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From: Bill Wendling <morbo@google.com>
To: dwarves@vger.kernel.org, bpf@vger.kernel.org
Cc: arnaldo.melo@gmail.com, Bill Wendling <morbo@google.com>
Subject: [RFC 0/1] Combining CUs into a single hash table
Date: Fri, 12 Feb 2021 13:16:06 -0800	[thread overview]
Message-ID: <20210212211607.2890660-1-morbo@google.com> (raw)

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


             reply	other threads:[~2021-02-12 21:17 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-12 21:16 Bill Wendling [this message]
2021-02-12 21:16 ` [RFC PATCH 1/1] dwarf_loader: have all CUs use a single hash table Bill Wendling
2021-02-23 20:44 ` [RFC 0/1] Combining CUs into " Bill Wendling
2021-02-23 20:54   ` Arnaldo Carvalho de Melo
2021-03-14  7:05   ` Yonghong Song
2021-03-14  8:28     ` Bill Wendling
2021-03-14 23:33       ` Yonghong Song

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