Weighted round robin

Weighted round robin (WRR) is a scheduling discipline. Each packet flow or connection has its own packet queue in a network interface card. It is the simplest approximation of generalized processor sharing (GPS). While GPS serves infinitesimal amounts of data from each nonempty queue, WRR serves a number of packets for each nonempty queue: number = normalized(weight / meanpacketsize).

Algorithm

WRR mechanism (pseudo-code):

// calculate number of packets to be served each round by connections

min = find smallest weight

for each flow f
   f.packets_to_be_served = f.weight / min

// main loop
loop
   for each non-empty flow queue f
      min(f.packets_to_be_served, f.packets_waiting).times do
         servePacket f.getPacket

Weaknesses

To obtain a normalized set of weights, required to approximate GPS, a mean packet size must be known. In IP networks with their variable packet size the mean has to be estimated which makes good GPS approximation hard to achieve in practice. Another weakness of WRR is that it cannot guarantee fair link sharing.

Deficit round robin (DRR) is a variation of WRR that achieves better GPS approximation without knowing the mean packet size of each connection in advance. There are more effective scheduling disciplines which handle both of these problems mentioned above (e.g. weighted fair queuing (WFQ)).

See also

• scheduling discipline