[Rd] Fourteen patches to speed up R
Radford Neal
radford at cs.toronto.edu
Fri Sep 3 19:27:19 CEST 2010
I've continued to work on speeding up R, and now have a collection of
fourteen patches, some of which speed up particular functions, and
some of which reduce general interpretive overhead. The total speed
improvement from these patches is substantial. It varies a lot from
one R program to the next, of course, and probably from one machine to
the next, but speedups of 25% can be expected in many programs, and
sometimes much more (though sometimes less as well).
The fourteen patches work for revision r52822 of the development
version of R (I haven't check against any changes in the last few
days), and also for release 2.11.1. These patches, along with
some documentation, are attached as speed-patches.tar.
I also wrote a number of timing test programs, which are attached as
speed-tests.tar.
I've included below the documentation on what each patch does, which
is also in "doc" in speed-patches.tar. Note that I fixed a few minor
bugs along the way.
There looks to be scope for more improvements in various parts of the
R interpreter that I didn't get to. I'll have to put this on hold for
now, however, to spend my time preparing for the coming teaching term.
I'd be happy to hear of any comments on these patches, though,
including information on how much they speed up typical programs, on
various machines.
Radford Neal
-----------------------------------------------------------------------
These patches to the R source for improving speed were written by
Radford M. Neal, Sept. 2010.
See the README file for how to install them.
Below, I describe these patches (in alphabetical order), indicate what
improvement they produce, and also mention any potential issues with
using the patch, and bugs that the patches incidently fix.
The timing improvements discussed below are what is obtained by
applying each patch individually, on an Intel system running Ubuntu
Linux with Gcc version 4.2.4. The total improvement from all patches
is much bigger, though in a few instances a patch can diminish the
effect of another patch, by reducing the magnitude of the
inefficiencies that the other patch eliminates. Note though, that the
percentage improvement for a given absolute improvement gets bigger as
when other patches reduce overall time.
For r52822, the total time for all tests in the accompanying speed
test suite is 674 seconds. This is reduced to 487 seconds with all
patches applied, a reduction of 28%. Particular R programs will, of
course, see widely varying reductions depending on what operations
they mostly do.
patch-dollar
Speeds up access to lists, pairlists, and environments using the
$ operator. The speedup comes mainly from avoiding the overhead of
calling DispatchOrEval if there are no complexities, from passing
on the field to extract as a symbol, or a name, or both, as available,
and then converting only as necessary, from simplifying and inlining
the pstrmatch procedure, and from not translating string multiple
times.
Relevant timing test script: test-dollar.r
This test shows about a 40% decrease in the time needed to extract
elements of lists and environments.
Changes unrelated to speed improvement:
A small error-reporting bug is fixed, illustrated by the following
output with r52822:
> options(warnPartialMatchDollar=TRUE)
> pl <- pairlist(abc=1,def=2)
> pl$ab
[1] 1
Warning message:
In pl$ab : partial match of 'ab' to ''
Some code is changed at the end of R_subset3_dflt because it seems
to be more correct, as discussed in code comments.
patch-evalList
Speeds up a large number of operations by avoiding allocation of
an extra CONS cell in the procedures for evaluating argument lists.
Relevant timing test scripts: all of them, but will look at test-em.r
On test-em.r, the speedup from this patch is about 5%.
patch-fast-base
Speeds up lookup of symbols defined in the base environment, by
flagging symbols that have a base environment definition recorded
in the global cache. This allows the definition to be retrieved
quickly without looking in the hash table.
Relevant timing test scripts: all of them, but will look at test-em.r
On test-em.r, the speedup from this patch is about 3%.
Issue: This patch uses the "spare" bit for the flag. This bit is
misnamed, since it is already used elsewhere (for closures). It is
possible that one of the "gp" bits should be used instead. The
"gp" bits should really be divided up for faster access, and so that
their present use is apparent in the code.
In case this use of the "spare" bit proves unwise, the patch code is
conditional on FAST_BASE_CACHE_LOOKUP being defined at the start of
envir.r.
patch-fast-spec
Speeds up lookup of function symbols that begin with a character
other than a letter or ".", by allowing fast bypass of non-global
environments that do not contain (and have never contained) symbols
of this sort. Since it is expected that only functions will be
given names of this sort, the check is done only in findFun, though
it could also be done in findVar.
Relevant timing test scripts: all of them, but will look at test-em.r
On test-em.r, the speedup from this patch is about 8%.
Issue: This patch uses the "spare" bit to flag environments known
to not have symbols starting with a special character. See remarks
on patch-fast-base.
In case this use of the "spare" bit proves unwise, the patch code is
conditional on FAST_SPEC_BYPASS being defined at the start of envir.r.
patch-for
Speeds up for loops by not allocating new space for the loop
variable every iteration, unless necessary.
Relevant timing test script: test-for.r
This test shows a speedup of about 5%.
Change unrelated to speed improvement:
Fixes what I consider to be a bug, in which the loop clobbers a
global variable, as demonstrated by the following output with r52822:
> i <- 99
> f <- function () for (i in 1:3) { print(i); if (i==2) rm(i); }
> f()
[1] 1
[1] 2
[1] 3
> print(i)
[1] 3
patch-matprod
Speeds up matrix products, including vector dot products. The
speed issue here is that the R code checks for any NAs, and
does the multiply in the matprod procedure (in array.c) if so,
since BLAS isn't trusted with NAs. If this check takes about
as long as just doing the multiply in matprod, calling a BLAS
routine makes no sense.
Relevant time test script: test-matprod.r
With no external BLAS, this patch speeds up long vector-vector
products by a factor of about six, matrix-vector products by a
factor of about three, and some matrix-matrix products by a
factor of about two.
Issue: The matrix multiply code in matprod using an LDOUBLE
(long double) variable to accumulate sums, for improved accuracy.
On a SPARC system I tested on, operations on long doubles are
vastly slower than on doubles, so that the patch produces a
large slowdown rather than an improvement. This is also an issue
for the "sum" function, which also uses an LDOUBLE to accumulate
the sum. Perhaps an ordinarly double should be used in these
places, or perhaps the configuration script should define LDOUBLE
as double on architectures where long doubles are extraordinarily
slow.
Due to this issue, not defining MATPROD_CAN_BE_DONE_HERE at the
start of array.c will disable this patch.
patch-parens
Speeds up parentheses by making "(" a special operator whose
argument is not evaluated, thereby bypassing the overhead of
evalList. Also slightly speeds up curly brackets by inlining
a function that is stylistically better inline anyway.
Relevant test script: test-parens.r
In the parens part of test-parens.r, the speedup is about 9%.
patch-protect
Speeds up numerous operations by making PROTECT, UNPROTECT, etc.
be mostly macros in the files in src/main. This takes effect
only for files that include Defn.h after defining the symbol
USE_FAST_PROTECT_MACROS. With these macros, code of the form
v = PROTECT(...) must be replaced by PROTECT(v = ...).
Relevant timing test scripts: all of them, but will look at test-em.r
On test-em.r, the speedup from this patch is about 9%.
patch-save-alloc
Speeds up some binary and unary arithmetic operations by, when
possible, using the space holding one of the operands to hold
the result, rather than allocating new space. Though primarily
a speed improvement, for very long vectors avoiding this allocation
could avoid running out of space.
Relevant test script: test-complex-expr.r
On this test, the speedup is about 5% for scalar operands and about
8% for vector operands.
Issues: There are some tricky issues with attributes, but I think
I got them right. This patch relies on NAMED being set correctly
in the rest of the code. In case it isn't, the patch can be disabled
by not defining AVOID_ALLOC_IF_POSSIBLE at the top of arithmetic.c.
patch-square
Speeds up a^2 when a is a long vector by not checking for the
special case of an exponent of 2 over and over again for every
vector element.
Relevant test script: test-square.r
The time for squaring a long vector is reduced in this test by a
factor of more than five.
patch-sum-prod
Speeds up the "sum" and "prod" functions by not checking for NA
when na.rm=FALSE, and other detailed code improvements.
Relevant test script: test-sum-prod.r
For sum, the improvement is about a factor of 2.5 when na.rm=FALSE,
and about 10% when na.rm=TRUE.
Issue: See the discussion of patch-matprod regarding LDOUBLE.
There is no change regarding this issue due to this patch, however.
patch-transpose
Speeds up the transpose operation (the "t" function) from detailed
code improvements.
Relevant test script: test-transpose.r
The improvement for 200x60 matrices is about a factor of two.
There is little or no improvement for long row or column vectors.
patch-vec-arith
Speeds up arithmetic on vectors of the same length, or when on
vector is of length one. This is done with detailed code improvements.
Relevant test script: test-vec-arith.r
On long vectors, the +, -, and * operators are sped up by about
20% when operands are the same length or one operand is of length one.
Rather mysteriously, when the operands are not length one or the
same length, there is about a 20% increase in time required, though
this may be due to some strange C optimizer peculiarity or some
strange cache effect, since the C code for this is the same as before,
with negligible additional overhead getting to it. Regardless, this
case is much less common than equal lengths or length one.
There is little change for the / operator, which is much slower than
+, -, or *.
patch-vec-subset
Speeds up extraction of subsets of vectors or matrices (eg, v[10:20]
or M[1:10,101:110]). This is done with detailed code improvements,
some increased fast treatment of common cases, and some avoidance of
unnecessary duplication.
Relevant test script: test-vec-subset.r
There are lots of tests in this script. The most dramatic improvement
is for extracting many rows and columns of a large array, where the
improvement is by about a factor of four. Extracting many rows from
one column of a matrix is sped up by about 30%. Extracting a large
part of a vector is sped up by about 20%. Several other operations
have improvements of 10% or more.
Changes unrelated to speed improvement:
Fixes two latent bugs where the code incorrectly refers to NA_LOGICAL
when NA_INTEGER is appropriate and where LOGICAL and INTEGER types
are treated as interchangeable. These cause no problems at the moment,
but would if representations were changed.
Issues: The current code duplicates a vector of indexes when
duplication seems unnecessary. As far as I can see, the only reason
for this is so that it can remove attributes, which is helpful only
for string subscripts, given how the routine to handle them returns
information via an attribute. If this is the only reason, as I concluded,
the duplication can easily be avoided, so I avoided it. But perhaps
I don't understand something, since there are a fair number of
interactions going on with this code. I also removed a layer of
procedure call overhead that seemed to be doing nothing. Probably
it used to do something, but no longer does, but if instead it is
preparation for some future use, then removing it would be a mistake.
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