DrHook will be replaced by Vernier in the UM code at the next UM release.

[mo-andymalcolm]

TL:DR - DrHook will be replaced by Vernier in the UM code at the next UM
release.

Longer explanation

DrHook is currently used in the UM for routine level profiling. However
this has some disadvantages. The overhead is quite high and there are a
number of problems when it comes to licensing to other users.

The Vernier profiling tool has been developed in-house as an open source
project. This has been developed to have a lower overhead and also no licensing problems.

UM ticket #7834 will implement the changes, however there are some
factors that need to be considered.

Calls to DrHook will be removed from all repositories.
The UM will call Vernier calipers directly, however the other
repositories used by the UM (Jules, UKCA, CASIM, Socrates, ..) are also
used by LFRic which does not use direct calls. As such a modified copy
of the LFRic timer_mod file will be added to the UM and calls in the
other repositories will have their profiling calls contact vernier
via the timer_mod interface. These code changes can be mainly automated
by the use of scripts and tests have shown this is a sensible solution.

The outstanding work is the technical changes needed in the UM build
system and the rose-stem system to swap from using drhook to vernier.
There is also a slight problem with vernier in createbc but that should
be solvable.

If anyone has any comments or questions about this proposed change please get in
contact.

Thanks
Andy Malcolm