pahole - Finding holes in kernel structs

[Max Laier]

On Thursday 12 February 2009 15:08:22 Andrew Brampton wrote:
> So I ran the tool pahole over a 7.1 FreeBSD Kernel, and found that
> many of the struct had holes, and some of which could be rearranged to
> fill the gap.

Interesting tool ...

>               I've made the list available here[2]. So my questions
> are two fold:
>
> 1) Is it worth my time trying to rearrange structs? If so do you think
> many of my patches would be accepted?
>
> 2) Is there a way to find out the most heavily used structs? There are
> ~3600 structs, and ~2000 holes, it might be a waste of my time fixing
> the structs which are only used once.

That's the tricky part.  Rearranging the structs itself is not that difficult, 
but identifying which should be rearranged and if, how ... that's the problem.  
The fact that gaps might be different for 64 vs. 32 bit architectures has 
already been mentioned.

In addition one needs to keep in mind that changing a struct layout is a ABI 
change.  So if we do identify structs that we want to change we should do them 
all at once to keep the different versions down to a minimum.

So to answer your first question, submitting 101 patches to rearrange 101 
structs is certainly a wasted effort.  However, if you take a good look at the 
2000 holes, identify an interesting subset and submit a patch to fix that 
subset ... that would be a worthwhile effort ... IMHO.

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