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Date: Mon, 16 Apr 2001 17:45:57 -0700
From: george anzinger <george@mvista.com>
Organization: Monta Vista Software
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To: Mark Salisbury <gonar@mediaone.net>
CC: Mark Salisbury <mbs@mc.com>, Jamie Lokier <lk@tantalophile.demon.co.uk>,
        Ben Greear <greearb@candelatech.com>,
        Horst von Brand <vonbrand@sleipnir.valparaiso.cl>,
        linux-kernel@vger.kernel.org,
        high-res-timers-discourse@lists.sourceforge.net
Subject: Re: No 100 HZ timer!
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Mark Salisbury wrote:
> 
> > Given a system speed, there is a repeating timer rate which will consume
> > 100% of the system in handling the timer interrupts.  An attempt will
> > be made to detect this rate and adjust the timer to prevent system
> > lockup.  This adjustment will look like timer overruns to the user
> > (i.e. we will take a percent of the interrupts and record the untaken
> > interrupts as overruns)
> 
> just at first blush, there are some things in general but I need to read
> this again and more closely....
> 
> but, with POSIX timers, there is a nifty little restriction/protection built
> into the spec regarding the re-insertion of short interval repeating timers.
> that is: a repeating timer will not be re-inserted until AFTER the
> associated signal handler has been handled.

Actually what it says is: "Only a single signal shall be queued to the
process for a given timer at any point in time.  When a timer for which
a signal is still pending expires, no signal shall be queued, and a
timer overrun shall occur."

It then goes on to talk about the overrun count and how it is to be
managed.

What I am suggesting is that the system should detect when these
interrupts would come so fast as to stall the system and just set up a
percent of them while bumping the overrun count as if they had all
occured.

George

> 
> this has some interesting consequences for signal handling and signal
> delivery implementations, but importantly, it ensures that even a flood of
> POSIX timers with very short repeat intervals will be handled cleanly.
> 
> I will get more detailed comments to you tomorrow.
> 
> Mark Salisbury
> 
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