Re: RCU related performance regression in 3.3

From: Pascal Chapperon
Date: Thu May 10 2012 - 04:40:18 EST

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:


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

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
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.


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