Re: [PATCH dwarves 3/3] dwarf_loader: add option to merge more dwarf cu's into one pahole cu

On Wed, Mar 24, 2021 at 11:53 PM Yonghong Song [off-list ref] wrote:

This patch added an option "merge_cus", which will permit
to merge all debug info cu's into one pahole cu.
For vmlinux built with clang thin-lto or lto, there exist
cross cu type references. For example, you could have
  compile unit 1:
     tag 10:  type A
  compile unit 2:
     ...
       refer to type A (tag 10 in compile unit 1)
I only checked a few but have seen type A may be a simple type
like "unsigned char" or a complex type like an array of base types.

There are two different ways to resolve this issue:
(1). merge all compile units as one pahole cu so tags/types
     can be resolved easily, or
(2). try to do on-demand type traversal in other debuginfo cu's
     when we do die_process().
The method (2) is much more complicated so I picked method (1).
An option "merge_cus" is added to permit such an operation.

Merging cu's will create a single cu with lots of types, tags
and functions. For example with clang thin-lto built vmlinux,
I saw 9M entries in types table, 5.2M in tags table. The
below are pahole wallclock time for different hashbits:
command line: time pahole -J --merge_cus vmlinux
      # of hashbits            wallclock time in seconds
          15                       460
          16                       255
          17                       131
          18                       97
          19                       75
          20                       69
          21                       64
          22                       62
          23                       58
          24                       64

What were the numbers for different hashbits without --merge_cus?

Note that the number of hashbits 24 makes performance worse
than 23. The reason could be that 23 hashbits can cover 8M
buckets (close to 9M for the number of entries in types table).
Higher number of hash bits allocates more memory and becomes
less cache efficient compared to 23 hashbits.

This patch picks # of hashbits 21 as the starting value
and will try to allocate memory based on that, if memory
allocation fails, we will go with less hashbits until
we reach hashbits 15 which is the default for
non merge-cu case.

Signed-off-by: Yonghong Song <redacted>
---
 dwarf_loader.c | 90 ++++++++++++++++++++++++++++++++++++++++++++++++++
 dwarves.h      |  2 ++
 pahole.c       |  8 +++++
 3 files changed, 100 insertions(+)

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