> Hello!
>
> > Low priority connections can clog the accept queue only when there are no
> > high priority connection requests coming along. As soon as a slot becomes empty
> > in the accept queue, it becomes available for a high priority connection.
>
> And in presence of persistent low priority traffic, high priority connection
> will not have any chances to take this slot. When high priority connection
> arrives all the slots are permanently busy with low ones.
>
> > If that happens, TCP SYN policing can be employed to limit the rate of low
> > priority connections getting into accept queue.
>
The aim of TCP SYN policing is to prevent server overload by discarding
connection requests early when the server system is about to reach overload.
One of the indicators of overload might be that the accept queue is
close to being filled up, there is little CPU time etc.
In these cases, TCP SYN policing should adapt (i.e. lower) the acceptance rates.
In such an adaptive system, the accept queue is not supposed to be completely
filled, thus low priority connections are not able to starve high priority
connections. By the way, different acceptance rates can be given
to different priority classes.
A more detailed discussion than on the website can be found
in the paper "In-kernel mechanisms for adaptive control of
overloaded web servers", available at
http://wwwtgs.cs.utwente.nl/Docs/eunice/summerschool/papers/programme.html
This paper discusses TCP SYN policing and prioritized listen queue.
Cheers,
Thiemo
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