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From: jfh@rpp386.cactus.org (John F Haugh II)
Newsgroups: comp.unix.wizards
Subject: comp_t fields (was: Re: ccpu)
Message-ID: <19230@rpp386.cactus.org>
Date: 21 Apr 91 19:42:35 GMT
References: <115829.28580@timbuk.cray.com> <16226.280f85ed@levels.sait.edu.au>
Reply-To: jfh@rpp386.cactus.org (John F Haugh II)
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In article <16226.280f85ed@levels.sait.edu.au> xtdn@levels.sait.edu.au writes:
>Ray.Moody@Cray.Com writes:
>> Ccpu is used compute a processes percent CPU utilization.  It is a
>> constant traditionally set to exp(-1/20).  It is unusual in that it is
>> one of the very few floating point numbers in the kernel.
>
>What about the comp_t fields in struct acct?  They, while not floats,
>are apparently floating point fields.

So this doesn't turn into a discussion of floating point fields in the
UNIX kernel ...

The comp_t fields are not "floating point fields" in the traditional
sense.  They are a scaled representation of 32 bit integers that the
system uses for various process accounting information.  The function
for decoding one of these gadgets is

	long
	uncompress(comp_t value)
	{
		return (value & 0x1fff) << ((value & 0xc000) >> 13);
	}

The compress function is slightly more complex, but left as an
exercise for the reader.

As an aside, AIX/370 makes considerable use of floating point in the
kernel, so watch out for those generalizations!
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