Re: [lm-sensors] Discrepancy between reported readings from

From: Jean Delvare <khali@linux-fr.org>
To: lm-sensors@vger.kernel.org
Subject: Re: [lm-sensors] Discrepancy between reported readings from
Date: Mon, 25 Oct 2010 07:16:02 +0000	[thread overview]
Message-ID: <20101025091602.34b08cdc@endymion.delvare> (raw)
In-Reply-To: <AANLkTik8hx=ej8V7ti_=izz0u+daOxxy5PmBjX0ZQv8W@mail.gmail.com>

Hi Arun,

On Sun, 24 Oct 2010 21:40:38 -0400, Arun Raghavan wrote:
> Thanks again for the prompt response. One more question regarding
> coretemp: any idea how I can probe the sensor in my own source code?
> I'm hoping to be able to probe a few times a second, while the
> lm-sensors usage guide says max. probing frequency should be at most
> 0.3-0.5 Hz.

Out of curiosity, which "usage guide" are you referring to?

> I'm trying to measure thermal response and hence
> transients. Temperature rises rather too rapidly for this once every
> 2-3 seconds restriction; there's no intermediate reading between idle
> temperature (~28C) and loaded (~55C). Even when I overclock to
> deliberately induce more dissipation, I don't get an intermediate
> reading between the same base temperature and 70C!

There's no reason why overclocking would help. Overclocking will
generate more heat, but the time it takes to generate it won't change.

> Please let me know if the 2-3s resolution is a limitation of the
> sensors API or the kernel module itself. Any tips on how I might be
> able to measure with greater frequency (apart from hooking up an
> external probe and thermometer)?

There's no limitation at the sensors API level. Each kernel driver has
its own cache lifetime depending on various things, including the
number of registers to read, how fast or slow register access is, and
the driver design.

In the case of the coretemp driver, the 1 second lifetime is pretty
arbitrary, as there are only a few registers to read (one per core) and
register access is very fast (CPU MSRs). Fenghua, Guenter, do you think
it would make sense to shorten the cache lifetime to 0.5 or even 0.25
second?

Arun, back to your immediate issue, you have two options, either
rebuild the coretemp driver with a shorter cache lifetime (replace HZ
with HZ/10 in function coretemp_update_device) or access the MSR
yourself from user-space (but then you lose all the power of
libsensors.)

-- 
Jean Delvare
http://khali.linux-fr.org/wishlist.html

_______________________________________________
lm-sensors mailing list
lm-sensors@lm-sensors.org
http://lists.lm-sensors.org/mailman/listinfo/lm-sensors