linux-kernel - Re: [RFC][PATCH] Introduce CLOCK_REALTIME

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Date:	Sat, 18 Jul 2009 14:09:39 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	john stultz <johnstul@...ibm.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...x.de>, Andi Kleen <ak@...e.de>,
	nikolag@...ibm.com, Darren Hart <dvhltc@...ibm.com>
Subject: Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE


* john stultz <johnstul@...ibm.com> wrote:

> Hey all,
> 
> 	After talking with some application writers who want very 
> fast, but not fine-grained timestamps, I decided to try to 
> implement a new clock_ids to clock_gettime(): 
> CLOCK_REALTIME_COARSE and CLOCK_MONOTONIC_COARSE which returns the 
> time at the last tick. This is very fast as we don't have to 
> access any hardware (which can be very painful if you're using 
> something like the acpi_pm clocksource), and we can even use the 
> vdso clock_gettime() method to avoid the syscall. The only trade 
> off is you only get low-res tick grained time resolution.
> 
> This isn't a new idea, I know Ingo pushed a patch (see commit 
> 5899a0f044f3c80e9f7262ec5bc7164773a4c28e) a little while ago that 
> made the vsyscall gettimeofday() return coarse grained time when 
> the vsyscall64 sysctrl was set to 2. However this affects all 
> applications on a system.

Note, that patch is an -rt commit, right? I.e. not yet upstream.

> With this method, applications can choose the proper 
> speed/granularity trade-off for themselves.
> 
> This is a first pass on this implementation, and while I did test 
> it, the box I tested it with did not have a glibc new enough to 
> utilize the vdso clock_gettime(), so there may still be issues 
> there. I'll find a newer box for testing shortly.
> 
> Any thoughts or feedback will be appreciated!

Looks good. I think we should offer both methods: your patch as an 
unconditional 'coarse time' approximator always available 
everywhere, plus the vsyscall redirector as well from -rt, to allow 
admins/users to tweak in a global way on apps that cannot be 
changed.

	Ingo
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