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SubjectRe: [PATCH v2 4/8] objtool: add undwarf debuginfo generation

* Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> 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.

So, out of curiosity, I'm wondering where that slowdown comes from: on modern x86
CPUs indexing by units of 6 bytes ought to be just as fast as indexing by 8 bytes,
unless I'm missing something? Is it maybe the not naturally aligned 32-bit words?

Or maybe there's some bad case of a 32-bit word crossing a 64-byte cache line
boundary that hits some pathological aspect of the CPU? We could probably get
around any such problems by padding by 2 bytes on 64-byte boundaries - that's only
a ~3% data size increase. The flip side would be a complication of the data
structure and its accessors - which might cost more in terms of code generation
efficiency than it buys us to begin with ...

Also, there's another aspect besides RAM footprint: a large data structure that is
~20% smaller means 20% less cache footprint: which for cache cold lookups might
matter more than the direct computational cost.

Thanks,

Ingo

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