Initial 6.1 questions
rwatson at FreeBSD.org
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
Universty of Cambridge
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