From tom@@@k@||ber@ @end|ng |rom gm@||@com  Wed Apr 13 12:26:31 2022
From: tom@@@k@||ber@ @end|ng |rom gm@||@com (Tomas Kalibera)
Date: Wed, 13 Apr 2022 12:26:31 +0200
Subject: [Rd] 
 Matrix issues when building R with znver3 architecture under GCC 11
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On 4/13/22 11:20, Kieran Short wrote:
> Hi Tomas,
>
> Many thanks for your thorough response, it is very much appreciated 
> and what you say makes perfect sense to me.
>
> I was relying on the in-built R compilation checks, I have been 
> working on the assumption that everything on the R side is correct 
> (including the matrix package).
>
> Indeed, R 4.1.3 builds and "make check-all" passes with the more 
> general?-march=x86-64 architecture compiled with -O3 optimizations (in 
> my hands, on the Zen3 system). So I had no underlying reason not to 
> believe R or its packages were the problem when -march=znver3 was 
> trialed. I found it interesting that it was only the one 
> factorizing.R?script in the Matrix suite that failed (out of the 
> seemingly hundreds of remaining checks overall which passed). So I was 
> more wondering if there might have been prior knowledge within the 
> brain's trust on this list that "oh the factorizing.R matrix test does 
> ABC error when R or the package is compiled with GCC using?XYZ flags". 
> As you'll read ahead, you can say that now. :)
Right, but something must be broken. You might get specific comments 
from the Matrix package maintainer, but it would help at least 
minimizing that failing example to some commands you can run in R 
console, and showing the differences in outputs.
>
> I don't think I have the capability to determine the root trigger in R 
> itself, the package, or the compiler (whichever one, or combination,? 
> it actually?is). However, assuming R isn't the issue, I have done is 
> go through the GCC optimizations and I have now isolated the culprit 
> optimization which crashes factorizing.R.
>
> It is "-fexpensive-optimizations".
>
> If I use?"-fno-expensive-optimizations" paired with -O2 or -O3 
> optimizations, all "make check-all" checks pass. So I can build a 
> fully checked and passed R 4.1.3 under my environment now with:
>
> ~/R/R-4.1.3/configure CC=gcc-11.2 CXX=g++-11.2 FC=gfortran-11.2 
> CXXFLAGS="-O3 -march=znver3 -fno-expensive-optimizations -flto" 
> CFLAGS="-O3 -march=znver3 -fno-expensive-optimizations -flto" 
> FFLAGS="-O3 -march=znver3 -fno-expensive-optimizations -flto" 
> --enable-memory-profiling --enable-R-shlib
Ok. The default optimization options used by R on selected current and 
future versions of GCC and clang also get tested via checking all of 
CRAN contributed packages. This testing sometimes finds errors not 
detected by "make check-all", including bugs in GCC. You would need a 
lot of resources to run these checks, though. In my experience it is not 
so rare that a bug (in R or GCC) only affects a very small number of 
packages, often even only one.
> I'm yet to benchmark whether the loss of that particular optimization 
> flag negates the advantages of using znver3 as a core architecture 
> target over a -x86-64 target in the first place.
> So I think I've solved my own problem (at least, it appears that way 
> based on the checks).
> So the remaining question is, what method or package does the 
> development team use (if any) for testing the speed of various base R 
> calculations?

That depends on the developer and the calculations, and on your goals - 
what you want to measure or show. I don't have a simple advice. If you 
are considering this for your own work, I'd recommend measuring some of 
your workloads. Also you can extrapolate from your workloads (from where 
time is spent in them) what would be a relevant benchmark. For example, 
if most time is spent in BLAS, then it is about finding a good optimized 
implementation (and for that checking the impact of the optimizations). 
Similarly, if it is some R package (base, recommended, or contributed), 
it may be using a computational kernel written in C or Fortran, 
something you could test separately or with a specific benchmark. I 
think it would be unlikely that CPU-specific C compiler optimizations 
would substantially speed up the R interpreter itself.

For just deciding whether -fno-expensive-optimization negates the gains, 
you might look at some general computational/other benchmarks (not R). 
If it negated it even on benchmarks used by others to present the gains, 
then it probably is not worth it.

One of the things I did in the past was looking at timings of selected 
CRAN packages (longer running examples, packages with most reverse 
dependencies) and then looking into the reasons for the individual 
bigger differences. That was when looking at the impacts of the 
byte-code compiler. Unlikely worth the effort in this case. Also, 
primarily, I think the bug should be traced down and fixed, wherever it 
is. Only then the measuring would make sense.

Best
Tomas



>
> best regards,
> Kieran
>
> On Wed, Apr 13, 2022 at 4:00 PM Tomas Kalibera 
> <tomas.kalibera at gmail.com> wrote:
>
>     Hi Kieran,
>
>     On 4/12/22 02:36, Kieran Short wrote:
>     > Hello,
>     >
>     > I'm new to this list, and have subscribed particularly because
>     I've come
>     > across an issue with building R from source with an AMD-based Zen
>     > architecture under GCC11. Please don't attack me for my linux
>     operating
>     > system choice, but it is Ubuntu 20.04 with Linux Kernel 5.10.102.1 -
>     > microsoft-standard-WSL2. I've built GCC11 using GCC8 (the
>     standard GCC
>     > under Ubuntu20.04 WSL release), under Windows11 with wslg.
>     WSL2/g runs as a
>     > hypervisor with ports to all system resources including display,
>     GPU (cuda,
>     > etc).
>     >
>     > The reason why I am posting this email is that I am trying to
>     compile R
>     > using the AMD Zen3 platform architecture rather than x86/64,
>     because it has
>     > processor-specific optimizations that improve performance over
>     the standard
>     > x86/64 in benchmarks. The Zen3 architecture optimizations are
>     not available
>     > in earlier versions of GCC (actually, they have possibly been
>     backported to
>     > GCC10 now). Since Ubuntu 20.04 doesn't have GCC11, I compiled
>     the GCC11
>     > compiler using the native GCC8.
>     >
>     > The GCC11 I have built can build R 4.1.3 with a standard x86-64
>     > architecture and pass all tests with "make check-all".
>     > I configured that with:
>     >> ~/R/R-4.1.3/configure CC=gcc-11.2 CXX=g++-11.2 FC=gfortran-11.2
>     > CXXFLAGS="-O3 -march=x86-64" CFLAGS="-O3 -march=x86-64" FFLAGS="-O3
>     > -march=x86-64" --enable-memory-profiling --enable-R-shlib
>     > and built with
>     >> make -j 32 -O
>     >> make check-all
>     > ## PASS.
>     >
>     > So I can build R in my environment with GCC11.
>     > In configure, I am using references to "gcc-11.2"
>     "gfortran-11.2" and
>     > "g++-11.2" because I compiled GCC11 compilers with these suffixes.
>     >
>     > Now, I'm using a 32 thread (16 core) AMD Zen3 CPU (a 5950x), and
>     want to
>     > use it to its full potential. Zen3 optimizations are available as a
>     > -march=znver3 option n GCC11. The znver3 optimizations improve
>     performance
>     > in Phoronix Test Suite benchmarks (I'm not aware of anyone that has
>     > compiled R with them). See:
>     >
>     https://www.phoronix.com/scan.php?page=article&item=amd-5950x-gcc11
>     <https://www.phoronix.com/scan.php?page=article&item=amd-5950x-gcc11>
>     >
>     > However, the R 4.1.3 build (made with "make -j 32 -O"),
>     configured with
>     > -march=znver3, produces an R that fails "make check-all".
>     >
>     >> ~/R/R-4.1.3/configure CC=gcc-11.2 CXX=g++-11.2 FC=gfortran-11.2
>     > CXXFLAGS="-O2 -march=znver3" CFLAGS="-O2 -march=znver3" FFLAGS="-O2
>     > -march=znver3" --enable-memory-profiling --enable-R-shlib
>     > or
>     >> ~/R/R-4.1.3/configure CC=gcc-11.2 CXX=g++-11.2 FC=gfortran-11.2
>     > CXXFLAGS="-O3 -march=znver3" CFLAGS="-O3 -march=znver3" FFLAGS="-O3
>     > -march=znver3" --enable-memory-profiling --enable-R-shlib
>     >
>     > The fail is always in the factorizing.R Matrix.R tests, and in
>     particular,
>     > there are a number of errors and a fatal error.
>     > I have attached the output because I cannot really understand
>     what is going
>     > wrong. But results returned from matrix calculations are
>     obviously odd with
>     > -march=znver3 in GCC 11. There is another backwards-compatible
>     architecture
>     > option "znver2" and this has EXACTLY the same result.
>     >
>     > While there are other warrnings and errors (many in assert.EQ()
>     ), the
>     > factorizing.R script continues. The fatal error (at line 2662 in the
>     > attached factorizing.Rout.fail text file) is:
>     >
>     >> ## problematic rank deficient rankMatrix() case -- only seen in
>     large
>     > cases ??
>     >> Z. <- readRDS(system.file("external", "Z_NA_rnk.rds",
>     package="Matrix"))
>     >> tools::assertWarning(rnkZ. <- rankMatrix(Z., method = "qr")) #
>     gave errors
>     > Error in assertCondition(expr, classes, .exprString = d.expr) :
>     >? ? Failed to get warning in evaluating rnkZ. <- rankMatrix(Z.,
>     method? ...
>     > Calls: <Anonymous> -> assertCondition
>     > Execution halted
>     >
>     > Can anybody shed light on what might be going on here? 'make
>     check-all'
>     > passes all the other checks. It is just factorizing.R in Matrix
>     that fails
>     > (other matrix tests run ok).
>     > Sorry this is a bit long-winded, but I thought details might be
>     important.
>
>     R gets used and tested most with the default optimizations,
>     without use
>     of model-specific instructions and with -O2 (GCC). It happens time to
>     time that some people try other optimization options and run into
>     problems. In principle, there are these cases (seen before):
>
>     (1) the test in R package (or R) is wrong - it (unintentionally)
>     expects
>     behavior which has been observed in builds with default
>     optimizations,
>     but is not necessarily the only correct one; in case of numerical
>     tolerances set empirically, they could simply be too tight
>
>     (2) the algorithm in R package or R has a bug - the result is really
>     wrong and it is because the algorithm is (unintentionally) not
>     portable
>     enough, it (unintentionally) only works with default optimizations or
>     lower; in case of numerical results, this can be because it
>     expects more
>     precision from the floating point computations than mandated by
>     IEEE, or
>     assumes behavior not mandated
>
>     (3) the optimization by design violates some properties the algorithm
>     knowingly depends on; with numerical computations, this can be a
>     sort of
>     "fast" (and similarly referred to) mode which violates IEEE floating
>     point standard by design, in the aim of better performance; due to
>     the
>     nature of the algorithm depending on IEEE, and poor luck, the results
>     end up completely wrong
>
>     (4) there is a bug in the C or Fortran compiler (GCC as we use
>     GCC) that
>     only exhibits with the unusual optimizations; the compiler produces
>     wrong code
>
>     So, when you run into a problem like this and want to get that fixed,
>     the first thing is to identify which case of the above it is, in
>     case of
>     1 and 2 also differentiate between base R and a package (and which
>     concrete package). Different people maintain these things and you
>     would
>     ideally narrow down the problem to a very small, isolated,
>     reproducible
>     example to support your claim where the bug is. If you do this right,
>     the problem can often get fixed very fast.
>
>     Such an example for (1) could be: few lines of standalone R code
>     using
>     Matrix that produces correct results, but the test is not happy. With
>     pointers to the real check in the tests that is wrong. And an
>     explanation why the result is wrong.
>
>     For (2)-(4) it would be a minimal standalone C/Fortran example
>     including
>     only the critical function/part of algorithm that is not correct/not
>     portable/not compiled correctly, with results obtained with
>     optimizations where it works and where it doesn't. Unless you find an
>     obvious bug in R easy to explain (2), when the example would not
>     have to
>     be standalone. With such standalone C example, you could easily
>     test the
>     results with different optimizations and compilers, it is easier to
>     analyze, and easier to produce a bug report for GCC. What would
>     make it
>     harder in this case is that it needs special hardware, but you could
>     still try with the example, and worry about that later (one option is
>     running in an emulator, and again a standalone example really helps
>     here). In principle, as it needs special hardware, the chances
>     someone
>     else would do this work is smaller. Indeed, if it turns out to be
>     (3),
>     it is unlikely to get resolved, but at least would get isolated (you
>     would know what not to run).
>
>     As a user, if you run into a problem like this and do not want to
>     get it
>     fixed, but just work it around somehow. First, it may be dangerous,
>     possibly one would get incorrect results from computations, but
>     say in
>     applications where they are verified externally. You could try
>     disabling
>     individual specific optimization until the tests pass. You could try
>     with later versions of gcc-11 (even unreleased) or gcc-12. Still,
>     a lot
>     of this is easier with a small example, too. You could ignore the
>     failing test. And it may not be worth it - it may be that you
>     could get
>     your speedups in a different, but more reliable way.
>
>     Using wsl2 on its own should not necessarily be a problem and the way
>     you built gcc from the description should be ok, but at some point it
>     would be worth checking under Linux and running natively - because
>     even
>     if these are numerical differences, they could be in principle
>     caused by
>     running on Windows (or in wsl2), at least in the past such
>     differences
>     were seen (related to (2) above). I would recommend checking on Linux
>     natively once you have at least a standalone R example.
>
>     Best
>     Tomas
>
>
>     >
>     > best regards,
>     > Kieran
>     > ______________________________________________
>     > R-devel at r-project.org mailing list
>     > https://stat.ethz.ch/mailman/listinfo/r-devel
>
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[Rd] Matrix issues when building R with znver3 architecture under GCC 11
Tomas Kalibera