linux-kernel - Re: division and cpu usage

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Message-ID: <Pine.LNX.4.64.0708280843030.11442@fbirervta.pbzchgretzou.qr>
Date:	Tue, 28 Aug 2007 08:45:00 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Luka Napotnik <luka.napotnik@...il.com>
cc:	"linux-os (Dick Johnson)" <linux-os@...logic.com>,
	linux-kernel@...r.kernel.org
Subject: Re: division and cpu usage


On Aug 28 2007 00:07, Luka Napotnik wrote:
>>>> 2. I'm trying to get the percentage of CPU used for a certain
>>>> task_struct and figured the following formula:
>>>>
>>>> (task->utime + task->stime) / jiffies
>
>This formula just doesn't work. I have a task with 99% CPU (top) but the
>result of that formula is random (4, sometimes 8). What am I doing
>wrong? Please help.

You are calculating the CPU time the process has used since the start.
That is definitely not the same as the "%CPU" that top shows, more like
the "TIME+" top shows by default.

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