MAXPY isn't really a BLAS 1 since it can reuse some data in certain vectors.
> On Apr 4, 2017, at 10:25 AM, Filippo Leonardi <filippo.l...@gmail.com> wrote: > > I really appreciate the feedback. Thanks. > > That of deadlock, when the order of destruction is not preserved, is a point > I hadn't thought of. Maybe it can be cleverly addressed. > > PS: If you are interested, I ran some benchmark on BLAS1 stuff and, for a > single processor, I obtain: > > Example for MAXPY, with expression templates: > BM_Vector_petscxx_MAXPY/8 38 ns 38 ns 18369805 > BM_Vector_petscxx_MAXPY/64 622 ns 622 ns 1364335 > BM_Vector_petscxx_MAXPY/512 281 ns 281 ns 2477718 > BM_Vector_petscxx_MAXPY/4096 2046 ns 2046 ns 349954 > BM_Vector_petscxx_MAXPY/32768 18012 ns 18012 ns 38788 > BM_Vector_petscxx_MAXPY_BigO 0.55 N 0.55 N > BM_Vector_petscxx_MAXPY_RMS 7 % 7 % > Direct call to MAXPY: > BM_Vector_PETSc_MAXPY/8 33 ns 33 ns 20973674 > BM_Vector_PETSc_MAXPY/64 116 ns 116 ns 5992878 > BM_Vector_PETSc_MAXPY/512 731 ns 731 ns 963340 > BM_Vector_PETSc_MAXPY/4096 5739 ns 5739 ns 122414 > BM_Vector_PETSc_MAXPY/32768 46346 ns 46346 ns 15312 > BM_Vector_PETSc_MAXPY_BigO 1.41 N 1.41 N > BM_Vector_PETSc_MAXPY_RMS 0 % 0 % > > And 3x speedup on 2 MPI ranks (not much communication here, anyway). I am now > convinced that this warrants some further investigation/testing. > > > On Tue, 4 Apr 2017 at 01:08 Jed Brown <j...@jedbrown.org> wrote: > Matthew Knepley <knep...@gmail.com> writes: > > >> BLAS. (Here a interesting point opens: I assume an efficient BLAS > >> > >> implementation, but I am not so sure about how the different BLAS do > >> things > >> > >> internally. I work from the assumption that we have a very well tuned BLAS > >> > >> implementation at our disposal). > >> > > > > The speed improvement comes from pulling vectors through memory fewer > > times by merging operations (kernel fusion). > > Typical examples are VecMAXPY and VecMDot, but note that these are not > xGEMV because the vectors are independent arrays rather than single > arrays with a constant leading dimension. > > >> call VecGetArray. However I will inevitably foget to return the array to > >> > >> PETSc. I could have my new VecArray returning an object that restores the > >> > >> array > >> > >> when it goes out of scope. I can also flag the function with [[nodiscard]] > >> to > >> > >> prevent the user to destroy the returned object from the start. > >> > > > > Jed claims that this pattern is no longer preferred, but I have forgotten > > his argument. > > Jed? > > Destruction order matters and needs to be collective. If an error > condition causes destruction to occur in a different order on different > processes, you can get deadlock. I would much rather have an error > leave some resources (for the OS to collect) than escalate into > deadlock. > > > We have had this discussion for years on this list. Having separate names > > for each type > > is really ugly and does not achieve what we want. We want smooth > > interoperability between > > objects with different backing types, but it is still not clear how to do > > this. > > Hide it internally and implicitly promote. Only the *GetArray functions > need to be parametrized on numeric type. But it's a lot of work on the > backend.