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

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Date:	Sat, 30 Oct 2010 23:50:04 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	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 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..


> Moreover, is there anyone still using this code ? LTTng uses the Generic
> Ring Buffer library which completely deprecates relay.c. Perf and Ftrace
> each have their own ring buffers, without dependency on relay.c.
> 
> BLK_DEV_IO_TRACE seems to still select RELAY. Has it completed its
> transition to either Ftrace or Perf ? Depending on Jens, moving blktrace
> relay dependency to the Generic Ring Buffer Library might be a good
> option to consider.
> 

I admit I have no idea if this code is actually still used, but as long as 
it's in the kernel I think we should strive to make it as good as possible 
- no? If there are still users this is an improvement, if there are no 
users (who knows if there are out-of-tree ones?) then it should probably 
just go away (but even then this patch does no harm - except for a bit of 
churn).


-- 
Jesper Juhl <jj@...osbits.net>             http://www.chaosbits.net/
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