linux-kernel - Re: [PATCH v1 0/2] perf,x86: add Intel RAPL PMU support

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Date:	Mon, 7 Oct 2013 18:29:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, ak@...ux.intel.com,
	acme@...hat.com, jolsa@...hat.com, zheng.z.yan@...el.com
Subject: Re: [PATCH v1 0/2] perf,x86: add Intel RAPL PMU support

On Mon, Oct 07, 2013 at 06:09:15PM +0200, Stephane Eranian wrote:
> The counters all count in the same unit. The perf_events API
> exposes all RAPL counters as 64-bit integers counting in unit
> of 1/2^32 Joules (or 0.23 nJ). User level tools must convert
> the counts by multiplying them by 0.23 and divide 10^9 to
> obtain Joules.  The reason for this is that the kernel avoids
> doing floating point math whenever possible because it is
> expensive (user floating-point state must be saved). The method
> used avoids kernel floating-point and minimizes the loss of
> precision (bits). Thanks to PeterZ for suggesting this approach.
> 
> To convert the raw count in Watt: W = C * 0.23 / (1e9 * time)

Right, so the output is in 32.32 fixed point. So if you want to convert
to double you'd do something like: 

  double watt = ldexp(counter, -32);


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