Each tracepoint will basically give a partial ordering (if you make it so,
of course - and on x86 it's hard to avoid it).
And with many trace-points, you can narrow down ordering if you're lucky.
But say that you have code like
CPU#1 CPU#2
trace_a trace_c
.. ..
trace_b trace_d
and since each CPU itself is obviously strictly ordered, you a priori know
that a < b, and c < d. But your trace buffer can look many different ways:
- a -> b -> c -> d
c -> d -> a -> b
Now you do know that what happened between c and d must all have
happened entirely after/before the things that happened between
a and b, and there is no overlap.
This is only assuming the x86 full memory barrier from a "lock xadd" of
course, but those are the semantics you'd get on x86. On others, the
ordering might not be that strong.
- a -> c -> b -> d
a -> c -> d -> b
With these trace point orderings, you really don't know anything at all
about the order of any access that happened in between. CPU#1 might
have gone first. Or not. Or partially. You simply do not know.
Well, x86 defines a memory order. But what I'm trying to explain is that
memory order still doesn't actually specify what happens to the code that
actually does tracing! The trace is only going to show the order of the
tracepoints, not the _other_ memory accesses. So you'll have *some*
information, but it's very partial.
And the thing is, all those other memory accesses are the ones that do all
the real work. You'll know they happened _somewhere_ between two
tracepoints, but not much more than that.
This is why timestamps aren't really any worse than sequence numbers in
all practical matters. They'll get you close enough that you can consider
them equivalent to a cache-coherent counter, just one that you don't have
to take a cache miss for, and that increments on its own!
Quite a lot of CPU's have nice, dependable, TSC's that run at constant
frequency.
And quite a lot of traces care a _lot_ about real time. When you do IO
tracing, the problem is almost never about lock ordering or anything like
that. You want to see how long a request took. You don't care AT ALL how
many tracepoints were in between the beginning and end, you care about how
many microseconds there were!
Linus
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