netdev - Re: [PATCH] [RESEND] iproute2 : invalid burst/cburst calculation with hrtimers

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Date:	Thu, 5 Feb 2009 07:38:32 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Denys Fedoryschenko <denys@...p.net.lb>
Cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	netdev@...r.kernel.org
Subject: Re: [PATCH] [RESEND] iproute2 : invalid burst/cburst calculation
	with hrtimers

On Thu, Feb 05, 2009 at 01:24:05AM +0200, Denys Fedoryschenko wrote:
> On Thursday 05 February 2009 00:49:29 Jarek Poplawski wrote:
> > On Thu, Feb 05, 2009 at 12:26:46AM +0200, Denys Fedoryschenko wrote:
> > ...
> >
> > > Maybe we can put note in documentation or even htb help output, that user
> > > can increase precision with hrtimers enabled, by setting HZ variable?
> >
> > Hmm... I rather meant using burst and cburst params. But hrtimers are
> > very hardware dependent, so it's hard to advise anything.
> >
> > Jarek P.
> 
> By default burst/cburst calculated depends on timer resolution, as i 
> understood, by taking 4th parameter of /proc/net/psched, which as i 
> understood - precision of hrtimer .
> 
> With hrtimers it is nanosecond resolution, precision is very high, but 
> overhead too high too. And as i understand, maybe it can be problem, that htb 
> still uses HZ variable in some cases like:
> 	return q->now + 2 * PSCHED_TICKS_PER_SEC / HZ;

I think HZ doesn't matter much in this place, because if it's ever
triggered there is something really wrong and precision is lost.
Anyway, this place was changed a bit in net-next.

I guess I could misunderstand your previous question. If you mean this
previous thread, then yes, IMHO CONFIG_HZ should matter for precision
(especially places where similar loops use all such jiffies in "normal"
situations). On the other hand, maybe I'm wrong, but using HZ 1000
with HIGH_RES_TIMERS looks quite obvious to me...

Jarek P.
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