[dtrace-discuss] What am I missing in the dtrace script?

[Bart Smaalders]

Brian D. Horn wrote:
> Wee Yeh Tan wrote:
>> On 7/13/07, Brian D. Horn <Brian.Horn at sun.com> wrote:
>>> Ok, then the question becomes "Where is documented that aggregates are
>>> MP safe, but scalars are
>>> not MP safe?"
>> Hah... I sort of saw that coming.  I recall seeing this somewhere
>> though I cannot remember exactly where.  Didn't see it in Bryan's
>> DTrace gotchas.
>>
>> Does this warrant a bug against the documentation?  Perhaps a note in
>> the variables section saying that "Global variables are not MP safe."?
>>
> I don't know whether it is a bug or not yet.  I may have missed some
> piece of documentation or seen old documentation.  It also begs the
> question of "Why was it decided that scalars should not be MP safe?"
> (assuming that is was decided as such!).

The question here isn't whether "scalaers are mt-safe", it's whether
or not the ++ and -- operators are atomic.

Would you expect

foo = foo + 1;

to work safely?

How about:

bar = foo;

foo = bar + 1;

There's no implicit locking in DTrace.  Aggregations are specifically
designed to be safe, and are documented as such:

> For performance-related questions, aggregated data is often more 
 > useful than individual data points. DTrace provides several built-in
> aggregating functions. When an aggregating function is applied to 
> subsets of a collection of data, then applied again to the results 
> of the analysis of those subsets, the results are identical to the 
> results returned by the aggregating function when it is applied to 
> the collection as a whole.
> 
> The DTrace facility stores a running count of data items for 
> aggregations. The aggregating functions store only the current 
> intermediate result and the new element that the function is being 
> applied to. The intermediate results are allocated on a per-CPU basis.
> Because this allocation scheme does not require locks, the implementation 
> is inherently scalable.



-- 
Bart Smaalders			Solaris Kernel Performance
barts at cyber.eng.sun.com		http://blogs.sun.com/barts