cvs commit: src/sys/net if.h

[Robert Watson]

On Tue, 12 Jun 2007, Alfred Perlstein wrote:

>>> AFAICT FreeBSD can't currently benefit from this as there is no cpu 
>>> affinity for connections. I may be wrong, but I see lower 
>>> single-connection throughput using a receive queue per core than using a 
>>> single receive queue. RSS is done by hashing a TCP tuple (I'm deliberately 
>>> vague because at least with cxgb there are multiple combinations, the 
>>> default is the standard 4-tuple) to a receive queue.
>>
>> If you're looking at concurrent TCP input processing, the tcbinfo lock is 
>> likely one source of overhead due to high contention.  I had hoped to make 
>> further progress on this for 7.0 (it's already better than 6.0 in a number 
>> of ways), but the instability of 7.x over the last month scuttled that 
>> project. It will have to be an 8.0 thing, but perhaps we can look at an MFC 
>> if that goes well.  I have some initial protyping but have been waiting for 
>> TCP to settle down again a bit before really digging in.
>
> Robert, have you added placeholder fields to objects that require them for 
> support?  This would help the MFC effort.

I'm not yet at a point where I'm comfortable enough with the prototyping to 
believe that I have all the right things to put in.  I've had to refactor the 
pbcinfo structure, for example.  It would also be nice to have multiple timer 
threads so that timers can run on the same CPUs as the tcpcb is normally 
processed on.  This is basically, however, a weak affinity model designed to 
limit lock contention, not eliminate the possibility of cross-CPU execution. 
What we need now is a feedback system so that the scheduler can more 
intelligently weight choices.

Robert N M Watson
Computer Laboratory
University of Cambridge