tc-fq(8)

NAME

   Fair Queuing (FQ) - Traffic Pacing

SYNOPSIS

   tc  qdisc  ...  fq  [  limit PACKETS ] [ flow_limit PACKETS ] [ quantum
   BYTES ] [ initial_quantum BYTES ] [ maxrate RATE ] [ buckets  NUMBER  ]
   [ pacing | nopacing ]

DESCRIPTION

   FQ (Fair Queue) is a classless packet scheduler meant to be mostly used
   for locally generated traffic.  It is  designed  to  achieve  per  flow
   pacing.   FQ  does  flow  separation,  and  is  able  to respect pacing
   requirements set by TCP stack.  All packets belonging to a  socket  are
   considered  as  a  'flow'.   For  non  local packets (router workload),
   packet rxhash is used as fallback.

   An  application  can  specify  a  maximum   pacing   rate   using   the
   SO_MAX_PACING_RATE  setsockopt  call.  This packet scheduler adds delay
   between packets to respect rate limitation set by TCP stack.

   Dequeueing happens in a round-robin fashion.  A special FIFO  queue  is
   reserved  for  high  priority packets ( TC_PRIO_CONTROL priority), such
   packets are always dequeued first.

   FQ is non-work-conserving.

   TCP pacing is good for flows  having  idle  times,  as  the  congestion
   window  permits  TCP stack to queue a possibly large number of packets.
   This removes the 'slow start after idle' choice,  badly  hitting  large
   BDP  flows  and  applications  delivering  chunks of data such as video
   streams.

PARAMETERS

   limit
   Hard limit on the real queue size. When  this  limit  is  reached,  new
   packets  are  dropped.  If the value is lowered, packets are dropped so
   that the new limit is met. Default is 10000 packets.

   flow_limit
   Hard limit on the maximum number of packets queued per  flow.   Default
   value is 100.

   quantum
   The  credit  per  dequeue  RR round, i.e. the amount of bytes a flow is
   allowed to dequeue at once. A larger value means a longer  time  period
   before  the  next  flow  will  be served.  Default is 2 * interface MTU
   bytes.

   initial_quantum
   The initial sending rate credit, i.e. the amount of bytes a new flow is
   allowed  to  dequeue  initially.   This  is specifically meant to allow
   using IW10 without added delay.  Default is 10 *  interface  MTU,  i.e.
   15140 for 'standard' ethernet.

   maxrate
   Maximum  sending  rate  of  a flow.  Default is unlimited.  Application
   specific setting via SO_MAX_PACING_RATE is ignored only if it is larger
   than this value.

   buckets
   The  size  of  the  hash  table  used  for flow lookups. Each bucket is
   assigned a red-black tree for efficient  collision  sorting.   Default:
   1024.

   [no]pacing
   Enable or disable flow pacing. Default is enabled.

EXAMPLES

   #tc qdisc add dev eth0 root fq
   #tc -s -d qdisc
   qdisc  fq  8003:  dev  eth0  root refcnt 2 limit 10000p flow_limit 100p
   buckets 1024 quantum 3028 initial_quantum 15140
    Sent 503727981 bytes 1146972 pkt (dropped  0,  overlimits  0  requeues
   54452)
    backlog 0b 0p requeues 54452
     1289 flows (1289 inactive, 0 throttled)
     0 gc, 31 highprio, 27411 throttled

SEE ALSO

   tc(8), socket(7)

AUTHORS

   FQ was written by Eric Dumazet.

---

Free and Open Source Software