pahole - Finding holes in kernel structs

[Kostik Belousov]

On Thu, Feb 12, 2009 at 02:08:22PM +0000, Andrew Brampton wrote:
> I found this useful tool called pahole[1]. It basically finds holes
> within structs, so for example on my 64bit machine this struct:
> 
> struct test {
>    int foo;
>    const char *bar;
>    int blah;
> }
> 
> Would have a hole between foo and bar of 4 bytes because both for and
> bar have been aligned on a 8 byte boundary, and the struct would also
> have 4 bytes of padding on the end. However, if I simply moved blah
> between foo and bar then the struct has shrunk by 8 bytes, which could
> be a good thing. This could also help keep structs within single cache
> lines, and just generally keep memory usage to a minimum when the
> struct is used many times (for example in an array).
> 
> So I ran the tool pahole over a 7.1 FreeBSD Kernel, and found that
Did you ported it to FreeBSD, or run on the Linux host ?

> many of the struct had holes, and some of which could be rearranged to
> fill the gap. 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.
> 
> thanks
> Andrew
> 
> [1] http://lwn.net/Articles/206805/
> [2] http://bramp.net/projects/kernel.pahole.bz2 (~260kB)
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