64bit ticks, was Re: Changing p_swtime and td_slptime to ticks

[Sam Leffler]

Jeff Roberson wrote:
> On Tue, 18 Sep 2007, Jeff Roberson wrote:
>
>> On Tue, 18 Sep 2007, Andre Oppermann wrote:
>>
>>> Jeff Roberson wrote:
>>>> On Mon, 17 Sep 2007, Julian Elischer wrote:
>>>>
>>>>> Jeff Roberson wrote:
>>>>>> Enclosed is a patch that fixes swapping with ULE.  ULE has never 
>>>>>> properly set p_swtime and td_slptime which are used by the 
>>>>>> swapout/swapin code to select the appropriate thread to swap.
>>>>>
>>>>> I have not looked at in the depth required, but 2 points that I 
>>>>> was unable
>>>>> to check to my satisfaction before I got called away for work....
>>>>>
>>>>> 1/ the source of the ticks is a monotonically increasing count 
>>>>> that never
>>>>> goes backwards or changes?
>>>>
>>>> ticks is incremented each time hardclock() is called.  That's it.
>>>>
>>>>>
>>>>> 2/ nothing that used to be accounted in seconds becomes accounted 
>>>>> for in ticks?
>>>>
>>>> I scale back to seconds where it is required.  Really I think ticks 
>>>> would be the better metric in vm_glue.c but didn't want to make any 
>>>> drastic changes.
>>>
>>> ticks is 2^31 on x86 and at HZ=1000 is wraps within a reasonable
>>> short uptime.  You have to make sure that your code handles that
>>> correctly or you run into lots of strange effects which are almost
>>> impossible to reproduce.  In TCP we've got bitten by that.
>>
>> Thanks Andre, this is a good point.  For the td_slptime I don't think 
>> it's of practical concern.  However, for swtime I think I will 
>> convert it then to seconds from boot.
>
> Is there a good reason for not making ticks 64bit?  math involving 
> this value is relatively infrequent.  Bruce?  Any comments?  It'd sure 
> let us forget all of these counter wrapping problems.

ticks is used a lot.  I'd rather set hz back to 100 by default.  This 
approach is a perfect example of ignoring low-end platforms.

    Sam