linux-kernel - Re: [PATCH] hwmon: coretemp: use list instead of fixed size array for temp data

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Message-ID: <20150717233338.0ae52805@endymion.delvare>
Date:	Fri, 17 Jul 2015 23:33:38 +0200
From:	Jean Delvare <jdelvare@...e.de>
To:	"Odzioba, Lukasz" <lukasz.odzioba@...el.com>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	"Yu, Fenghua" <fenghua.yu@...el.com>, lm-sensors@...sensors.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: coretemp: use list instead of fixed size array
 for temp data

Hi Lukasz,

On Fri, 17 Jul 2015 19:23:44 +0000, Odzioba, Lukasz wrote:
> On Friday, July 17, 2015 8:02 PM Guenter Roeck wrote:
> > Please explain why krealloc() won't work, why using krealloc(() would
> > result in a larger memory footprint than using lists, and why disabling
> > CPUs would require any action in the first place.
> 
> It will work, but it can use more memory for cpus with many cores.
> If you have just one core visible to the kernel with id 59 
> (i.e. the rest are disabled by hardware) out of 60-core cpu then you
> have to allocate an array of 60 pointers instead of just one element of

Arrays of pointers are cheap. You can fit 512 pointers in a single
memory page.

> the list. Of course you can say that for cpu with just one core list will
> use 3x the memory needed by array and that's true. I see no point in 
> arguing which case is more important, let's move on.

I see the point in arguing: the example above is just silly and does
not match any real-world application. Nobody buys a 60-core CPU to run
it with a single core enabled.

When you have two or more alternative implementations possible,
thinking in terms of the most common cases is a key to make the right
decision. Thinking about servers with a lots of CPU cores versus
embedded devices with few cores and tight memory constraints, that is
useful. Making up corner cases is not.

-- 
Jean Delvare
SUSE L3 Support
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