteed throughput a separate class (1:n) is
+         * created. Imagine a class is a box. Whenever a packet ends up in a
+         * class it is stored in this box until a kernel sends it in which case
+         * it is removed from box. Packets are placed into boxes based on rules
+         * (filters) - e.g. depending on destination IP/MAC address. If there is
+         * no rule to be applied, root qdisc have a default where such packets
+         * go (1:2 in this case). Packets come in over and over again and boxes
+         * get filled more and more. Imagine that kernel sends packets just once
+         * a second. So it starts to traverse through this tree. It starts with
+         * root qdisc and over 1:1 it gets to 1:2. It sends packets up to its
+         * 'rate'. Then it takes 1:3 and again sends packets up to its 'rate'.
+         * And the whole process is repeated until 1:n is processed. So now we
+         * have ensured each class its guaranteed bandwidth. If the sum of sent
+         * data doesn't exceed 'rate' in 1:1 class, we can go further and send
+         * more packets. The rest of available bandwidth is distributed to
+         * 1:2,1:3...1:n classes by ratio of their 'rate'. As soon as root
+         * 'rate' limit is reached or there are no more packets to send, we stop
+         * sending and wait another second. Each class has SFQ qdisc which
+         * shuffles packets in boxes stochastically, so one sender could not
+         * starve others.
+         *
+         * Therefore, whenever we want to plug a new guaranteed interface, we
+         * need to create a new class and adjust 'rate' of 1:2 class. When
+         * unplugging we do the exact opposite - remove associated class, and
+         * adjust the 'rate'.
+         *
+         * This description is rather longer and you'd better read it before you
+         * start digging into this :)
+         */
         if (hierarchical_class) {
-            /* If we are creating hierarchical class, all non guaranteed traffic
-             * goes to 1:2 class which will adjust 'rate' dynamically as NICs with
-             * guaranteed throughput are plugged and unplugged. Class 1:1 is there
-             * so we don't exceed the maximum limit for network. For each NIC with
-             * guaranteed throughput a separate classid will be created.
-             * NB '1:' is just a shorter notation of '1:0' */
             virCommandFree(cmd);
             cmd = virCommandNew(TC);
             virCommandAddArgList(cmd, "class", "add", "dev", ifname, "parent",
-- 
1.7.8.6


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