freebsd stable - Jul 2020 - BerkeleyDB Hash vs Btree on FreeBSD and Linux

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?
> 
> 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.

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