machdep.cpu_idle_hlt and SMP perf?

[Andrew Gallatin]

John Baldwin writes:
 > On Tuesday 07 February 2006 17:15, Andrew Gallatin wrote:
 > > John Baldwin writes:
 > >  > On Monday 06 February 2006 17:37, Andrew Gallatin wrote:
 > >  > > John Baldwin writes:
 > >  > >  > On Monday 06 February 2006 14:46, Andrew Gallatin wrote:
 > >  > >  > > Andre Oppermann writes:
 > >  > >  > >  > Andrew Gallatin wrote:
 > >  > >  > >  > > Why dooes machdep.cpu_idle_hlt=1 drop my 10GbE network rx
 > >  > >  > >  > > performance by a considerable amount (7.5Gbs -> 5.5Gbs)?
 > >  > >  >
 > >  > >  > You may be seeing problems because it might simply take a while for
 > >  > >  > the CPU to wake up from HLT when an interrupt comes in.  The 4BSD
 > >  > >  > scheduler tries to do IPIs to wakeup any sleeping CPUs when it
 > >  > >  > schedules a new thread, but that would add higher latency for
 > >  > >  > ithreads than just preempting directly to the ithread.  Oh, you
 > >  > >  > have to turn that on, it's off by default
 > >  > >  > (kern.sched.ipiwakeup.enabled=1).
 > >  > >
 > >  > > Hmm..  It seems to be on by default.  Unfortunately, it does not seem
 > >  > > to help.
 > >  >
 > >  > I'm not sure.
 > >
 > > One thing which really helps is disabling preemption.  If I do that,
 > > I get 7.7Gb/sec with machdep.cpu_idle_hlt=1.  This is slightly better
 > > than machdep.cpu_idle_hlt=0 and no PREEMPTION.
 > >
 > > BTW, net.isr.direct=1 in all testing.
 > 
 > Do you have very little userland activity in this test?

Essentially none.  netserver just sits in a loop, reading from the
socket and throwing the data away.

Drew