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From: bert hubert <ahu@ds9a.nl>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] HTB not that exact
Date: Sat, 08 Dec 2001 11:52:15 +0000	[thread overview]
Message-ID: <marc-lartc-100781240318403@msgid-missing> (raw)
In-Reply-To: <marc-lartc-100766696815760@msgid-missing>

On Fri, Dec 07, 2001 at 06:57:39PM +0200, Sami Farin wrote:
> Ahu wrote:
> > Both I think. There are some indications that raising HZ may not work, as it
> > may not raise the resolution of the timers in the Linux Kernel - I am
> > investigating this! So try keeping HZ stable, and raising the size of your
> > bucket.
> 
> would it  make any difference if you do...

Ok - I've since investigated this. Shapers use timers internally to block
themselves for set periods of time. These timers appear to have HZ
resolution, but a comment in the very good Linux Device Drivers book
                    http://www.xml.com/ldd/chapter/book/
appeared to indicate that raising HZ would not raise their resolution.

I contacted Jonathan Corbet, one of the authors, and he checked and found
that the book was incorrect: raising HZ *does* increase timer resolution. 

> -#define PSCHED_CLOCK_SOURCE    PSCHED_JIFFIES
> +#define PSCHED_CLOCK_SOURCE    PSCHED_CPU
> 
> I didn't see PSCHED_CLOCK_SOURCE in the 2.4routing docs...
> Can you tell more about PSCHED_CLOCK_SOURCE ?

As far as I can see, this only increases the *resolution*, not *precision*.
In other words, you can't specify shorter delays this way but you can
measure a delay that already happened far more precisely.

For example, if you decide to delay for 1 jiffy, 10ms, you may in fact have
waited 14ms. PSCHED_JIFFIES will not see this - 1.4=1 jiffy has passed.
PSCHED_CPU *will* see it, and let the right amount of packets out.

But I'm hoping that Martin Devera can check this for me, devik?

Regards,

bert

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  parent reply	other threads:[~2001-12-08 11:52 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2001-12-06 19:28 [LARTC] HTB not that exact Stefan Rompf
2001-12-06 19:48 ` bert hubert
2001-12-06 20:33 ` Daniel Ryczek
2001-12-06 21:35 ` Stefan Rompf
2001-12-07 11:46 ` david
2001-12-07 16:57 ` Sami Farin
2001-12-08  8:51 ` Martin Devera
2001-12-08 11:52 ` bert hubert [this message]
2001-12-08 11:53 ` bert hubert
2001-12-08 15:18 ` devik

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