# Installation guide for the *ELPA* library#

## Preamble ##

This file provides documentation on how to build the *ELPA* library in **version ELPA-2018.11.001**.
With release of **version ELPA-2017.05.001** the build process has been significantly simplified,
which makes it easier to install the *ELPA* library.

The old, obsolete legacy API will be deprecated in the future !
Allready now, all new features of ELPA are only available with the new API. Thus, there
is no reason to keep the legacy API arround for too long.

The release ELPA 2018.11.001 will be the last release, where the legacy API is
enabled by default (and can be disabled at build time).
With release ELPA 2019.05.001 the legacy API will be disabled by default, however,
can be still switched on at build time.
Most likely with the release ELPA 2019.11.001 the legacy API will be deprecated and not supported anymore.

The release of ELPA 2018.11.001 changes the ABI and API, since more functions have an error argument. In C this error argument is required, in Fortran it is optional.

## How to install *ELPA* ##

First of all, if you do not want to build *ELPA* yourself, and you run Linux,
it is worth having a look at the [*ELPA* webpage*] (http://elpa.mpcdf.mpg.de)
and/or the repositories of your Linux distribution: there exist
pre-build packages for a number of Linux distributions like Fedora,
Debian, and OpenSuse. More, will hopefully follow in the future.

If you want to build (or have to since no packages are available) *ELPA* yourself,
please note that *ELPA* is shipped with a typical "configure" and "make"
autotools procedure. This is the **only supported way** how to build and install *ELPA*.


If you obtained *ELPA* from the official git repository, you will not find
the needed configure script! You will have to create the configure scipt with autoconf.


## (A): Installing *ELPA* as library with configure ##

*ELPA* can be installed with the build steps
- configure
- make
- make check   | or make check CHECK_LEVEL=extended
- make install

Please look at configure --help for all available options.

An excerpt of the most important (*ELPA* specific) options reads as follows:

| configure option                     | description                                           |
|:------------------------------------ |:----------------------------------------------------- |
|  --disable-legacy                    | do not build legacy API, will be build as default     |
|  --enable-openmp                     | use OpenMP threading, default no.                     |
|  --enable-redirect                   | for ELPA test programs, allow redirection of <br> stdout/stderr per MPI taks in a file <br> (useful for timing), default no. |
|  --enable-single-precision           | build with single precision version                   |
|  --disable-timings                   | more detailed timing, default yes <br> **If disabled some features like autotune will <br> not work anymmore !** |
|  --disable-band-to-full-blocking     | build ELPA2 with blocking in band_to_full <br> (default:enabled) |
|  --disable-mpi-module                | do not use the Fortran MPI module, <br> get interfaces by 'include "mpif.h') |
|  --disable-generic                   | do not build GENERIC kernels, default: enabled        |
|  --disable-sse                       | do not build SSE kernels, default: enabled            |
|  --disable-sse-assembly              | do not build SSE_ASSEMBLY kernels, default: enabled   |
|  --disable-avx                       | do not build AVX kernels, default: enabled            |
|  --disable-avx2                      | do not build AVX2 kernels, default: enabled           |
|  --enable-avx512                     | build AVX512 kernels, default: disabled               |
|  --enable-gpu                        | build GPU kernels, default: disabled                  |
|  --enable-bgp                        | build BGP kernels, default: disabled                  |
|  --enable-bgq                        | build BGQ kernels, default: disabled                  |
|  --with-mpi=[yes|no]                 | compile with MPI. Default: yes                        |
|  --with-cuda-path=PATH               | prefix where CUDA is installed [default=auto]         |
|  --with-cuda-sdk-path=PATH           | prefix where CUDA SDK is installed [default=auto]     |
|  --with-GPU-compute-capability=VALUE | use compute capability VALUE for GPU version, <br> default: "sm_35" |
|  --with-fixed-real-kernel=KERNEL     | compile with only a single specific real kernel.      |
|  --with-fixed-complex-kernel=KERNEL  | compile with only a single specific complex kernel.   |
|  --with-gpu-support-only             | Compile and always use the GPU version                |
|  --enable-scalapack-tests            | build SCALAPACK test cases for performance <br> omparison, needs MPI, default no. |
|  --enable-autotuning                 | enables autotuning functionality, default yes         |
|  --enable-c-tests                    | enables the C tests for elpa, default yes             |
| --disable-assumed-size               | do NOT use assumed-size Fortran arrays. default use   |
| --enable-scalapack-tests             | build also ScalaPack tests for performance comparison; needs MPI |
| --disable-Fortran2008-features       | disable Fortran 2008 if compiler does not support it  |
| --enable-pyhton                      | build and install python wrapper, default no          |
| --enable-python-tests                | enable python tests, default no.                      |


We recommend that you do not build ELPA in its main directory but that you use it
in a sub-directory:

mkdir build
cd build

../configure [with all options needed for your system, see below]

In this way, you have a clean separation between original *ELPA* source files and the compiled
object files

Please note, that it is necessary to set the **compiler options** like optimisation flags etc.
for the Fortran and C part.
For example sth. like this is a usual way: ./configure FCFLAGS="-O2 -mavx" CFLAGS="-O2 -mavx"
For details, please have a look at the documentation for the compilers of your choice.

**Note** that most kernels can only be build if the correct compiler flags for this kernel (e.g. AVX-512)
have been enabled.


### Choice of building with or without MPI ###

It is possible to build the *ELPA* library with or without MPI support.

Normally *ELPA* is build with MPI, in order to speed-up calculations by using distributed
parallelisation over several nodes. This is, however, only reasonably if the programs
calling the *ELPA* library are already MPI parallized, and *ELPA* can use the same
block-cyclic distribution of data as in the calling program.

Programs which do not support MPI parallelisation can still make use of the *ELPA* library if it
has also been build without MPI support.

If you want to build *ELPA* with MPI support, please have a look at "A) Setting of MPI compiler and libraries".
For builds without MPI support, please have a look at "B) Building *ELPA* without MPI support".
**NOTE** that if *ELPA* is build without MPI support, it will be serial unless the OpenMP parallization is
explicitely enabled.

Please note, that it is absolutely supported that both versions of the *ELPA* library are build
and installed in the same directory.

#### A) Setting of MPI compiler and libraries ####

In the standard case *ELPA* needs a MPI compiler and MPI libraries. The configure script
will try to set this by itself. If, however, on the build system the compiler wrapper
cannot automatically found, it is recommended to set it by hand with a variable, e.g.

configure FC=mpif90

In some cases, on your system different MPI libraries and compilers are installed. Then it might happen
that during the build step an error like "no module mpi" or "cannot open module mpi" is given.
You can disable that the  *ELPA* library uses a MPI modules (and instead uses MPI header files) by
adding

--disable-mpi-module

to the configure call.

Please continue reading at "C) Enabling GPU support"


#### B) Building *ELPA* without MPI support ####

If you want to build *ELPA* without MPI support, add

--with-mpi=no

to your configure call.

You have to specify which compilers should be used with e.g.,

configure FC=gfortran --with-mpi=no

**DO NOT specify a MPI compiler here!**

Note, that the installed *ELPA* library files will be suffixed with
"_onenode", in order to discriminate this build from possible ones with MPI.


Please continue reading at "C) Enabling GPU support"

### Enabling GPU support ###

The *ELPA* library can be build with GPU support. If *ELPA* is build with GPU
support, users can choose at RUNTIME, whether to use the GPU version or not.

For GPU support, NVIDIA GPUs with compute capability >= 3.5 are needed.

GPU support is set with

--enable-gpu

It might be necessary to also set the options (please see configure --help)

--with-cuda-path
--with-cuda-sdk-path
--with-GPU-compute-capability

Please continue reading at "D) Enabling OpenMP support".


### Enabling OpenMP support ###

The *ELPA* library can be build with OpenMP support. This can be support of hybrid
MPI/OpenMP parallelization, since *ELPA* is build with MPI support (see A ) or only
shared-memory parallization, since *ELPA* is build without MPI support (see B).

To enable OpenMP support, add

--enable-openmp

as configure option.

Note that as in case with/without MPI, you can also build and install versions of *ELPA*
with/without OpenMP support at the same time.

However, the GPU choice at runtime is not compatible with OpenMP support.

Please continue reading at "E) Standard libraries in default installation paths".


### Standard libraries in default installation paths ###

In order to build the *ELPA* library, some (depending on the settings during the
configure step) libraries are needed.

Typically these are:
  - Basic Linear Algebra Subroutines (BLAS)                   (always needed)
  - Lapack routines                                           (always needed)
  - Basic Linear Algebra Communication Subroutines (BLACS)    (only needed if MPI support was set)
  - Scalapack routines                                        (only needed if MPI support was set)
  - a working MPI library                                     (only needed if MPI support was set)
  - a working OpenMP library                                  (only needed if OpenMP support was set)
  - a working CUDA/cublas library                             (only needed if GPU support was set)

If the needed library are installed on the build system in standard paths (e.g. /usr/lib64)
in the most cases the *ELPA* configure step will recognize the needed libraries
automatically. No setting of any library paths should be necessary.

If your configure steps finish succcessfully, please continue at "G) Choice of ELPA2 compute kernels".
If your configure step aborts, or you want to use libraries in non standard paths please continue at
"F) Non standard paths or non standard libraries".

### Non standard paths or non standard libraries ###

If standard libraries are on the build system either installed in non standard paths, or
special non standard libraries (e.g. *Intel's MKL*) should be used, it might be necessary
to specify the appropriate link-line with the **SCALAPACK_LDFLAGS** and **SCALAPACK_FCFLAGS**
variables.

For example, due to performance reasons it might be benefical to use the *BLAS*, *BLACS*, *LAPACK*,
and *SCALAPACK* implementation from *Intel's MKL* library.

Together with the Intel Fortran Compiler the call to configure might then look like:

configure SCALAPACK_LDFLAGS="-L$MKL_HOME/lib/intel64 -lmkl_scalapack_lp64 -lmkl_intel_lp64 -lmkl_sequential \
                             -lmkl_core -lmkl_blacs_intelmpi_lp64 -lpthread -lm -Wl,-rpath,$MKL_HOME/lib/intel64" \
	  SCALAPACK_FCFLAGS="-L$MKL_HOME/lib/intel64 -lmkl_scalapack_lp64 -lmkl_intel_lp64 -lmkl_sequential \
	                      -lmkl_core -lmkl_blacs_intelmpi_lp64 -lpthread -lm -I$MKL_HOME/include/intel64/lp64"

and for *INTEL MKL* together with *GNU GFORTRAN* :

configure SCALAPACK_LDFLAGS="-L$MKL_HOME/lib/intel64 -lmkl_scalapack_lp64 -lmkl_gf_lp64 -lmkl_sequential \
                             -lmkl_core -lmkl_blacs_intelmpi_lp64 -lpthread -lm -Wl,-rpath,$MKL_HOME/lib/intel64" \
	  SCALAPACK_FCFLAGS="-L$MKL_HOME/lib/intel64 -lmkl_scalapack_lp64 -lmkl_gf_lp64 -lmkl_sequential \
	                     -lmkl_core -lmkl_blacs_intelmpi_lp64 -lpthread -lm -I$MKL_HOME/include/intel64/lp64"


Please, for the correct link-line refer to the documentation of the correspondig library. In case of *Intel's MKL* we
suggest the [Intel Math Kernel Library Link Line Advisor] (https://software.intel.com/en-us/articles/intel-mkl-link-line-advisor).


### Choice of ELPA2 compute kernels ###

ELPA 2stage can be used with different implementations of compute intensive kernels, which are architecture dependent.
Some kernels (all for x86_64 architectures) are enabled by default (and must be disabled if you do not want them),
others are disabled by default and must be enabled if they are wanted.

One can enable "kernel classes" by setting e.g.

--enable-avx2 


This will try to build all the AVX2 kernels. Please see configure --help for all options

With

--disable-avx2

one chan choose not to build the AVX2 kernels.


During the configure step all possible kernels will be printed, and whether they will be enabled or not.

It is possible to build *ELPA* with as many kernels as desired, the user can then choose at runtime which
kernels should be used.

It this is not desired, it is possible to build *ELPA* with only one (not necessary the same) kernel for the
real and complex valued case, respectively. This can be done with the "--with-fixed-real-kernel=NAME" or
"--with-fixed-complex-kernel=NAME" configure options. For details please do a "configure --help"

#### Cross compilation ####

The ELPA library does _not_ supports cross-compilation by itself, i.e. compilation of the ELPA library on an architecture wich is not
identical than the architecture ELPA should be used on.

Whenever a cross-compilation situation might occur, great care has to be taken during the build process by the user.

At the moment we see two potential pitfalls:

1.) The "build architecure" is inferior to the "target" architecture (w.r.t. the instructions sets)

In this case, at the moment, the ELPA library can only be build with instructions sets supported on the build
system. All later instruction sets will _not_ be used in the compilation. This case might lead to less optimal
performance compared to the case that ELPA is build directly on the target system.

For example, if the "build architecture" consists of an HASWELL node (supporting up to Intel's AVX2 instruction set) and the 
"target architecture" is a Skylake node (supporting Intel's AVX-512 instruction set) than the AVX-512 kernels can not be build
This will lead to a performance degradation on the Skylake nodes, but is otherwise harmless (no chrashes).


2.) The "build architecure" is superior to the "target" architecture (w.r.t. the instructions sets)

This case is a critical one, since ELPA will by default build with instructions sets which are not supported on the target
system. This will lead to crashes, if during build the user does not take care to solve this issue.

For example, if the "build architecture" supports Intels' AVX-2 instruction set and the 
"target architecture" does only support Intel's AVX instruction set, then by default ELPA will be build with AVX-2 instruction set
and this will also be used at runtime (since it improves the performance). However, at the moment, since the target system does not support
AVX-2 instructions this will lead to a crash.

One can avoid this unfortunate situation by disabling instructions set which are _not_ supported on the target system.
In the case above, setting

--disable-avx2

during build, will remdy this problem.


### Doxygen documentation ###
A doxygen documentation can be created with the "--enable-doxygen-doc" configure option

### Some examples ###

#### Intel cores supporting AVX2 (Hasell and newer) ####

We recommend that you build ELPA with the Intel compiler (if available) for the Fortran part, but
with GNU compiler for the C part.

1. Building with Intel Fortran compiler and GNU C compiler:

Remarks:
  - you have to know the name of the Intel Fortran compiler wrapper
  - you do not have to specify a C compiler (with CC); GNU C compiler is recognized automatically
  - you should specify compiler flags for Intel Fortran compiler; in the example only "-O3 -xAVX2" is set
  - you should be careful with the CFLAGS, the example shows typical flags

FC=mpi_wrapper_for_intel_Fortran_compiler CC=mpi_wrapper_for_gnu_C_compiler ./configure FCFLAGS="-O3 -xAVX2" CFLAGS="-O3 -march=native -mavx2 -mfma -funsafe-loop-optimizations -funsafe-math-optimizations -ftree-vect-loop-version -ftree-vectorize" --enable-option-checking=fatal SCALAPACK_LDFLAGS="-L$MKLROOT/lib/intel64 -lmkl_scalapack_lp64 -lmkl_intel_lp64 -lmkl_sequential -lmkl_core -lmkl_blacs_intelmpi_lp64 -lpthread " SCALAPACK_FCFLAGS="-I$MKL_HOME/include/intel64/lp64"


2. Building with GNU Fortran compiler and GNU C compiler:

Remarks: 
  - you have to know the name of the GNU Fortran compiler wrapper
  - you DO have to specify a C compiler (with CC); GNU C compiler is recognized automatically
  - you should specify compiler flags for GNU Fortran compiler; in the example only "-O3 -march=native -mavx2 -mfma" is set
  - you should be careful with the CFLAGS, the example shows typical flags

FC=mpi_wrapper_for_gnu_Fortran_compiler CC=mpi_wrapper_for_gnu_C_compiler ./configure FCFLAGS="-O3 -march=native -mavx2 -mfma" CFLAGS="-O3 -march=native -mavx2 -mfma  -funsafe-loop-optimizations -funsafe-math-optimizations -ftree-vect-loop-version -ftree-vectorize" --enable-option-checking=fatal SCALAPACK_LDFLAGS="-L$MKLROOT/lib/intel64 -lmkl_scalapack_lp64 -lmkl_gf_lp64 -lmkl_sequential -lmkl_core -lmkl_blacs_intelmpi_lp64 -lpthread " SCALAPACK_FCFLAGS="-I$MKL_HOME/include/intel64/lp64"


2. Building with Intel Fortran compiler and Intel C compiler:

Remarks:
  - you have to know the name of the Intel Fortran compiler wrapper
  - you have to specify the Intel C compiler
  - you should specify compiler flags for Intel Fortran compiler; in the example only "-O3 -xAVX2" is set
  - you should be careful with the CFLAGS, the example shows typical flags

FC=mpi_wrapper_for_intel_Fortran_compiler CC=mpi_wrapper_for_intel_C_compiler ./configure FCFLAGS="-O3 -xAVX2" CFLAGS="-O3 -xAVX2" --enable-option-checking=fatal SCALAPACK_LDFLAGS="-L$MKLROOT/lib/intel64 -lmkl_scalapack_lp64 -lmkl_intel_lp64 -lmkl_sequential -lmkl_core -lmkl_blacs_intelmpi_lp64 -lpthread " SCALAPACK_FCFLAGS="-I$MKL_HOME/include/intel64/lp64"