pahole - Finding holes in kernel structs

[Harald Servat]

On Thu, Feb 12, 2009 at 3:13 PM, Kostik Belousov <kostikbel at gmail.com>wrote:

> 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
>

Interesting utility Andrew!

Remember that size of some types depend on the memory ABI (32 or 64 bits),
so this influences on the result of this utility.


>
> > [1] http://lwn.net/Articles/206805/
> > [2] http://bramp.net/projects/kernel.pahole.bz2 (~260kB)
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