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SubjectRe: [PATCH v2 4/8] objtool: add undwarf debuginfo generation
On Thu, Jun 29, 2017 at 10:06:52AM -0500, Josh Poimboeuf wrote:
> On Thu, Jun 29, 2017 at 04:46:18PM +0200, Ingo Molnar wrote:
> >
> > * Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > > > Plus, shouldn't we use __packed for 'struct undwarf' to minimize the
> > > > structure's size (to 6 bytes AFAICS?) - or is optimal packing of the main
> > > > undwarf array already guaranteed on every platform with this layout?
> > >
> > > Ah yes, it should definitely be packed (assuming that doesn't affect performance
> > > negatively).
> >
> > So if I count that correctly that should shave another ~1MB off a typical ~4MB
> > table size?
>
> Here's what my Fedora kernel looks like *before* the packed change:
>
> $ eu-readelf -S vmlinux |grep undwarf
> [15] .undwarf_ip PROGBITS ffffffff81f776d0 011776d0 0012d9d0 0 A 0 0 1
> [16] .undwarf PROGBITS ffffffff820a50a0 012a50a0 0025b3a0 0 A 0 0 1
>
> The total undwarf data size is ~3.5MB.
>
> There are 308852 entries of two parallel arrays:
>
> * .undwarf (8 bytes/entry) = 2470816 bytes
> * .undwarf_ip (4 bytes/entry) = 1235408 bytes
>
> If we pack undwarf, reducing the size of the .undwarf entries by two
> bytes, it will save 308852 * 2 = 617704.
>
> So the savings will be ~600k, and the typical size will be reduced to ~3MB.

Just for the record, while packing the struct from 8 to 6 bytes did save
600k, it also made the unwinder ~7% slower. I think that's probably an
ok tradeoff, so I'll leave it packed in v3.

--
Josh

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