On 3/2/07, Patrick McHardy <[EMAIL PROTECTED]> 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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