[Valgrind-developers] Bug 464103 DHAT user histogram requests

[Paul F. <pj...@wa...>]

Hi

I'd like some feedback on

https://bugs.kde.org/show_bug.cgi?id=464103

This allows the user to specify that a block of memory should be 
histogrammed. It's intended for blocks larger than the existing 1024 
byte limit - recently at work I was profiling and the top data structure 
was about 1600 bytes so I couldn't see the histogram straight way - I 
rebuilt dhat with a limit of 2048 to see it. Since this is a user 
request it shouldn't bee too overwhelming (either for runtime memory or 
the size of the html generated).

I'll do a bit of trial and error and put in some upper limit. About 2500 
will fit on my screen, so probably somewhere between 10k and 25k. For 
such large arrays someone needs to write a nice zoomable GUI.

One change I'll probably make straight away is to add an argument for 
the initial count (normally 0 for malloc or 1 for calloc or std::vector 
with an initial size and value). Or possibly have 2 macros like 
DHAT_HISTOGRAM_MEMORY_UNINIT and  DHAT_HISTOGRAM_MEMORY_INIT


I've also been thinking about an array version of these user requests.

For instance say we have 'struct Node' which is 80 bytes.

If a user has

std::vector<Node> nodeVec(500);
or
struct Node *nodes = malloc(500*sizeof(struct Node));

Then this would be too big for the DHAT histogram. Just increasing the 
limiat as with the 1st user request isn't very useful either. So what 
I'm thinking of is a user request

DHAT_HISTOGRAM_ARRAY

that takes 3 arguments
- pointer to memory
- number of elements
- size of each element

That would look like this

std::vector<Node> nodeVec(500);
DHAT_HISTOGRAM_ARRAY(nodeVec.data(), nodeVec.size(), sizeof(Node));

In DHAT this would find and throw away the old block. A bit inefficient 
but I don't see an alternative. Then create number_of_elements subblocks.

I did think about automating that. The only alloc function where that 
would be possible is calloc. malloc new and new[] just have the overall 
size available.

A+
Paul