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

Linus Torvalds
2008-10-17T00:13:38Z

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