Am 30.05.19 um 13:55 schrieb Jeff King:

On Wed, May 29, 2019 at 07:54:44PM +0200, René Scharfe wrote:

quoted

Am 29.05.19 um 03:17 schrieb Jeff King:

quoted

But here the problem is in the tree, not the blob. So we're not finding
suspect blobs, but rather re-checking each tree. And no matter what we
do (whether it's visiting the object again, or creating a set or mapping
with the object names) is going to be linear there. And a repository
with a symlink in the root tree is going to revisit or put in our
mapping every single root tree.

That's true, potentially it needs remember and/or reprocess all trees,
meaning this check may double the run time of fsck in the worst case.
Example from the wild: The kernel repo currently has 36 symlinks and
6+ million objects are checked in total, and the symlink check processes
18943 trees_with_symlinks entries there.

That sounds about right. It's basically every version of every tree that
has a symlink. Did it make a noticeable difference in timing? Indexing
the whole kernel history is already a horribly slow process. :)

Right, I didn't notice a difference -- no patience for watching that
thing to the end.  But here are some numbers for v2.21.0 vs. master with
the patch:

Benchmark #1: git fsck
  Time (mean ± σ):     307.775 s ±  9.054 s    [User: 307.173 s, System: 0.448 s]
  Range (min … max):   294.052 s … 322.931 s    10 runs

Benchmark #2: ~/src/git/git fsck
  Time (mean ± σ):     319.754 s ±  2.255 s    [User: 318.927 s, System: 0.671 s]
  Range (min … max):   316.376 s … 323.747 s    10 runs

Summary
  'git fsck' ran
    1.04 ± 0.03 times faster than '~/src/git/git fsck'

Seeing only a single CPU core being stressed for that long is a bit sad
to see.  Checking individual objects should be relatively easy to
parallelize, shouldn't it?

René