[WLANnews] Optimierung der Bandbreitenverteilung/Latenz mit fq_codel

[Felix Fietkau]

On 2013-08-25 5:24 PM, Ruben Kelevra wrote:
> Das hier klingt anders:
> 
> "However, today's Linux implementation of CoDel is imperfect: there
> are typically (at least) one or more packets of buffering under the
> Linux qdisc, in the device driver (or one packet in htb) even if BQL
> is available. This means that the "head drop" of CoDel's design is not
> actually a true head drop, but several packets back in in the actual
> queue (since there is no packet loss at the device driver interface),
> and that CoDel's square root computation is not exactly correct. These
> effects are vanishingly small at 1Gbps or higher, but when used at low
> speeds, even one packet of buffering is very significant; today's
> fq_codel and codel qdiscs do not try to compensate for what can be
> significant sojourn time of these packets at low bandwidth. So you
> might have to "tune" the qdiscs in ways (e.g. the target) that in
> principle the CoDel algorithm should not require when used at low
> bandwidths. We hope to get this all straightened out someday soon, but
> knowing exactly how much buffering is under a qdisc is currently
> difficult and it isn't clear when this will happen.
> 
> When running it at 1GigE and lower, today it helps to change a few
> parameters given limitations in today's Linux implementation and
> underlying device drivers."
> 
> Quelle:
> 
> http://www.bufferbloat.net/projects/codel/wiki/Best_Practices_for_Benchmarking_CoDel_and_FQ_CoDel
Das passt doch mit dem zusammen wovon ich gesprochen habe. Die Queue von
der in dem Absatz die Rede ist, ist die Hardware-Queue (*under* Linux
qdisc, nicht 'in').
Diese Hardware-Queue wird von der txqueuelen nicht beeinflusst. Die
txqueuelen bezieht sich auf die maximale qdisc Queue Länge.

- Felix