linux-kernel - sem_otime trashing

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Date:	Sat, 01 Jun 2013 21:02:51 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>
Subject: sem_otime trashing

Hi Rik,

I finally managed to get EFI boot, i.e. I'm now able to test on my i3 
(2core+HT).

With semscale (i.e.: just overhead, perform semop=0 operations), the 
scalability from 1 to 2 cores is good, but not linear:
# semscale 10 | grep "interleave 2"
> Cpus 1, interleave 2 delay 0: 35502103 in 10 secs
> Cpus 2, interleave 2 delay 0: 53990954 in 10 secs
---
  +53% when adding the 2nd core
(interleave 2 to force to use different cores)

Did you consider moving sem_otime into the individual semaphores?
I did that (gross patch attached), and the performance is significantly 
better:

# semscale 10 | grep "interleave 2"
Cpus 1, interleave 2 delay 0: 35585634 in 10 secs
Cpus 2, interleave 2 delay 0: 70410230 in 10 secs
      ---
     +99% scalability when adding the 2nd core

Unfortunately I won't be able to read my mails next week, but the effect 
was too significant not to share it immediately.

--
     Manfred

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