I have a number of issues with our current locking regime and our
propensity for disabling interrupts. I have in mind some ideas for
reducing interrupt disabling and eliminating scheduling contention
except in the case of one cpu stealing a thread from another cpu's
runqueue. I'll try to dash that off early this evening. This should
also greatly reduce the overhead of timer interrupts.
-Kip
On 6/13/06, Robert Watson <[EMAIL PROTECTED]> wrote:
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
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