file: /proc/sys/net/core/default_qdisc
variable: net.core.default_qdisc
Official reference

The default queuing discipline to use for network devices. This allows overriding the default of pfifo_fast with an alternative. Since the default queuing discipline is created without additional parameters so is best suited to queuing disciplines that work well without configuration like stochastic fair queue (sfq), CoDel (codel) or fair queue CoDel (fq_codel). Don’t use queuing disciplines like Hierarchical Token Bucket or Deficit Round Robin which require setting up classes and bandwidths. Note that physical multiqueue interfaces still use mq as root qdisc, which in turn uses this default for its leaves. Virtual devices (like e.g. lo or veth) ignore this setting and instead default to noqueue. Default: pfifo_fast