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

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Message-ID: <55A94303.5040805@roeck-us.net>
Date:	Fri, 17 Jul 2015 11:01:39 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	"Odzioba, Lukasz" <lukasz.odzioba@...el.com>,
	Jean Delvare <jdelvare@...e.de>
CC:	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hwmon: coretemp: use list instead of fixed size array
 for temp data

On 07/17/2015 10:28 AM, Odzioba, Lukasz wrote:
> From: Guenter Roeck [mailto:linux@...ck-us.net]
> On Friday, July 17, 2015 6:55 PM Guenter Roeck wrote:
>
>> You don't really explain why your approach would be better than
>> allocating an array of pointers to struct temp_data and increasing
>> its size using krealloc if needed.
>
> Let's consider two cases of such implementation:
> a) we use array of pointers with O(n) access algorithm
> b) we use array of pointers with O(1) access algorithm
>
> In both cases an array will have greater memory footprint unless
> we implement reallocation ourselves when cpus are disabled which will
> make code harder to maintain.

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.

> Case b) does not handle huge core ids and sparse enumeration well -
> it is still to discuss whether we really need it since there is no
> such hardware yet.
>
"yet" is a key term here. Presumably you have insider information.
Unless you can share this information, I don't see the point of
replacing an O(1) algorithm with O(n), especially since there
is a relatively simple alternative available to support more CPUs.

> I am not saying that my solution is the best of possible ones.
> I am saying that "the best" can vary depending on which criteria do you
> choose from (time, memory, clean code...). Some may say that O(n) is
> fine unless we have thousands of cores and this code is not on hot path,
> others may be concerned more about memory on small/old devices.
> I don't see holy grail here, If you see one please let me know.
>

Unless you clarify that Intel will introduce CPU IDs which can not be used
as array index because they are too sparse, I don't really see how the list
solution would consume less memory than an array of pointers, even if the
array is somewhat sparse. After all, a list consumes at least two pointers
per entry.

Thanks,
Guenter

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