Profiling semantics

[Simon Marlow]

On 17/01/2012 09:30, Roman Cheplyaka wrote:
> * Simon Marlow<marlowsd at gmail.com>  [2012-01-03 11:00:58+0000]
>> I'd be interested in hearing feedback, particularly if you find a
>> case where costs are attributed somewhere that you didn't expect, or
>> the stack looks wrong.
>
> What I often find counter-intuitive is how the 'entries' count is
> computed.
>
> Sometimes I see 0 entries for some SCC while its children have non-zero
> entries counts. I guess this has to do with inlining? Still, I don't see
> why it should be so -- if it's possible to compute metrics like time and
> allocations for such an SCC, why shouldn't it be possible to compute
> entries?
>
> Another case is when a function has a large number of entries, although
> in reality it's entered only once (I can confirm that using trace, for
> example). My guess would be that its SCC got inherited by some recursive
> function, but again, this is very confusing.
>
> And because of things like that, I don't usually trust the entries
> counts even when they look realistic.
>
> I observe this with GHC 7.0.3. Do you think this was improved in GHC
> 7.4.1?

This is improved a *lot* with 7.4.1.  In fact, we know for sure that it 
was completely broken before that.  Please try with 7.4.1 and let me 
know if you still find anything strange.

Cheers,
	Simon



> (Unfortunately, it's going to be hard to make a minimal test case, and
> it's also not trivial to simply test with a newer GHC. But if you
> say there's a chance, I'll put effort into that.)
>