netdev - Re: [PATCH iproute2 1/2] tc_core: Use double in tc_core

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Message-ID: <4A2E6BA4.30002@trash.net>
Date:	Tue, 09 Jun 2009 16:03:16 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Jarek Poplawski <jarkao2@...il.com>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	Antonio Almeida <vexwek@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Martin Devera <devik@....cz>,
	Eric Dumazet <dada1@...mosbay.com>,
	Vladimir Ivashchenko <hazard@...ncoudi.com>,
	Badalian Vyacheslav <slavon@...telecom.ru>
Subject: Re: [PATCH iproute2 1/2] tc_core: Use double in tc_core_time2tick()

Jarek Poplawski wrote:
> Change 'time' parameters of tc_core_time2tick() and tc_core_time2big()
> from unsigned to double. It is especially needed to use in
> tc_calc_rtable() for kernels with increased psched ticks resolution,
> but even without this, it looks reasonable to avoid rounding here.

I would prefer to keep using integers, which are a lot easier to
analyze for rounding errors and overflows, besides having constant
granularity.

> -unsigned tc_core_time2tick(unsigned time)
> +unsigned tc_core_time2tick(double time)
>  {
> -	return time*tick_in_usec;
> +	return time * tick_in_usec + 0.5;

ceil()?

>  }

It seems inconsistent to have the time2tick() function take a
double, but return an unsigned from tick2time(). If we're going
to change this, please keep them symetrical (you could even use
floor() in tick2time() to make it more explicit).

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