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From: jeh@cmkrnl.com (Jamie Hanrahan)
Newsgroups: comp.os.ms-windows.programmer.nt.kernel-mode,comp.realtime
Subject: Re: Windows NT 4.0 Interrupt Latencies
Message-ID: <1997Nov15.233820.8308@cmkrnl>
Date: 15 Nov 97 23:38:20 PST
References: <345FAABB.6FDF@ndsuk.com> <63qto6$g5b@news.jhu.edu>  <64kp1d$4i4@news3.euro.net>
Organization: Kernel Mode Systems, San Diego, CA
Lines: 35
Xref: news.mitre.org comp.os.ms-windows.programmer.nt.kernel-mode:5142 comp.realtime:23359

In article <64kp1d$4i4@news3.euro.net>, fbemelx@euronet.nl (Frank Bemelman) writes:
> getrid_ofthispart_roberts@ishtar.med.jhu.edu (Dale Roberts) wrote:
> 
> [snip]
>>>In almost all systems one is at the mercy of other code running at the
>>>highest priority of the processor.
> 
>>Of course.  But in any useful realtime system (which, again, NT is NOT) 
>>*YOU* can choose which code runs at the highest priority.  I'm just 
>>pointing out that, in NT, you do not have this level of control.  Some 
>>other device driver can have a higher priority than yours, and you cannot 
>>control this.
> 
> How does the 'other device driver' controls it's priority being higher
> than yours ?

It doesn't.  NT assigns interrupt request levels (let's not say 
"priority" here, as "priority" is an attribute of a thread, not an 
interrupt) to the various interrupts. 

So, if NT happens to assign an IRQL to your device's interrupt that's
not the highest IRQL of all device interrupts, you are at the mercy of
all the higher-IRQL devices' drivers.  Even if yours is the
highest-IRQL device interrupt, several NT functions such as the
interval timer and the interprocessor interrupt have higher IRQLs than
*any* device, and while NT takes care to not spend too much time in
those ISRs, they still result in some jitter in the interrupt latency
to the highest-IRQL device interrupt. 

	--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
        Internet: jeh@cmkrnl.com (JH645)  CompuServe: 74140,2055  
drivers, internals, networks, applications, and training for VMS and Windows NT
NT driver FAQ, links, and other information:  http://www.cmkrnl.com/

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