Initial 6.1 questions

Andrew R. Reiter arr at
Mon Jun 12 23:21:09 UTC 2006

Sorry to reply to myself ...

On Mon, 12 Jun 2006, Andrew R. Reiter 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.
::> Kris
::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.
::David Xu
:Given that it seems that various scenarios for locking bottlenecks can 
:occur on various systems with different numbers of CPUs.  Has there been 
:any research done on providing "best fit" profiles for varied N cpu 

Meaning at compile time certain profiles are taken for a given system to 
provide a good effort at providing a "best fit" for locking with their 

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