Re: [RFC patch 15/15] LTTng timestamp x86

[Linus Torvalds]

On Thu, 16 Oct 2008, Linus Torvalds wrote:
> 
> Perhaps more importantly - if the TSC really are out of whack, that just 
> means that now all your timestamps are worthless, because the value you 
> calculate ends up having NOTHING to do with the timestamp. So you cannot 
> even use it to see how long something took, because it may be that you're 
> running on the CPU that runs behind, and all you ever see is the value of 
> LTT_MIN_PROBE_DURATION.

If it isn't clear: the alternative is to just always use local timestamps.

At least that way the timestamps mean _something_. You can get the 
difference between two events when they happen on the same CPU, and it is 
about as meaningful as it can be.

Don't even _try_ to make a global clock.

Yes, to be able to compare across CPU's you'd need to have extra 
synchronization information (eg offset and frequency things), but quite 
frankly, the "global TSC" thing is already worse than even a totally 
non-synchronized TSC for the above reasons. 

			Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/