Messages in this thread |  | | Date | Sun, 15 Sep 2002 03:16:33 -0300 | From | Werner Almesberger <> | Subject | Re: CBQ implementation issues on linux |
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ashwani kumar mehra wrote: > iii) A late 1999 doc that i found on the web says the dequeue > code is also invoked via qdisc_run_queues (in sch_generic.c), thru > a bottom half handler(net_bh). but am unable to locate it in the > 2.4.18 code. am i missing something obvious or has the same been > reimplemented using tasklets/soft interrupts? maybe a call to > qdisc_restart?
This has changed a little. You may want to have a look at the updated yet unfinished version of that ancient document:
ftp://icaftp.epfl.ch/pub/people/almesber/junk/tc-04FEB2001-0.tar.gz (file tc/tc.ps)
Be warned that this "updated version" is for 2.4.0, so it's not all that up to date anymore. But then, the traffic control code hasn't changed very much in 2.4.
> 1. buffering at driver level: large buffering at driver level may > negate the effect of the scheduler by delaying higher priority > packets. ALTQ uses a token bucket regulator after the scheduler to > counter this problem. however i've been unable to find something > similar or any comment on this issue with regards to linux.
You could use something acting as a shaper (i.e. CBQ or HTB) as the root qdisc, then cram all the other queuing inside it.
> 2. timer granularity issues: a driver will only interrupt after > its buffer gets emptied. however the scheduler may be invoked in > between by trigerring it on events like packet enqueue, expiry of > watchdog timers, etc. In case of timer expiry events, the timer > granularity remains 10 ms, so wont this lead to a bursty dequeue > on the next timer tick?
Yes, it does :-( Increasing HZ helps a little, and so do the opportunistic checks (i.e. trying to dequeue whenever we're handling an interrupt anyway). Unfortunately, the latter don't improve worst-case burstiness, which is an issue if the next network element in the path happens to be particularly picky.
- Werner
-- _________________________________________________________________________ / Werner Almesberger, Buenos Aires, Argentina wa@almesberger.net / /_http://www.almesberger.net/____________________________________________/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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