[PATH] ALTQ(9) codel algorithm implementation

[Julian Elischer]

On 6/14/13 6:08 PM, Luigi Rizzo wrote:
> On Fri, Jun 14, 2013 at 01:51:25PM +0400, Gleb Smirnoff wrote:
>>    Ermal,
> ...
>> I'm afraid we can't grow mbuf packet header with 8 bytes just to satisfy
>> the ALTQ codel algo, which would definitely have a limited usage among
>> FreeBSD users. Thus, "enqueue_time" should go into mbuf_tags(9) not into
>> mbuf packet header.
in -current there are currently 2 pad bytes as I just grew it by 16 
bits and wanted to get it 32 bit aligned. is there something you can 
do with 2 bytes to make the overhead less than a tag?
tags are not as expensive as they seem however (last I checked).


> not to take positions one way or the other, but getting and releasing
> a tag on every packet is going to kill performance.
>
> If i remember well, 2-3 years ago at bsdcan there was discussion
> (and mention of some pending work, jeffr maybe ?)
> on providing some leading space in the mbuf so one could put there
> tags (e.g. ipfw and dummynet ones) without having to allocate them.
> Not sure where is this.

yes we discussed this and nothing came of it but it's still a valid 
point of discussion.
>
> The other thing with codel is that it needs a fairly coarse
> timer resolution (100us..1ms) to work so one might be happy
> with shorter timestamps (e.g. 4 bytes) if space allows them.
>
> cheers
> luigi
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