Re: [RFC 0/1] Combining CUs into a single hash table

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Feb 23, 2021 at 12:44:58PM -0800, Bill Wendling escreveu:
> Bump for exposure.

While preparing my presentation for devconf.cz I stumbled on a problem
with split btf, I want to first bisect this before publishing...

I'll move this to the front of my priority list and inform here about it
ASAP.

- Arnaldo

 
> On Fri, Feb 12, 2021 at 1:16 PM Bill Wendling <morbo@google.com> wrote:
> >
> > Hey gang,
> >
> > I would like your feedback on this patch.
> >
> > This patch creates one hash table that all CUs share. The impetus for this
> > patch is to support clang's LTO (Link-Time Optimizations). Currently, pahole
> > can't handle the DWARF data that clang produces, because the CUs may refer to
> > tags in other CUs (all of the code having been squozen together).
> >
> > One solution I found is to process the CUs in two steps:
> >
> >   1. add the CUs into a single hash table, and
> >   2. perform the recoding and finalization steps in a a separate step.
> >
> > The issue I'm facing with this patch is that it balloons the runtime from
> > ~11.11s to ~14.27s. It looks like the underlying cause is that some (but not
> > all) hash buckets have thousands of entries each. I've bumped up the
> > HASHTAGS__BITS from 15 to 16, which helped a little. Bumping it up to 17 or
> > above causes a failure.
> >
> > A couple of things I thought of may help. We could increase the number of
> > buckets, which would help with distribution. As I mentioned though, that seemed
> > to cause a failure. Another option is to store the bucket entries in a
> > non-list, e.g. binary search tree.
> >
> > I wanted to get your opinions before I trod down one of these roads.
> >
> > Share and enjoy!
> > -bw
> >
> > Bill Wendling (1):
> >   dwarf_loader: have all CUs use a single hash table
> >
> >  dwarf_loader.c | 45 +++++++++++++++++++++++++++++++++------------
> >  1 file changed, 33 insertions(+), 12 deletions(-)
> >
> > --
> > 2.30.0.478.g8a0d178c01-goog
> >

-- 

- Arnaldo