linux-kernel - Re: [PATCH 1/2] lib: Provide generic atomic64

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Message-ID: <4A34F564.2010500@redhat.com>
Date:	Sun, 14 Jun 2009 16:04:36 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Paul Mackerras <paulus@...ba.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	benh@...nel.crashing.org, akpm@...ux-foundation.org,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] lib: Provide generic atomic64_t implementation

Paul Mackerras wrote:
> Avi Kivity writes:
>
>   
>> An alternative implementation using 64-bit cmpxchg will recover most of 
>> the costs of hashed spinlocks.  I assume most serious 32-bit 
>> architectures have them?
>>     
>
> Have a 64-bit cmpxchg, you mean?  x86 is the only one I know of, and
> it already has an atomic64_t implementation using cmpxchg8b (or
> whatever it's called).
>   

Yes (and it is cmpxchg8b).  I'm surprised powerpc doesn't have DCAS support.

> My thinking is that the 32-bit non-x86 architectures will be mostly
> UP, so the overhead is just an interrupt enable/restore.  Those that
> are SMP I would expect to be small SMP -- mostly just 2 cpus and maybe
> a few 4-way systems.
>   

The new Nehalems provide 8 logical threads in a single socket.  All 
those threads share a cache, and they have cmpxchg8b anyway, so this 
won't matter.

-- 
error compiling committee.c: too many arguments to function

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