Re: Mainline kernel OLTP performance update

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Jan 15, 2009 at 09:44:42AM -0800, Andrew Morton wrote:
> > Me too.  Anecdotally, I haven't noticed this in my lab machines, but
> > what I have noticed is on someone else's laptop (a hyperthreaded atom)
> > that I was trying to demo powertop on was that IPI reschedule interrupts
> > seem to be out of control ... they were ticking over at a really high
> > rate and preventing the CPU from spending much time in the low C and P
> > states.  To me this implicates some scheduler problem since that's the
> > primary producer of IPI reschedules ... I think it wouldn't be a
> > significant extrapolation to predict that the scheduler might be the
> > cause of the above problem as well.
> > 
> 
> Good point.
> 
> The context switch rate actually went down a bit.
> 
> I wonder if the Intel test people have records of /proc/interrupts for
> the various kernel versions.

I think Chinang does, but he's out of office today.  He did say in an
earlier reply:

> I took a quick look at the interrupts figure between 2.6.24 and 2.6.27.
> i/o interuputs is slightly down in 2.6.27 (due to reduce throughput).
> But both NMI and reschedule interrupt increased.  Reschedule interrupts
> is 2x of 2.6.24.

So if the reschedule interrupt is happening twice as often, and the
context switch rate is basically unchanged, I guess that means the
scheduler is doing a lot more work to get approximately the same
results.  And that seems like a bad thing.

Again, it's worth bearing in mind that these are all RT tasks, so the
underlying problem may be very different from the one that both James and
I have observed with an Atom laptop running predominantly non-RT tasks.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."