USERS_GUIDE.md 17 KB
Newer Older
Andreas Marek's avatar
Andreas Marek committed
1
## Users guide for the *ELPA* library ##
Andreas Marek's avatar
Andreas Marek committed
2

3
4
5
6
7
This document provides the guide for using the *ELPA* library with the new API (API version 20170403 or higher).
If you want to use the deprecated legacy API (we strongly recommend against this), please refer to the document
[USERS_GUIDE_DEPRECATED_LEGACY_API.md] (USERS_GUIDE_DEPRECATED_LEGACY_API.md).

If you need instructions on how to build *ELPA*, please look at [INSTALL.md] (INSTALL.md).
Andreas Marek's avatar
Andreas Marek committed
8
9
10
11
12

### Online and local documentation ###

Local documentation (via man pages) should be available (if *ELPA* has been installed with the documentation):

13
For example "man elpa2_print_kernels" should provide the documentation for the *ELPA* program, which prints all
Andreas Marek's avatar
Andreas Marek committed
14
the available kernels.
Andreas Marek's avatar
Andreas Marek committed
15

Andreas Marek's avatar
Andreas Marek committed
16
Also a [online doxygen documentation] (http://elpa.mpcdf.mpg.de/html/Documentation/ELPA-2018.11.001/html/index.html)
Andreas Marek's avatar
Andreas Marek committed
17
18
for each *ELPA* release is available.

Andreas Marek's avatar
Andreas Marek committed
19

20
### API of the *ELPA* library ###
Andreas Marek's avatar
Andreas Marek committed
21

22
23
With release 2017.05.001 of the *ELPA* library the interface has been rewritten substantially, in order to have a more generic 
interface and to avoid future interface changes.
Andreas Marek's avatar
Andreas Marek committed
24
25

For compatibility reasons the interface defined in the previous release 2016.11.001 is also still available
26
**IF AND ONLY IF** *ELPA* has been build with support of this legacy interface.
Andreas Marek's avatar
Andreas Marek committed
27

28
The legacy API defines all the functionality as it has been defined in *ELPA* release 2016.11.011. Note, however,
29
that all future features of *ELPA* will only be accessible via the new API defined in release 2017.05.001 or later.
Andreas Marek's avatar
Andreas Marek committed
30

31
32
33
As mentioned, we advise against it, but if you want to use the legacy API please look at the document 
[USERS_GUIDE_DEPRECATED_LEGACY_API.md] (USERS_GUIDE_DEPRECATED_LEGACY_API.md).

34
35
36
37
38
39
40
41
42
43
44
45
46

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.


47
48
49
50
51
52
53
54
55
56
### Table of Contents: ###

- I)   General concept of the *ELPA* API
- II)  List of supported tunable parameters
- III) List of computational routines
- IV)  Using OpenMP threading
- V)   Influencing default values with environment variables
- VI)   Autotuning

## I) General concept of the *ELPA* API ##
Andreas Marek's avatar
Andreas Marek committed
57

58
Using *ELPA* just requires a few steps:
Andreas Marek's avatar
Andreas Marek committed
59

Andreas Marek's avatar
Andreas Marek committed
60
61
- include elpa headers "elpa/elpa.h" (C-Case) or use the Fortran module "use elpa"

Andreas Marek's avatar
Andreas Marek committed
62
- define a instance of the elpa type
Andreas Marek's avatar
Andreas Marek committed
63

Andreas Marek's avatar
Andreas Marek committed
64
- call elpa_init
Andreas Marek's avatar
Andreas Marek committed
65

Andreas Marek's avatar
Andreas Marek committed
66
- call elpa_allocate to allocate an instance of *ELPA*
Andreas Marek's avatar
Andreas Marek committed
67
68
69
70
71
72
73
74
75
  note that you can define (and configure individually) as many different instances
  for ELPA as you want, e.g. one for CPU only computations and for larger matrices on GPUs

- use ELPA-type function "set" to set matrix and MPI parameters

- call the ELPA-type function "setup"

- set or get all possible ELPA tunable options with ELPA-type functions get/set

Andreas Marek's avatar
Andreas Marek committed
76
77
- call ELPA-type function solve or others

Andreas Marek's avatar
Andreas Marek committed
78
- if the ELPA object is not needed any more call ELPA-type function destroy
Andreas Marek's avatar
Andreas Marek committed
79

Andreas Marek's avatar
Andreas Marek committed
80
- call elpa_uninit at the end of the program
Andreas Marek's avatar
Andreas Marek committed
81

82
83
84
85
86
87
88
89
To be more precise a basic call sequence for Fortran and C looks as follows:

Fortran synopsis

```Fortran
 use elpa
 class(elpa_t), pointer :: elpa
 integer :: success
Andreas Marek's avatar
Andreas Marek committed
90

91
92
93
94
 if (elpa_init(20171201) /= ELPA_OK) then        ! put here the API version that you are using
    print *, "ELPA API version not supported"
    stop
  endif
Andreas Marek's avatar
Andreas Marek committed
95
96
97
98
99
100
  elpa => elpa_allocate(success)
  if (success != ELPA_OK) then
    ! react on the error
    ! we urge every user to always check the error codes
    ! of all ELPA functions
  endif
101
102
103
104
105
106
107
108
109
110

  ! set parameters decribing the matrix and it's MPI distribution
  call elpa%set("na", na, success)                          ! size of the na x na matrix
  call elpa%set("nev", nev, success)                        ! number of eigenvectors that should be computed ( 1<= nev <= na)
  call elpa%set("local_nrows", na_rows, success)            ! number of local rows of the distributed matrix on this MPI task 
  call elpa%set("local_ncols", na_cols, success)            ! number of local columns of the distributed matrix on this MPI task
  call elpa%set("nblk", nblk, success)                      ! size of the BLACS block cyclic distribution
  call elpa%set("mpi_comm_parent", MPI_COMM_WORLD, success) ! the global MPI communicator
  call elpa%set("process_row", my_prow, success)            ! row coordinate of MPI process
  call elpa%set("process_col", my_pcol, success)            ! column coordinate of MPI process
Andreas Marek's avatar
Andreas Marek committed
111

Andreas Marek's avatar
Andreas Marek committed
112
  success = elpa%setup()
113

114
115
116
117
  ! if desired, set any number of tunable run-time options
  ! look at the list of possible options as detailed later in
  ! USERS_GUIDE.md
  call e%set("solver", ELPA_SOLVER_2STAGE, success)
Andreas Marek's avatar
Andreas Marek committed
118

119
120
121
122
  ! set the AVX BLOCK2 kernel, otherwise ELPA_2STAGE_REAL_DEFAULT will
  ! be used
  call e%set("real_kernel", ELPA_2STAGE_REAL_AVX_BLOCK2, success)

123
124
125
126
  ! use method solve to solve the eigenvalue problem to obtain eigenvalues
  ! and eigenvectors
  ! other possible methods are desribed in USERS_GUIDE.md
  call e%eigenvectors(a, ev, z, success)
127

128
129
  ! cleanup
  call elpa_deallocate(e)
130

131
132
  call elpa_uninit()
```
Andreas Marek's avatar
Andreas Marek committed
133

134
135
136
C Synopsis:
```C
   #include <elpa/elpa.h>
137

138
139
   elpa_t handle;
   int error;
Andreas Marek's avatar
Andreas Marek committed
140

141
142
143
144
   if (elpa_init(20171201) != ELPA_OK) {                          // put here the API version that you are using
     fprintf(stderr, "Error: ELPA API version not supported");
     exit(1);
   }
Andreas Marek's avatar
Andreas Marek committed
145

146
   handle = elpa_allocate(&error);
Andreas Marek's avatar
Andreas Marek committed
147
148
149
150
151
   if (error != ELPA_OK) {
     /* react on the error code */
     /* we urge the user to always check the error codes of all ELPA functions */
   }

Andreas Marek's avatar
Andreas Marek committed
152

153
154
155
156
157
158
159
160
161
   /* Set parameters the matrix and it's MPI distribution */
   elpa_set(handle, "na", na, &error);                                           // size of the na x na matrix
   elpa_set(handle, "nev", nev, &error);                                         // number of eigenvectors that should be computed ( 1<= nev <= na)
   elpa_set(handle, "local_nrows", na_rows, &error);                             // number of local rows of the distributed matrix on this MPI task 
   elpa_set(handle, "local_ncols", na_cols, &error);                             // number of local columns of the distributed matrix on this MPI task
   elpa_set(handle, "nblk", nblk, &error);                                       // size of the BLACS block cyclic distribution
   elpa_set(handle, "mpi_comm_parent", MPI_Comm_c2f(MPI_COMM_WORLD), &error);    // the global MPI communicator
   elpa_set(handle, "process_row", my_prow, &error);                             // row coordinate of MPI process
   elpa_set(handle, "process_col", my_pcol, &error);                             // column coordinate of MPI process
Andreas Marek's avatar
Andreas Marek committed
162

163
   /* Setup */
Andreas Marek's avatar
Andreas Marek committed
164
   error = elpa_setup(handle);
Andreas Marek's avatar
Andreas Marek committed
165

166
167
168
   /* if desired, set any number of tunable run-time options */
   /* look at the list of possible options as detailed later in
      USERS_GUIDE.md */
Andreas Marek's avatar
Andreas Marek committed
169

170
   elpa_set(handle, "solver", ELPA_SOLVER_2STAGE, &error);
171
172
173
174
  
   // set the AVX BLOCK2 kernel, otherwise ELPA_2STAGE_REAL_DEFAULT will
   // be used
   elpa_set(handle, "real_kernel", ELPA_2STAGE_REAL_AVX_BLOCK2, &error)
Andreas Marek's avatar
Andreas Marek committed
175

176
177
178
   /* use method solve to solve the eigenvalue problem */
   /* other possible methods are desribed in USERS_GUIDE.md */
   elpa_eigenvectors(handle, a, ev, z, &error);
Andreas Marek's avatar
Andreas Marek committed
179

180
181
182
183
   /* cleanup */
   elpa_deallocate(handle);
   elpa_uninit();
```
Andreas Marek's avatar
Andreas Marek committed
184

185
## II) List of supported tunable parameters ##
Andreas Marek's avatar
Andreas Marek committed
186

187
The following table gives a list of all supported parameters which can be used to tune (influence) the runtime behaviour of *ELPA* ([see here if you cannot read it in your editor] (https://gitlab.mpcdf.mpg.de/elpa/elpa/wikis/USERS_GUIDE))
Andreas Marek's avatar
Andreas Marek committed
188

189
190
191
192
193
194
195
196
197
198
199
| Parameter name | Short description     | default value               | possible values         | since API version | 
| :------------- |:--------------------- | :-------------------------- | :---------------------- | :---------------- | 
| solver         | use ELPA 1 stage <br>  or 2 stage solver | ELPA_SOLVER_1STAGE          | ELPA_SOLVER_1STAGE <br> ELPA_SOLVER_2STAGE      | 20170403          |
| gpu            | use GPU (if build <br> with GPU support)| 0                           | 0 or 1             | 20170403          | 
| real_kernel    | real kernel to be <br> used in ELPA 2 | ELPA_2STAGE_REAL_DEFAULT    | see output of <br> elpa2_print_kernels    | 20170403          |
| complex kernel | complex kernel to <br>  be used in ELPA 2 | ELPA_2STAGE_COMPLEX_DEFAULT | see output of <br>  elpa2_print_kernels     | 20170403          |
| omp_threads    | OpenMP threads used <br> (if build with OpenMP <br> support) | 1 | >1 | 20180525 |
| qr | Use QR decomposition in <br> ELPA 2 real | 0 | 0 or 1 |  20170403  |
| timings | Enable time <br> measurement | 1 | 0 or 1 |  20170403  |
| debug | give debug information | 0 | 0 or 1 | 20170403  |
       
200

201
## III) List of computational routines ##
202

Andreas Marek's avatar
Andreas Marek committed
203
The following compute routines are available in *ELPA*: Please have a look at the man pages or  [online doxygen documentation] (http://elpa.mpcdf.mpg.de/html/Documentation/ELPA-2018.11.001/html/index.html) for details.
204
205


206
207
208
209
210
211
212
213
214
| Name         | Purpose                                                                 | since API version |
| :----------- | :---------------------------------------------------------------------- | :---------------- |
| eigenvectors | solve std. eigenvalue problem <br> compute eigenvalues and eigenvectors | 20170403  |
| eigenvalues  | solve std. eigenvalue problem <br> compute eigenvalues only             | 20170403  |
| generalized_eigenvectors | solve generalized eigenvalule problem <br> compute eigenvalues and eigenvectors | 20180525 |
| generalized_eigenvalues  | solve generalized eigenvalule problem <br> compute eigenvalues only             | 20180525 |
| hermitian_multiply       | do (real) a^T x b <br> (complex) a^H x b                                        | 20170403 |
| cholesky                 | do cholesky factorisation                                                       | 20170403 |
| invert_triangular        | invert a upper triangular matrix                                                | 20170403 |
215
| solve_tridiagonal        | solve EVP for a tridiagonal matrix                                              | 20170403 |
216
217


218
## IV) Using OpenMP threading ##
219

220
221
222
If *ELPA* has been build with OpenMP threading support you can specify the number of OpenMP threads that *ELPA* will use internally.
Please note that it is **mandatory**  to set the number of threads to be used with the OMP_NUM_THREADS environment variable **and**
with the **set method** 
223

224
225
226
```Fortran
call e%set("omp_threads", 4, error)
```
227

228
**or the *ELPA* environment variable**
229

230
export ELPA_DEFAULT_omp_threads=4 (see Section V for an explanation of this variable).
231

232
Just setting the environment variable OMP_NUM_THREADS is **not** sufficient.
233

234
This is necessary to make the threading an autotunable option.
235

236
## V) Influencing default values with environment variables ##
237

238
239
For each tunable parameter mentioned in Section II, there exists a default value. This means, that if this parameter is **not explicitly** set by the user by the
*ELPA* set method, *ELPA* takes the default value for the parameter. E.g. if the user does not set a solver method, than *ELPA* will take the default "ELPA_SOLVER_1STAGE".
240

241
The user can change this default value by setting an enviroment variable to the desired value.
242

243
244
245
246
The name of this variable is always constructed in the following way:
```
ELPA_DEFAULT_tunable_parameter_name=value
```
247

248
, e.g. in case of the solver the user can
249

250
251
252
```
export ELPA_DEFAULT_solver=ELPA_SOLVER_2STAGE
```
253

254
in order to define the 2stage solver as the default.
255

256
257
258
259
260
261
262
**Important note**
The default valule is completly ignored, if the user has manually set a parameter-value pair with the *ELPA* set method!
Thus the above environemnt variable will **not** have an effect, if the user code contains a line
```Fortran
call e%set("solver",ELPA_SOLVER_1STAGE,error)
```
.
263

264
## VI) Using autotuning ##
265

266
267
Since API version 20171201 *ELPA* supports the autotuning of some "tunable" parameters (see Section II). The idea is that if *ELPA* is called multiple times (like typical in
self-consistent-iterations) some parameters can be tuned to an optimal value, which is hard to set for the user. Note, that not every parameter mentioned in Section II can actually be tuned with the autotuning. At the moment, only the parameters mentioned in the table below are affected by autotuning.
268

269
There are two ways, how the user can influence the autotuning steps:
270

271
272
273
1.) the user can set one of the following autotuning levels
- ELPA_AUTOTUNE_FAST
- ELPA_AUTOTUNE_MEDIUM
274

275
276
Each level defines a different set of tunable parameter. The autouning option will be extended by future releases of the *ELPA* library, at the moment the following
sets are supported: 
277

278
279
280
281
282
283
| AUTOTUNE LEVEL          | Parameters                                              |
| :---------------------- | :------------------------------------------------------ |
| ELPA_AUTOTUNE_FAST      | { solver, real_kernel, complex_kernel, omp_threads }    |
| ELPA_AUTOTUNE_MEDIUM    | all of abvoe + { gpu, partly gpu }                      |
| ELPA_AUTOTUNE_EXTENSIVE | all of above + { various blocking factors, stripewidth, |
|                         | intermediate_bandwidth }                                |
284

285
286
2.) the user can **remove** tunable parameters from the list of autotuning possibilites by explicetly setting this parameter,
e.g. if the user sets in his code 
287

288
289
290
291
```Fortran
call e%set("solver", ELPA_SOLVER_2STAGE, error)
```
**before** invoking the autotuning, then the solver is fixed and not considered anymore for autotuning. Thus the ELPA_SOLVER_1STAGE would be skipped and, consequently, all possible autotuning parameters, which depend on ELPA_SOLVER_1STAGE.
292

293
The user can invoke autotuning in the following way:
294
295


296
Fortran synopsis
297

298
299
300
301
302
303
304
```Fortran
 ! prepare elpa as you are used to (see Section I)
 ! only steps for autotuning are commentd
 use elpa
 class(elpa_t), pointer :: elpa
 class(elpa_autotune_t), pointer :: tune_state   ! create an autotuning pointer
 integer :: success
305

306
307
308
309
 if (elpa_init(20171201) /= ELPA_OK) then
    print *, "ELPA API version not supported"
    stop
  endif
Andreas Marek's avatar
Andreas Marek committed
310
  elpa => elpa_allocate(success)
311

312
313
314
315
316
317
318
319
320
  ! set parameters decribing the matrix and it's MPI distribution
  call elpa%set("na", na, success)
  call elpa%set("nev", nev, success))
  call elpa%set("local_nrows", na_rows, success)
  call elpa%set("local_ncols", na_cols, success)
  call elpa%set("nblk", nblk, success)
  call elpa%set("mpi_comm_parent", MPI_COMM_WORLD, success)
  call elpa%set("process_row", my_prow, success)
  call elpa%set("process_col", my_pcol, success)
321

Andreas Marek's avatar
Andreas Marek committed
322
  success = elpa%setup()
323

Andreas Marek's avatar
Andreas Marek committed
324
  tune_state => e%autotune_setup(ELPA_AUTOTUNE_MEDIUM, ELPA_AUTOTUNE_DOMAIN_REAL, success)   ! prepare autotuning, set AUTOTUNE_LEVEL and the domain (real or complex)
325

326
327
  ! do the loop of subsequent ELPA calls which will be used to do the autotuning
  do i=1, scf_cycles
Andreas Marek's avatar
Andreas Marek committed
328
    unfinished = e%autotune_step(tune_state, success)   ! check whether autotuning is finished; If not do next step
329

330
331
332
    if (.not.(unfinished)) then
      print *,"autotuning finished at step ",i
    endif
333

Andreas Marek's avatar
Andreas Marek committed
334
    call e%eigenvectors(a, ev, z, success)       ! do the normal computation
335

336
  enddo
337

Andreas Marek's avatar
Andreas Marek committed
338
  call e%autotune_set_best(tune_state, success)         ! from now use the values found by autotuning
339

340
341
  call elpa_autotune_deallocate(tune_state)    ! cleanup autotuning object 
```
342

343
344
345
346
C Synopsis
```C
   /* prepare ELPA the usual way; only steps for autotuning are commented */
   #include <elpa/elpa.h>
347

348
349
350
   elpa_t handle;
   elpa_autotune_t autotune_handle;                               // handle for autotuning
   int error;
351

352
353
354
355
   if (elpa_init(20171201) != ELPA_OK) { 
     fprintf(stderr, "Error: ELPA API version not supported");
     exit(1);
   }
356

357
   handle = elpa_allocate(&error);
358

359
360
361
362
363
364
365
366
367
368
369
   /* Set parameters the matrix and it's MPI distribution */
   elpa_set(handle, "na", na, &error);
   elpa_set(handle, "nev", nev, &error);
   elpa_set(handle, "local_nrows", na_rows, &error);
   elpa_set(handle, "local_ncols", na_cols, &error);
   elpa_set(handle, "nblk", nblk, &error);
   elpa_set(handle, "mpi_comm_parent", MPI_Comm_c2f(MPI_COMM_WORLD), &error);
   elpa_set(handle, "process_row", my_prow, &error);
   elpa_set(handle, "process_col", my_pcol, &error);
   /* Setup */
   elpa_setup(handle);
370

371
   autotune_handle = elpa_autotune_setup(handle, ELPA_AUTOTUNE_FAST, ELPA_AUTOTUNE_DOMAIN_REAL, &error);   // create autotune object
372

373
374
   // repeatedl call ELPA, e.g. in an scf iteration
   for (i=0; i < scf_cycles; i++) {
375

Andreas Marek's avatar
Andreas Marek committed
376
     unfinished = elpa_autotune_step(handle, autotune_handle, &error);      // check whether autotuning finished. If not do next step
377

378
379
380
     if (unfinished == 0) {
       printf("ELPA autotuning finished in the %d th scf step \n",i);
      }
381
382


383
384
385
      /* do the normal computation */
      elpa_eigenvectors(handle, a, ev, z, &error);
   }
Andreas Marek's avatar
Andreas Marek committed
386
   elpa_autotune_set_best(handle, autotune_handle &error);  // from now on use values used by autotuning
387
388
389
   elpa_autotune_deallocate(autotune_handle);        // cleanup autotuning
   
```
390

391
  
392
393
394