Initial 6.1 questions

[Robert Watson]

On Tue, 13 Jun 2006, David Xu wrote:

> On Tuesday 13 June 2006 04:32, Kris Kennaway wrote:
>> On Mon, Jun 12, 2006 at 09:08:12PM +0100, Robert Watson wrote:
>>> On Mon, 12 Jun 2006, Scott Long wrote:
>>>> I run a number of high-load production systems that do a lot of network
>>>> and filesystem activity, all with HZ set to 100.  It has also been shown
>>>> in the past that certain things in the network area where not fixed to
>>>> deal with a high HZ value, so it's possible that it's even more
>>>> stable/reliable with an HZ value of 100.
>>>>
>>>> My personal opinion is that HZ should gop back down to 100 in 7-CURRENT 
>>>> immediately, and only be incremented back up when/if it's proven to be 
>>>> the right thing to do. And, I say that as someone who (errantly) pushed 
>>>> for the increase to 1000 several years ago.
>>>
>>> I think it's probably a good idea to do it sooner rather than later.  It 
>>> may slightly negatively impact some services that rely on frequent timers 
>>> to do things like retransmit timing and the like.  But I haven't done any 
>>> measurements.
>>
>> As you know, but for the benefit of the list, restoring HZ=100 is often an 
>> important performance tweak on SMP systems with many CPUs because of all 
>> the sched_lock activity from statclock/hardclock, which scales with HZ and 
>> NCPUS.
>
> sched_lock is another big bottleneck, since if you 32 CPUs, in theory you 
> have 32X context switch speed, but now it still has only 1X speed, and there 
> are code abusing sched_lock, the M:N bits dynamically inserts a thread into 
> thread list at context switch time, this is a bug, this causes thread list 
> in a proc has to be protected by scheduler lock, and delivering a signal to 
> process has to hold scheduler lock and find a thread, if the proc has many 
> threads, this will introduce long scheduler latency, a proc lock is not 
> enough to find a thread, this is a bug, there are other code abusing 
> scheduler lock which really can use its own lock.

I've added Kip Macy to the CC, who is working with a patch for Sun4v that 
eliminates sched_lock.  Maybe he can comment some more on this thread?

Robert N M Watson
Computer Laboratory
Universty of Cambridge