BerkeleyDB Hash vs Btree on FreeBSD and Linux

[Li-Wen Hsu]

On Wed, Jul 1, 2020 at 8:07 PM Nick Kostirya via freebsd-stable
<freebsd-stable at freebsd.org> wrote:
>
> On Wed, 1 Jul 2020 18:20:19 +0800
> Li-Wen Hsu <lwhsu at freebsd.org> wrote:
>
> > On Wed, Jul 1, 2020 at 6:07 PM Nick Kostirya via freebsd-stable
> > <freebsd-stable at freebsd.org> wrote:
> > >
> > > Hello.
> > >
> > > I noticed that BerkeleyDB Hash is VERY slow compared to BerkeleyDB Btree on FreeBSD (UFS or ZFS).
> > > But they (Hash and Btree) have roughly the same performance on Linux.
> > >
> > > Why?
> >
> > It's not an easy question, do you have more information about the test
> > environment setup, and the statistics of the results?

Any information about this?

> > I'd recommend using some analysis tools like DTrace to check what it's busy for.
>
>
> The top show getblk status often.
>
> Please tell me what you can and how to look with DTrace.
>
> I use
>   dtrace -n '::: /execname == "a.out"/ { @[probefunc] = count(); }'
>
> but I do not see the difference between Hash and Btree.

I would say check the flame graph:
http://www.brendangregg.com/blog/2015-03-10/freebsd-flame-graphs.html
There is benchmarks/flamegraph port but I haven't used it for a while.

Li-Wen