linux-kernel - Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

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Date:	Tue, 06 Mar 2007 16:42:16 -0800
From:	Dan Hecht <dhecht@...are.com>
To:	tglx@...utronix.de
Cc:	Zachary Amsden <zach@...are.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, ak@...e.de,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>,
	Dan Hecht <dhecht@...are.com>
Subject: Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

On 03/06/2007 03:53 PM, Thomas Gleixner wrote:
>> 2) Virtual interrupts have a relatively high overhead as compared with 
>> native interrupts.  So, in vmitime, we wanted to be able to lower the 
>> timer interrupt rate at runtime, even if HZ is a compile time constant 
>> (and set to something high, like 1000hz).  While we could hack this in 
>> by using evt->min_delta_ns, it wouldn't really work since process time 
>> accounting would be wrong.  Instead, we should allow the 
>> tick_sched_timer in cases (c) and (d) to have runtime configurable 
>> period, and then scale the time value accordingly before passing to 
>> account_system_time.  This is probably something the Xen folks will want 
>> also, since I think Xen itself only gets 100hz hard timer, and so it can 
>> implement at best a oneshot virtual timer with 100hz resolution.  Any 
>> objections to us doing something like this?
> 
> Yes. It's gross hackery. 
> 
> 1) We want to have a cleanup of the tick assumptions _all_ over the
> place and this is going to be real hard work.
> 
> 2) As I said above. The time accounting for virtualization needs to be
> fixed in a generic way.
> 
> I'm not going to accept some weird hackery for virtualization, which is
> of exactly ZERO value for the kernel itself. Quite the contrary it will
> make the cleanup harder and introduce another hard to remove thing,
> which will in the worst case last for ever.
>

Okay, to confirm I'm on the same page as you, you want to move process 
time accounting from being periodic sampled based to being trace based? 
i.e. at the system-call/interrupt boundaries, read clocksource and 
compute directly the amount of system/user/process time?

Do you know if anyone has explored this?  I thought there was a 
discussion about this a while back but it was rejected due to the 
sample-based approach having much lower overheads on high system call 
rate workloads.
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