[petsc-users] Time cost by Vec Assembly

[Jed Brown]

Fande Kong <fdkong.jd at gmail.com> writes:

> On Fri, Oct 7, 2016 at 10:30 PM, Jed Brown <jed at jedbrown.org> wrote:
>
>> Barry Smith <bsmith at mcs.anl.gov> writes:
>> >     There is still something wonky here, whether it is the MPI
>> implementation or how PETSc handles the assembly. Without any values that
>> need to be communicated it is unacceptably that these calls take so long.
>> If we understood __exactly__ why the performance suddenly drops so
>> dramatically we could perhaps fix it. I do not understand why.
>>
>> I guess it's worth timing.  If they don't have MPI_Reduce_scatter_block
>> then it falls back to a big MPI_Allreduce.  After that, it's all
>> point-to-point messaging that shouldn't suck and there actually
>> shouldn't be anything to send or receive anyway.  The BTS implementation
>> should be much smarter and literally reduces to a barrier in this case.
>>
>
> Hi Jed,
>
> How to use the BTS implementation for Vec. For mat, we may just use
> "-matstash_bts"?

Should be -vec_assembly_bts