[LARTC] Transmit queue effect?

[LARTC] Transmit queue effect?

[Ashton Mills]

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Hey all,

Just a quick question -- does anyone know what impact the txquelen as displayed by ifconfig for a nic really has?

I noticed that, with an HTB qdisc installed (which is what, 128 packets by default?), the txqeuelen is still listed as 100 (ip link list). As an experiment I set off some downloads, and reduced the txqeuelen to 0 (ifconfig eth0 txqueuelen 0) with no effect at all on the stream.

According to man ifconfig:

txqueuelen length
              Set the length of the transmit queue of the device. It is useful
              to set this to small values  for  slower  devices  with  a  high
              latency  (modem links, ISDN) to prevent fast bulk transfers from
              disturbing interactive traffic like telnet too much.

Even with the default pfifo_fast qdisc all nics start with, reducing or setting the txqueuelen to 0 seems to have no effect. Should it be having an impact, and if not, then why is it there?

I'm optimising a link for certain streams, and using a pfifo with limit 25 on one of the classes. Wondering if any latency saved here is going to be lost further up the chain.

Regards,

Ashton

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