linux-kernel - Re: [PATCH] Optimize relay_alloc_page_array() slightly by using vzalloc rather than vmalloc and memset

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Date:	Mon, 1 Nov 2010 20:22:37 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Jesper Juhl <jj@...osbits.net>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tom Zanussi <zanussi@...ibm.com>,
	Karim Yaghmour <karim@...rsys.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] Optimize relay_alloc_page_array() slightly by using
 vzalloc rather than vmalloc and memset

On Sun, Oct 31, 2010 at 02:39:14PM -0400, Mathieu Desnoyers wrote:
>* Jesper Juhl (jj@...osbits.net) wrote:
>> On Sat, 30 Oct 2010, Mathieu Desnoyers wrote:
>> 
>> > * Jesper Juhl (jj@...osbits.net) wrote:
>> > > Hi,
>> > > 
>> > > We can optimize kernel/relay.c::relay_alloc_page_array() slightly by using 
>> > > vzalloc. The patch makes these changes:
>> > > 
>> > >  - use vzalloc instead of vmalloc+memset.
>> > >  - remove redundant local variable 'array'.
>> > >  - declare local 'pa_size' as const.
>> > 
>> > Hrm ? How does declaring a local variable as const helps the compiler in
>> > any way ?
>> > 
>> 
>> Hmm, probably not very much in this case (but it doesn't hurt either ;) - 
>> actually, removing the const yielded the exact same result, so it's 
>> "not at all" in this case). 
>> That bit came from my "build-in" tendency to declare stuff const when it 
>> obviously doesn't change/nor should. It's a habbit..
>
>Which looks to me like a misunderstanding of the C99 standard. What you
>do is:
>
>static struct page **relay_alloc_page_array(unsigned int n_pages)
>{
>	const size_t pa_size = n_pages * sizeof(struct page *);
>	...
>}
>
>So the compiler has no choice but to emit code that will fill in the
>value of pa_size at runtime, because it depends on "n_pages", a
>parameter received by the function. So pa_size is everything but
>constant.
>
>The C99 standard, section 6.7.3 (Type qualifiers) states:
>
>"The implementation may place a const object that is not volatile in a
>read-only region of storage. Moreover, the implementation need not
>allocate storage for such an object if its address is never used."
>


This is not enforced by C99. This is C, not C++. :)

>So maybe gcc is kind here and it just removes this const specifier
>without complaining, but a different compiler might be more strict and
>fail to compile because you would be dynamically assigning a value to a
>variable placed in read-only storage.
>

That compiler would be broken if it exists. Also, I doubt linux kernel
could be compiled with other compilers than gcc (except icc?).
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