Dynamic skew (Re: review of Projects/replay_cache_collision_avoidance, ending Jan. 12)

[Ken Raeburn]

On Dec 30, 2008, at 15:44, Nicolas Williams wrote:
> Time to boot is difficult to pin down, unfortunately.  There's time
> spent on hardware POST, BIOS, ... measurements of which may be
> unavailable.  To estimate POST and such time one needs to know  
> whether a
> boot was cold, warm or fast ("fast reboot" == old kernel loads new
> kernel and transfers control without taking a trip through the BIOS).
> So the conservative thing to do is to ignore time spent in POST, BIOS,
> ... and focus only on kernel start->rcache creation time.   
> Unfortunately
> there are no portable APIs that I know of for obtaining time to boot
> from the point the OS kernel started keeping time.

You also need to make sure you don't get confused in the case where  
the boot-time clock value is bogus, and later corrected by NTP, so  
that time()-boot_time may be significantly different from  
seconds_since_boot.  (Or, explicitly decide that you will trust the  
system clock to be accurate at startup.)

Ken