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<h4 class="subsectionHead"><span class="titlemark"><span
class="cmr-12">4.2 </span></span> <a
id="x16-150004.2"></a><span
class="cmr-12">GPU example</span></h4>
<!--l. 426--><p class="noindent" ><span
class="cmr-12">The code discussed here shows how to set up a program exploiting the combined GPU</span>
<span
class="cmr-12">capabilities of PSBLAS and AMG4PSBLAS. The code example is available in the</span>
<span
class="cmr-12">source distribution directory </span><span class="obeylines-h"><span class="verb"><span
class="cmtt-12">amg4psblas/examples/gpu</span></span></span><span
class="cmr-12">.</span>
<!--l. 431--><p class="indent" > <span
class="cmr-12">First of all, we need to include the appropriate modules and declare some auxiliary</span>
<span
class="cmr-12">variables:</span>
<!--l. 433--><p class="indent" > <a
id="x16-15001r5"></a><hr class="float"><div class="float"
>
<div class="center"
>
<!--l. 452--><p class="noindent" >
<div class="minipage"><pre class="verbatim" id="verbatim-10">
program amg_dexample_gpu
  use psb_base_mod
  use amg_prec_mod
  use psb_krylov_mod
  use psb_util_mod
  use psb_gpu_mod
  use data_input
  use amg_d_pde_mod
  implicit none
  .......
  ! GPU variables
  type(psb_d_hlg_sparse_mat) :: agmold
  type(psb_d_vect_gpu)       :: vgmold
  type(psb_i_vect_gpu)       :: igmold
 
</pre>
<!--l. 471--><p class="nopar" ></div></div>
<br /> <div class="caption"
><span class="id">Listing 5: </span><span
class="content">setup of a GPU-enabled test program part one.</span></div><!--tex4ht:label?: x16-15001r5 -->
</div><hr class="endfloat" />
<!--l. 478--><p class="indent" > <span
class="cmr-12">In this particular example we are choosing to employ a </span><span class="obeylines-h"><span class="verb"><span
class="cmtt-12">HLG</span></span></span> <span
class="cmr-12">data structure for</span>
<span
class="cmr-12">sparse matrices on GPUs; for more information please refer to the PSBLAS-EXT users’</span>
<span
class="cmr-12">guide.</span>
<!--l. 482--><p class="indent" > <span
class="cmr-12">We then have to initialize the GPU environment, and pass the appropriate MOLD</span>
<span
class="cmr-12">variables to the build methods (see also the PSBLAS and PSBLAS-EXT users’</span>
<span
class="cmr-12">guides).</span>
<!--l. 485--><p class="indent" > <a
id="x16-15002r6"></a><hr class="float"><div class="float"
>
<div class="center"
>
<!--l. 501--><p class="noindent" >
<div class="minipage"><pre class="verbatim" id="verbatim-11">
  call psb_init(ctxt)
  call psb_info(ctxt,iam,np)
  !
  ! BEWARE: if you have NGPUS  per node, the default is to
  ! attach to mod(IAM,NGPUS)
  !
  call psb_gpu_init(ictxt)
  ......
  t1 = psb_wtime()
  call prec%smoothers_build(a,desc_a,info, amold=agmold, vmold=vgmold, imold=igmold)
 
</pre>
<!--l. 516--><p class="nopar" ></div></div>
<br /> <div class="caption"
><span class="id">Listing 6: </span><span
class="content">setup of a GPU-enabled test program part two.</span></div><!--tex4ht:label?: x16-15002r6 -->
</div><hr class="endfloat" />
<!--l. 523--><p class="indent" > <span
class="cmr-12">Finally, we convert the input matrix, the descriptor and the vectors to use a</span>
<span
class="cmr-12">GPU-enabled internal storage format. We then preallocate the preconditioner</span>
<span
class="cmr-12">workspace before entering the Krylov method. At the end of the code, we close the</span>
<span
class="cmr-12">GPU environment</span>
<!--l. 527--><p class="indent" > <a
id="x16-15003r7"></a><hr class="float"><div class="float"
>
<div class="center"
>
<!--l. 557--><p class="noindent" >
<div class="minipage"><pre class="verbatim" id="verbatim-12">
  call desc_a%cnv(mold=igmold)
  call a%cscnv(info,mold=agmold)
  call psb_geasb(x,desc_a,info,mold=vgmold)
  call psb_geasb(b,desc_a,info,mold=vgmold)
  !
  ! iterative method parameters
  !
  call psb_barrier(ctxt)
  call prec%allocate_wrk(info)
  t1 = psb_wtime()
  call psb_krylov(s_choice%kmethd,a,prec,b,x,s_choice%eps,&
       & desc_a,info,itmax=s_choice%itmax,iter=iter,err=err,itrace=s_choice%itrace,&
       & istop=s_choice%istopc,irst=s_choice%irst)
  call prec%deallocate_wrk(info)
  call psb_barrier(ctxt)
  tslv = psb_wtime() - t1
  ......
  call psb_gpu_exit()
  call psb_exit(ctxt)
  stop
 
</pre>
<!--l. 584--><p class="nopar" ></div></div>
<br /> <div class="caption"
><span class="id">Listing 7: </span><span
class="content">setup of a GPU-enabled test program part three.</span></div><!--tex4ht:label?: x16-15003r7 -->
</div><hr class="endfloat" />
<!--l. 592--><p class="indent" > <span
class="cmr-12">It is very important to employ smoothers and coarsest solvers that are suited to the</span>
<span
class="cmr-12">GPU, i.e. methods that do NOT employ triangular system solve kernels. Methods that</span>
<span
class="cmr-12">satisfy this constraint include:</span>
<ul class="itemize1">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-12">JACOBI</span></span></span>
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-12">BJAC</span></span></span> <span
class="cmr-12">with the following methods on the local blocks:</span>
<ul class="itemize2">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-12">INVK</span></span></span>
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-12">INVT</span></span></span>
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-12">AINV</span></span></span></li></ul>
</li></ul>
<!--l. 604--><p class="noindent" ><span
class="cmr-12">and their </span><span
class="cmmi-12">ℓ</span><sub><span
class="cmr-8">1</span></sub> <span
class="cmr-12">variants.</span>
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