Re: [gentoo-dev] RFC: BLAS and LAPACK runtime switching

[Michał Górny]

On Tue, 2019-05-28 at 01:37 -0700, Mo Zhou wrote:
> Different BLAS/LAPACK implementations are expected to be compatible
> to each other in both the API and ABI level. They can be used as
> drop-in replacement to the others. This sounds nice, but the difference
> in SONAME hampered the gentoo integration of well-optimized ones.

If SONAMEs are different, then they are not compatible by definition.

> Assume a Gentoo user compiled a pile of packages on top of the reference
> BLAS and LAPACK, namely these reverse dependencies are linked against
> libblas.so.3 and liblapack.so.3 . When the user discovered that
> OpenBLAS provides much better performance, they'll have to recompile
> the whole reverse dependency tree in order to take advantage from
> OpenBLAS,
> because the SONAME of OpenBLAS is libopenblas.so.0 . When the user
> wants to try MKL (libmkl_rt.so), they'll have to recompile the whole
> reverse dependency tree again.
>
> This is not friendly to our earth.
>
> Goal
> ----
>
>   * When a program is linked against libblas.so or liblapack.so
>     provided by any BLAS/LAPACK provider, the eselect-based solution
>     will allow user to switch the underlying library without recompiling
>     anything.
>
>   * When a program is linked against a specific implementation, e.g.
>     libmkl_rt.so, the solution doesn't break anything.
>
> Solution
> --------
>
> Similar to Debian's update-alternatives mechanism, Gentoo's eselect
> is good at dealing with drop-in replacements as well. My preliminary
> investigation suggests that eselect is enough for enabling BLAS/LAPACK
> runtime switching. Hence, the proposed solution is eselect-based:
>
>   * Every BLAS/LAPACK implementation should provide generic library
>     and eselect candidate libraries at the same time. Taking netlib,
>     BLIS and OpenBLAS as examples:
>
>     reference:
>
>       usr/lib64/blas/reference/libblas.so.3 (SONAME=libblas.so.3)
>         -- default BLAS provider
>         -- candidate of the eselect "blas" unit
>         -- will be symlinked to usr/lib64/libblas.so.3 by eselect

/usr/lib64 is not supposed to be modified by eselect, it's package
manager area.  Yes, I know a lot of modules still do that but that's no
reason to make things worse when people are putting significant effort
to actually improve things.

>       usr/lib64/lapack/reference/liblapack.so.3 (SONAME=liblapack.so.3)
>         -- default LAPACK provider
>         -- candidate of the eselect "lapack" unit
>         -- will be symlinked to usr/lib64/liblapack.so.3 by eselect
>
>     blis (doesn't provide LAPACK):
>
>       usr/lib64/libblis.so.2  (SONAME=libblis.so.2)
>         -- general purpose
>
>       usr/lib64/blas/blis/libblas.so.3 (SONAME=libblas.so.3)
>         -- candidate of the eselect "blas" unit
>         -- will be symlinked to usr/lib64/libblas.so.3 by eselect
>         -- compiled from the same set of object files as libblis.so.2
>
>     openblas:
>
>       usr/lib64/libopenblas.so.0 (SONAME=libopenblas.so.0)
>         -- general purpose
>
>       usr/lib64/blas/openblas/libblas.so.3 (SONAME=libblas.so.3)
>         -- candidate of the eselect "blas" unit
>         -- will be symlinked to usr/lib64/libblas.so.3 by eselect
>         -- compiled from the same set of object files as
> libopenblas.so.0
>
>       usr/lib64/lapack/openblas/liblapack.so.3 (SONAME=liblapack.so.3)
>         -- candidate of the eselect "lapack" unit
>         -- will be symlinked to usr/lib64/liblapack.so.3 by eselect
>         -- compiled from the same set of object files as
> libopenblas.so.0
>
> This solution is similar to Debian's[3]. This solution achieves our
> goal,
> and it requires us to patch upstream build systems (same to Debian).
> Preliminary demonstration for this solution is available, see below.

So basically the three walls of text say in round-about way that you're
going to introduce custom hacks to recompile libraries with different
SONAME.  Ok.

>
> Is this solution reliable?
> --------------------------
>
> * A similar solution has been used by Debian for many years.
> * Many projects call BLAS/LAPACK libraries through FFI, including Julia.
>   (See Julia's standard library: LinearAlgebra)
>
> Proposed Changes
> ----------------
>
> 1. Deprecate sci-libs/{blas,cblas,lapack,lapacke}-reference from gentoo
>    main repo. They use exactly the same source tarball. It's not quite
>    helpful to package these components in a fine-grained manner. A
> single
>    sci-libs/lapack package is enough.

Where's the gain in that?

> 2. Merge the "cblas" eselect unit into "blas" unit. It is potentially
>    harmful when "blas" and "cblas" point to different implementations.
>    That means "app-eselect/eselect-cblas" should be deprecated.
>
> 3. Update virtual/{blas,cblas,lapack,lapacke}. BLAS/LAPACK providers
>    will be registered in their dependency information.
>
> Note, ebuilds for BLAS/LAPACK reverse dependencies are expected to work
> with these changes correctly without change. For example, my local
> numpy-1.16.1 compilation was successful without change.
>
> Preliminary Demonstration
> -------------------------
>
> The preliminary implementation is available in my personal overlay[4].
> A simple sanity test script `check-cpp.sh` is provided to illustrate
> the effectiveness of the proposed solution.
>
> The script `check-cpp.sh` compiles two C++ programs -- one calls general
> matrix-matrix multiplication from BLAS, while another one calls general
> singular value decomposition from LAPACK. Once compiled, this script
> will switch different BLAS/LAPACK implementations and run the C++
> programs
> without recompilation.
>
> The preliminary result is avaiable here[5]. (CPU=Power9, ARCH=ppc64le)
> From the experimental results, we find that
>
>   For (512x512) single precision matrix multiplication:
>    * reference BLAS takes ~360 ms
>    * BLIS takes ~70 ms
>    * OpenBLAS takes ~10 ms
>
>   For (512x512) single precision singular value decomposition:
>    * reference LAPACK takes ~1900 ms
>    * BLIS (+reference LAPACK) takes ~1500 ms
>    * OpenBLAS takes ~1100 ms
>
> The difference in computation speed illustrates the effectiveness of
> the proposed solution. Theoretically, any other package could take
> advantage from this solution without any recompilation as long as
> it's linked against a library with SONAME.

An actual ABI compliance test, e.g. done using abi-compliance-checker
would be more interesting.

>
> Acknowledgement
> ---------------
> This is an on-going GSoC-2019 Porject:
> https://summerofcode.withgoogle.com/projects/?sp-page=2#6268942782300160

It would probably have been better if the project was discussed before
GSoC.  I'm really against pushing a bad idea forward just because
someone set it for GSoC without discussing it first.

-- 
Best regards,
Michał Górny