netdev - Re: Netem tfifo implementation

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Date:	Fri, 2 Mar 2007 15:56:54 -0500
From:	"Ritesh Kumar" <ritesh@...unc.edu>
To:	"Patrick McHardy" <kaber@...sh.net>
Cc:	netdev@...r.kernel.org
Subject: Re: Netem tfifo implementation

On 3/2/07, Patrick McHardy <kaber@...sh.net> wrote:
> Ritesh Kumar wrote:
> > Hi,
> >    I recently saw the qdisc "tfifo" in the netem module
> > (net/sched/sch_netem.c) when I migrated some of my patches from 2.6.14
> > to 2.6.20. As I understand, tfifo helps in keeping the queue of
> > packets sorted according to their "time_to_send". [tfifo was not
> > present in 2.6.14 perhaps because arrival order of packets was always
> > equal to the departure order]. However, tfifo uses a linear search in
> > the packet queue to find where to enqueue the packet.
> >    Quite some time ago (2.6.14 era), I needed a similar functionality
> > from the netem module and I ended up coding a pointer based min-heap
> > for the same. I was wondering if the community was interested in using
> > the min-heap implementation to replace the linear search
> > implementation. I have tested the min-heap quite a few times and it
> > seems to work.
> >    The implementation is slightly non-trivial because it uses
> > pointers to maintain the heap structure instead if using good old
> > fixed size arrays. I did this mainly so that the limit of the netem
> > qdisc could be changed on the fly. However, because every sk_buff now
> > needs two pointers for its children nodes, I added an extra
> > (sk_buff*)next2 to struct sk_buff (sorry!). However, this can probably
> > be changed to a pointer inside netem_skb_cb.  Also, because I needed
> > this for personal work and 2.6.14 didn't contain tfifo, I basically
> > removed the embedded qdisc and made netem a classless qdisc with my
> > min heap as the native "queue" (sorry again! :) )
>
> The tfifo qdisc has a limit, why not just allocate a fixed-size heap
> based on that?
>
>

The tfifo queue limit itself can be changed and that creates the
problem. If we use a fixed heap (say implemented using a fixed size
array) then we will have to copy over all pointers from the first
array to a reallocated array whenever the queue limit is changed.
In retrospect, moving just a few 10s of kilobytes of data doesn't seem
that much of a problem... now I feel stupid having put so much effort
:).

Ritesh
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