Ömer Sinan Ağacan wrote:
(off-topic: I'm wondering why an empty tuple is passed to `getCurrentCSS`?)
See the comment on getCurrentCCS# in compiler/prelude/primops.txt.pp - it's a token to prevent GHC optimisations from floating the primop up (which obviously might change the call stack).
Now my first question is: Assuming stack traces are implemented as explained by Simon Marlow in his talk and slides, can we say that costs are always assigned to top cost-centre in the stack?
Cost is attributed to the current cost-centre *stack*. So "a/f" != "b/f".
As far as I can understand, current implementation is different from what's explained in Sansom and Jones, for example
The papers actually never introduce cost-centre stacks, as far as I know. It's generally better to check Sansom's PhD thesis, the GHC source code or the other sources I mentioned. There's been quite a bit of work on this...
* I can't see "SUB" cost-centre in list of "built-in cost-centres" in `rts/Profiling.c`.
As I understand it, the "SUB" cost centre refers to situations where the cost-centre stack does *not* get updated on function entry. So it never exists physically.
is annotated with `CCS_DONT_CARE`. Why is that?
That is an annotation on lambdas, and refers to what cost-centre stack their allocation cost should be counted on. As top-level functions and static constructors are allocated statically, they don't count for the heap profile, therefore "don't care". See the definition of GenStgRhs in compiler/stgSyn/StgSyn.lhs.
Also, I can see `_push_` and `_tick_`(what's this?) instructions placed in generated STG but no `_call_` instructions.
This is what actually manages cost-centre stack - "_push_" pushes the given cost-centre on top of the cost centre stack, whereas "_tick" just increases entry count. These two aspects have slightly different properties as far as transformations are concerned, and therefore often end up getting separated during optimisations. Not sure what "_call_" is suppose to be. What's the context?
There is also something like `CCCS` in generated STG but I have no ideas about this.
That's simply "current cost-centre stack". I like to think that the hint of silliness was intentional.
So in short I'm trying to understand how cost-centre related annotations are generated and how are they used.
Sure. Better place for quick queries might be #ghc on FreeNode though - my nick is petermw. Greetings, Peter Wortmann