Initial 6.1 questions

Robert Watson rwatson at
Mon Jun 12 20:02:52 UTC 2006

On Mon, 12 Jun 2006, Danial Thom wrote:

>> This is a design change that is in the process of being reconsidered.  I 
>> expect that HZ will not be 1000 in 7.x, but can't tell you whether it will 
>> go back to 100, or some middle ground.  There are a number of benefits to a 
>> higher HZ, not least is more accurate timing of some network timer events. 
>> Since I don't have my hands in the timer code, I can't speak to what the 
>> decision process here is, or when any change might happen, but I do expect 
>> to see some change.
> Will anything break if I tweek this downward?

No, shouldn't do.  I wouldn't go below 100 though, as things like process 
statistics, involuntary context switches, etc, are all affected.

>> Finally, there is a known performance problem involving loopback network 
>> traffic and preemption, which results in additional context switches.  You 
>> may want to try disabling preemption and see if/how that impacts your 
>> numbers. There has been seen quite a bit of discussion of this problem, and 
>> I expect to see a solution for it in the near future.  This problem does 
>> not manifest for remote traffic, only loopback traffic.
> I'm sending this traffic from an external device, receiving on an em 
> controller with blackhole set to 1. So I assume this loopback issue doesn't 
> apply to this test?

The above comments only refer to traffic being sent over if_loop interfaces or 
certain other deferred work scenarios. Basically, defering of work to the 
netisr from a user thread rather than an interrupt thread results in a 
premature context switch.

Robert N M Watson
Computer Laboratory
Universty of Cambridge

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