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Date: Fri, 3 Jun 2005 18:30:10 +0200
From: Andi Kleen <ak@suse.de>
To: Parag Warudkar <kernel-stuff@comcast.net>
Cc: john stultz <johnstul@us.ibm.com>, Nishanth Aravamudan <nacc@us.ibm.com>,
       Andi Kleen <ak@suse.de>, lkml <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH 3/4] new timeofday x86-64 arch specific changes (v. B1)
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On Thu, Jun 02, 2005 at 07:50:33PM -0400, Parag Warudkar wrote:
> On Thursday 02 June 2005 19:20, john stultz wrote:
> > Could you see if the slowness you're feeling is correlated to the
> > acpi_pm timesource?
> 
> Speaking of which, the below code from arch/i386/timer_pm.c looks particularly 
> more taxing to me - 3 times read from ioport in a loop - not sure how many 
> time that executes. 
> 
> static inline u32 read_pmtmr(void)
> {
>         u32 v1=0,v2=0,v3=0;
>         /* It has been reported that because of various broken
>          * chipsets (ICH4, PIIX4 and PIIX4E) where the ACPI PM time
>          * source is not latched, so you must read it multiple
>          * times to insure a safe value is read.
>          */
>         do {
>                 v1 = inl(pmtmr_ioport);
>                 v2 = inl(pmtmr_ioport);
>                 v3 = inl(pmtmr_ioport);
>         } while ((v1 > v2 && v1 < v3) || (v2 > v3 && v2 < v1)
>                         || (v3 > v1 && v3 < v2));
> 
> Shouldn't that loop be limited to the broken chipsets - why would correct 
> people with correctly working chipsets carry this extra burden? (Or is it 
> insignificant?)

It is not insignificant and makes a lot of difference. On the x86-64
version of pmtimer I dropped it completely and so far nobody 
complained.

However I wonder why this new time system is using pmtimer by default
at all. That is very broken because pmtimer is one of the slowest.
I would suggest to duplicate the time source selection I have
in the latest x86-64 (-rc5) time.c, that is optimal for all machines
I know about (except that you might need to add cyclone and a non TSC
fallback for i386)

-Andi