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

[Ingo Molnar]

* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx> 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