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Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected
From: Mike Galbraith <efault@gmx.de>
To: richard -rw- weinberger <richard.weinberger@gmail.com>
Cc: Andi Kleen <andi@firstfloor.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>
Date: Sun, 16 Sep 2012 06:48:12 +0200
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On Sat, 2012-09-15 at 19:08 +0200, richard -rw- weinberger wrote: 
> On Sat, Sep 15, 2012 at 6:36 PM, Mike Galbraith <efault@gmx.de> wrote:
> >> Just using futex is unfortunately not the answer either.
> >
> > Yes, postgress performs loads better with it's spinlocks, but due to
> > that, it necessarily _hates_ preemption.  How the is the scheduler
> > supposed to know that any specific userland task _really_ shouldn't be
> > preempted at any specific time, else bad things follow?
> 
> Why perform custom userspace spinlocks better than futex() based ones?
> I thought we have futex() to get rid of the custom ones...
> Makes futex() only sense when things like priority inheritance are needed?

Dunno.  Likely because data doesn't go cold when you spin a bit, but go
to sleep and the next guy may stomp cache flat with size XXL boots.

-Mike