Re: RCU related performance regression in 3.3

[Pascal Chapperon]

Le 05/05/2012 01:14, Paul E. McKenney a Ãcrit :
> On Fri, May 04, 2012 at 11:41:13PM +0200, Pascal Chapperon wrote:
> > Le 04/05/2012 17:04, Paul E. McKenney a Ãcrit :
> > > On Fri, May 04, 2012 at 04:42:54PM +0200, Pascal Chapperon wrote:
> > > > Le 01/05/2012 17:45, Paul E. McKenney a Ãcrit :
> > > >
> > > > > Here is my RCU_FAST_NO_HZ patch stack on top of v3.4-rc4.
> > > > >
> > > > > Or you can pull branch fnh.2012.05.01a from:
> > > > >
> > > > > 	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
> > > > >
> > > > > 							Thanx, Paul
> > > > I applied your global patch on top of v3.4-rc4. But the slowdown is
> > > > worse than before : boot sequence took 80s instead 20-30s (12s for
> > > > initramfs instead of 2s).
> > > >
> > > > I'll send you rcu tracing log in a second mail.
> > >
> > > Hmmm...  Well, I guess I am glad that I finally did something that
> > > had an effect, but I sure wish that the effect had been in the other
> > > direction!
> > >
> > > Just to make sure I understand: the difference between the 20-30s and
> > > the 80s is exactly the patch I sent you?
> > >
> > > 							Thanx, Paul
> > Yes. Exactly same kernel config as in previous results, I applied
> > your patch against v3.4-rc4, and sorry, the result is exactly what I
> > said;
> > I saw that your global patch was quite huge, and addresses things which
> > are not directly related with the initial patch (commit
> > 7cb92499000e3c86dae653077b1465458a039ef6); maybe a side effect?
> >
> > However, I'm ready to try this patch on my smaller laptop which
> > supports well CONFIG_FAST_NO_HZ=y and systemd, if you think it can
> > help ?
> >
> > Another thought: this issue as nothing to do with i7 Hyper-threading
> > capacities ? (as I test core2duo, Pentium ulv in same conditions and I
> > don't encountered any slowdown ?)
>
> Well, one possibility is that your setup starts the jiffies counter
> at some interesting value.  The attached patch (also against v3.4-rc4)
> applies a bit more paranoia to the initialization to handle this
> and other possibilities.
>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
I tried your new patch against v3.4-rc5 and saw no improvement :
still 75 s. for the boot sequence.

I'll send you the logs in a second mail.

Pascal


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