linux-kernel - Re: semaphore and mutex in current Linux kernel (3.2.2)

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Date:	Sun, 1 Apr 2012 14:19:43 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Chen, Dennis (SRDC SW)" <Dennis1.Chen@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>
Subject: Re: semaphore and mutex in current Linux kernel (3.2.2)


* Chen, Dennis (SRDC SW) <Dennis1.Chen@....com> wrote:

> Documentation/mutex-design.txt:
> 
> "- 'struct mutex' is smaller on most architectures: E.g. on x86,
>    'struct semaphore' is 20 bytes, 'struct mutex' is 16 bytes.
>    A smaller structure size means less RAM footprint, and better
>    CPU-cache utilization."
> ================================================================
>
> Now in my x86-64 32-bit Linux environment, 'struct semaphone' 
> is 16 bytes, 'struct mutex' is 20 bytes. So seems the RAM 
> footprint advantages are not there...

It got larger due to the adaptive spin-mutex performance 
optimization.

> For the performance advantages followed, I don't have the 
> ./test-mutex and maybe the testing environment, so haven't the 
> 1st hand data for this item...

Well, a way to reproduce that would be to find a lock_mutex 
intense workload ('perf top -g', etc.), and then changing back 
the underlying mutex to a semaphore, and measure the performance 
of the two primitives.

Thanks,

	Ingo
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