Re: [PATCH net-next 1/2] netem: rate-latency extension

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 24 novembre 2011 à 18:39 +0100, Hagen Paul Pfeifer a écrit :
> Currently netem is not in the ability to emulate channel bandwidth. Only static
> delay (and optional random jitter) can be configured.
> 
> To emulate the channel rate the token bucket filter (sch_tbf) can be used.  But
> TBF has some major emulation flaws. The buffer (token bucket depth/rate) cannot
> be 0. Also the idea behind TBF is that the credit (token in buckets) fills if
> no packet is transmitted. So that there is always a "positive" credit for new
> packets. In real life this behavior contradicts the law of nature where
> nothing can travel faster as speed of light. E.g.: on an emulated 1000 byte/s
> link a small IPv4/TCP SYN packet with ~50 byte require ~0.05 seconds - not 0
> seconds.
> 
> Netem is an excellent place to implement a rate limiting feature: static
> delay is already implemented, tfifo already has time information and the
> user can skip TBF configuration completely.
> 
> This patch implement rate latency feature which can be configured via
> tc. e.g:
> 
> 	tc qdisc add dev eth0 root netem ratelatency 10kbit
> 
> To emulate a link of 5000byte/s and add an additional static delay of 10ms:
> 
> 	tc qdisc add dev eth0 root netem delay 10ms ratelatency 5KBps
> 
> Note: similar to TBF the rate-latency extension is bounded to the kernel timing
> system. Depending on the architecture timer granularity, higher rates (e.g.
> 10mbit/s and higher) tend to transmission bursts. Also note: further queues
> living in network adaptors; see ethtool(8).
> 
> Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
> ---
>  include/linux/pkt_sched.h |    5 +++++
>  net/sched/sch_netem.c     |   40 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 45 insertions(+), 0 deletions(-)

I like this patch, this is a useful extension.

Only point is why you chose ratelatency instead of rate ?

We want to emulate a real link, and yes, a 1000 bytes packet must be
delayed _before_ we deliver it to the device, but its a detail of how
works netem.

The usual word we use to describe a 1Mbps link is "1Mbps rate" ;)