Date: Fri, 28 Jan 2005 09:24:39 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: George Anzinger <george@mvista.com>, LKML <linux-kernel@vger.kernel.org>,
       Doug Niehaus <niehaus@ittc.ku.edu>,
       Benedikt Spranger <bene@linutronix.de>
Subject: Re: High resolution timers and BH processing on -RT
Message-ID: <20050128082439.GA3984@elte.hu>
References: <1106871192.21196.152.camel@tglx.tec.linutronix.de> <20050128044301.GD29751@elte.hu> <1106900411.21196.181.camel@tglx.tec.linutronix.de>
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* Thomas Gleixner <tglx@linutronix.de> wrote:

> > is this due to algorithmic/PIT-programming overhead, or due to the noise
> > introduced by other, non-hard-RT timers? I'd guess the later from the
> > looks of it, but did your test introduce such noise (via networking and
> > application workloads?).
> 
> Right, it's due to noise by non-RT timers, which I enforced by adding
> networking and applications.
> 
> This adds random timer expires and admittedly the PIT reprogramming
> overhead is adding portions of that noise.

i havent seen your latest code - what is the basic data-structure? The
stock kernel has arrays of timers with increasing granularity and a
cascade mechanism to move timers down the arrays as they slowly expire -
but with a high-resolution API (1 usec accuracy?) how does the basic
data structure look like?

Is the "noise" due to timers expiring "at once" - but isnt it unlikely
for 'normal' timers to expire in exactly the same usec as the real
high-resolution one?

or is it that we have a 'group' of normal timers expiring, which, if
they happen to occur _just_ prior a HRT event will generate a larger
delay?

	Ingo
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