Em Fri, Mar 26, 2021 at 08:18:07AM -0700, Yonghong Song escreveu:
>
>
> On 3/26/21 7:41 AM, Arnaldo Carvalho de Melo wrote:
> > Em Wed, Mar 24, 2021 at 11:53:32PM -0700, Yonghong Song escreveu:
> > > 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
> > >
> > > 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.
> >
> > I'll probably add a way to specify the starting max_hashbits to be able
> > to use 'perf stat' to show what causes the performance difference.
>
> The problem is with hashtags__find(), esp. the loop
>
> uint32_t bucket = hashtags__fn(id);
> const struct hlist_head *head = hashtable + bucket;
>
> hlist_for_each_entry(tpos, pos, head, hash_node) {
> if (tpos->id == id)
> return tpos;
> }
>
> Say we have 8M types and (1 << 15) buckets, that means
> each bucket will 64 elements. So each lookup will traverse
> the loop 32 iterations on average.
>
> If we have 1 << 21 buckets, then each buckets will have 4 elements,
> and the average number of loop iterations for hashtags__find()
> will be 2.
>
> If the patch needs respin, I can add the above descriptions
> in the commit message.
I can add that, as a comment.
- Arnaldo
