On Tue, Apr 4, 2017 at 1:19 PM, Filippo Leonardi <filippo.l...@gmail.com> wrote:
> You are in fact right, it is the same speedup of approximatively 2.5x > (with 2 ranks), my brain rounded up to 3. (This was just a test done in 10 > min on my Workstation, so no pretence to be definite, I just wanted to have > an indication). > Hmm, it seems like PetscKernelAXPY4() is just not vectorizing correctly then. I would be interested to see your code. As you say, I am using OpenBLAS, so I wouldn't be surprised of those > results. If/when I use MKL (or something similar), I really do not expect > such an improvement). > > Since you seem interested (if you are interested, I can give you all the > details): the comparison I make, is with "petscxx" which is my template > code (which uses a single loop) using AVX (I force PETSc to align the > memory to 32 bit boundary and then I use packets of 4 doubles). Also notice > that I use vectors with nice lengths, so there is no need to "peel" the end > of the loop. The "PETSc" simulation is using PETSc's VecMAXPY. > Thanks, Matt > On Tue, 4 Apr 2017 at 19:12 Barry Smith <bsm...@mcs.anl.gov> wrote: > >> >> 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. >> >> -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener