linux-kernel - Re: [PATCH] Introduce jiffies_32 and related compare functions

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Date:	Tue, 12 Dec 2006 07:56:35 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
Cc:	akpm@...l.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Introduce jiffies_32 and related compare functions

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Tue, 12 Dec 2006 05:09:23 +0100
> 
>> We definitly *like* being able to use bigger timeouts on 64bits platforms.
>>
>> Not that they are mandatory since the same application should run fine on 
>> 32bits kernel. But as the standard type for 'tick timestamps' is 'unsigned 
>> long', a change would be invasive.
>>
>> Maybe some applications are now relying on being able to
>> sleep()/select()/poll() for periods > 30 days and only run on 64
>> bits kernels.
> 
> I think one possible target would be struct timer, at least
> in theory.
> 
> There is also a line of reasoning that says that on 64-bit
> platforms we have some flexibility to set HZ very large, if
> we wanted to at some point, and going to 32-bit jiffies
> storage for some things may eliminate that kind of flexibility.
>

Yes good point, and my understanding is that we go for a tickless kernel in 
2.6.21, or so.
I wonder if virtual HZ wont be sticked to a low value.

I suspect in the case HZ raises, we switch some/most uses of jiffies_32 to 
another  variable (xtime_32 or whatever), but keep the storage on 32bits...

But keeping 64bits values 'just because hardware allows us this kind of 
expenditure' seems not reasonable to me, but lazy...

Eric

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