diff options
Diffstat (limited to 'vendor/gmp-6.3.0/tune/README')
| -rw-r--r-- | vendor/gmp-6.3.0/tune/README | 501 |
1 files changed, 501 insertions, 0 deletions
diff --git a/vendor/gmp-6.3.0/tune/README b/vendor/gmp-6.3.0/tune/README new file mode 100644 index 0000000..f76407f --- /dev/null +++ b/vendor/gmp-6.3.0/tune/README @@ -0,0 +1,501 @@ +Copyright 2000-2002, 2004 Free Software Foundation, Inc. + +This file is part of the GNU MP Library. + +The GNU MP Library is free software; you can redistribute it and/or modify +it under the terms of either: + + * the GNU Lesser General Public License as published by the Free + Software Foundation; either version 3 of the License, or (at your + option) any later version. + +or + + * the GNU General Public License as published by the Free Software + Foundation; either version 2 of the License, or (at your option) any + later version. + +or both in parallel, as here. + +The GNU MP Library is distributed in the hope that it will be useful, but +WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY +or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License +for more details. + +You should have received copies of the GNU General Public License and the +GNU Lesser General Public License along with the GNU MP Library. If not, +see https://www.gnu.org/licenses/. + + + + + + GMP SPEED MEASURING AND PARAMETER TUNING + + +The programs in this directory are for knowledgeable users who want to +measure GMP routines on their machine, and perhaps tweak some settings or +identify things that can be improved. + +The programs here are tools, not ready to run solutions. Nothing is built +in a normal "make all", but various Makefile targets described below exist. + +Relatively few systems and CPUs have been tested, so be sure to verify that +results are sensible before relying on them. + + + + +MISCELLANEOUS NOTES + +--enable-assert + + Don't configure with --enable-assert, since the extra code added by + assertion checking may influence measurements. + +Direct mapped caches + + Some effort has been made to accommodate CPUs with direct mapped caches, + by putting data blocks more or less contiguously on the stack. But this + will depend on TMP_ALLOC using alloca, and even then it may or may not + be enough. + +FreeBSD 4.2 i486 getrusage + + This getrusage seems to be a bit doubtful, it looks like it's + microsecond accurate, but sometimes ru_utime remains unchanged after a + time of many microseconds has elapsed. It'd be good to detect this in + the time.c initializations, but for now the suggestion is to pretend it + doesn't exist. + + ./configure ac_cv_func_getrusage=no + +NetBSD 1.4.1 m68k macintosh time base + + On this system it's been found getrusage often goes backwards, making it + unusable (time.c getrusage_backwards_p detects this). gettimeofday + sometimes doesn't update atomically when it crosses a 1 second boundary. + Not sure what to do about this. Expect possible intermittent failures. + +SCO OpenUNIX 8 /etc/hw + + /etc/hw takes about a second to return the cpu frequency, which suggests + perhaps it's measuring each time it runs. If this is annoying when + running the speed program repeatedly then set a GMP_CPU_FREQUENCY + environment variable (see TIME BASE section below). + +Timing on GNU/Linux + + On Linux, timing currently uses the cycle counter. This is unreliable, + since the counter is not saved and restored at context switches (unlike + FreeBSD and Solaris where the cycle counter is "virtualized"). + + Using the clock_gettime method with CLOCK_PROCESS_CPUTIME_ID (posix) or + CLOCK_VIRTUAL (BSD) should be more reliable. To get clock_gettime + with glibc, one has to link with -lrt (which also drags in the pthreads + threading library). configure.in must be hacked to detect this and + arrange proper linking. Something like + + old_LIBS="$LIBS" + AC_SEARCH_LIBS(clock_gettime, rt, [AC_DEFINE(HAVE_CLOCK_GETTIME)]) + TUNE_LIBS="$LIBS" + LIBS="$old_LIBS" + + AC_SUBST(TUNE_LIBS) + + might work. + +Low resolution timebase + + Parameter tuning can be very time consuming if the only timebase + available is a 10 millisecond clock tick, to the point of being + unusable. This is currently the case on VAX and ARM systems. + + + + +PARAMETER TUNING + +The "tuneup" program runs some tests designed to find the best settings for +various thresholds, like MUL_TOOM22_THRESHOLD. Its output can be put +into gmp-mparam.h. The program is built and run with + + make tune + +If the thresholds indicated are grossly different from the values in the +selected gmp-mparam.h then there may be a performance boost in applicable +size ranges by changing gmp-mparam.h accordingly. + +Be sure to do a full reconfigure and rebuild to get any newly set thresholds +to take effect. A partial rebuild is enough sometimes, but a fresh +configure and make is certain to be correct. + +If a CPU has specific tuned parameters coming from a gmp-mparam.h in one of +the mpn subdirectories then the values from "make tune" should be similar. +But check that the configured CPU is right and there are no machine specific +effects causing a difference. + +It's hoped the compiler and options used won't have too much effect on +thresholds, since for most CPUs they ultimately come down to comparisons +between assembler subroutines. Missing out on the longlong.h macros by not +using gcc will probably have an effect. + +Some thresholds produced by the tune program are merely single values chosen +from what's a range of sizes where two algorithms are pretty much the same +speed. When this happens the program is likely to give somewhat different +values on successive runs. This is noticeable on the toom3 thresholds for +instance. + + + + +SPEED PROGRAM + +The "speed" program can be used for measuring and comparing various +routines, and producing tables of data or gnuplot graphs. Compile it with + + make speed + +(Or on DOS systems "make speed.exe".) + +Here are some examples of how to use it. Check the code for all the +options. + +Draw a graph of mpn_mul_n, stepping through sizes by 10 or a factor of 1.05 +(whichever is greater). + + ./speed -s 10-5000 -t 10 -f 1.05 -P foo mpn_mul_n + gnuplot foo.gnuplot + +Compare mpn_add_n and an mpn_lshift by 1, showing times in cycles and +showing under mpn_lshift the difference between it and mpn_add_n. + + ./speed -s 1-40 -c -d mpn_add_n mpn_lshift.1 + +Using option -c for times in cycles is interesting but normally only +necessary when looking carefully at assembler subroutines. You might think +it would always give an integer value, but this doesn't happen in practice, +probably due to overheads in the time measurements. + +In the free-form output the "#" symbol against a measurement means the +corresponding routine is fastest at that size. This is a convenient visual +cue when comparing different routines. The graph data files <name>.data +don't get this since it would upset gnuplot or other data viewers. + + + + +TIME BASE + +The time measuring method is determined in time.c, based on what the +configured host has available. A cycle counter is preferred, possibly +supplemented by another method if the counter has a limited range. A +microsecond accurate getrusage() or gettimeofday() will work quite well too. + +The cycle counters (except possibly on alpha) and gettimeofday() will depend +on the machine being otherwise idle, or rather on other jobs not stealing +CPU time from the measuring program. Short routines (those that complete +within a timeslice) should work even on a busy machine. + +Some trouble is taken by speed_measure() in common.c to avoid ill effects +from sporadic interrupts, or other intermittent things (like cron waking up +every minute). But generally an idle machine will be necessary to be +certain of consistent results. + +The CPU frequency is needed to convert between cycles and seconds, or for +when a cycle counter is supplemented by getrusage() etc. The speed program +will convert as necessary according to the output format requested. The +tune program will work with either cycles or seconds. + +freq.c knows how to get the frequency on some systems, or can measure a +cycle counter against gettimeofday() or getrusage(), but when that fails, or +needs to be overridden, an environment variable GMP_CPU_FREQUENCY can be +used (in Hertz). For example in "bash" on a 650 MHz machine, + + export GMP_CPU_FREQUENCY=650e6 + +A high precision time base makes it possible to get accurate measurements in +a shorter time. + + + + +EXAMPLE COMPARISONS - VARIOUS + +Here are some ideas for things that can be done with the speed program. + +There's always going to be a certain amount of overhead in the time +measurements, due to reading the time base, and in the loop that runs a +routine enough times to get a reading of the desired precision. Noop +functions taking various arguments are available to measure this. The +"overhead" printed by the speed program each time in its intro is the "noop" +routine, but note that this is just for information, it isn't deducted from +the times printed or anything. + + ./speed -s 1 noop noop_wxs noop_wxys + +To see how many cycles per limb a routine is taking, look at the time +increase when the size increments, using option -D. This avoids fixed +overheads in the measuring. Also, remember many of the assembler routines +have unrolled loops, so it might be necessary to compare times at, say, 16, +32, 48, 64 etc to see what the unrolled part is taking, as opposed to any +finishing off. + + ./speed -s 16-64 -t 16 -C -D mpn_add_n + +The -C option on its own gives cycles per limb, but is really only useful at +big sizes where fixed overheads are small compared to the code doing the +real work. Remember of course memory caching and/or page swapping will +affect results at large sizes. + + ./speed -s 500000 -C mpn_add_n + +Once a calculation stops fitting in the CPU data cache, it's going to start +taking longer. Exactly where this happens depends on the cache priming in +the measuring routines, and on what sort of "least recently used" the +hardware does. Here's an example for a CPU with a 16kbyte L1 data cache and +32-bit limb, showing a suddenly steeper curve for mpn_add_n at about 2000 +limbs. + + ./speed -s 1-4000 -t 5 -f 1.02 -P foo mpn_add_n + gnuplot foo.gnuplot + +When a routine has an unrolled loop for, say, multiples of 8 limbs and then +an ordinary loop for the remainder, it can happen that it's actually faster +to do an operation on, say, 8 limbs than it is on 7 limbs. The following +draws a graph of mpn_sub_n, to see whether times smoothly increase with +size. + + ./speed -s 1-100 -c -P foo mpn_sub_n + gnuplot foo.gnuplot + +If mpn_lshift and mpn_rshift have special case code for shifts by 1, it +ought to be faster (or at least not slower) than shifting by, say, 2 bits. + + ./speed -s 1-200 -c mpn_rshift.1 mpn_rshift.2 + +An mpn_lshift by 1 can be done by mpn_add_n adding a number to itself, and +if the lshift isn't faster there's an obvious improvement that's possible. + + ./speed -s 1-200 -c mpn_lshift.1 mpn_add_n_self + +On some CPUs (AMD K6 for example) an "in-place" mpn_add_n where the +destination is one of the sources is faster than a separate destination. +Here's an example to see this. ".1" selects dst==src1 for mpn_add_n (and +mpn_sub_n), for other values see speed.h SPEED_ROUTINE_MPN_BINARY_N_CALL. + + ./speed -s 1-200 -c mpn_add_n mpn_add_n.1 + +The gmp manual points out that divisions by powers of two should be done +using a right shift because it'll be significantly faster than an actual +division. The following shows by what factor mpn_rshift is faster than +mpn_divrem_1, using division by 32 as an example. + + ./speed -s 10-20 -r mpn_rshift.5 mpn_divrem_1.32 + + + + +EXAMPLE COMPARISONS - MULTIPLICATION + +mul_basecase takes a ".<r>" parameter. If positive, it gives the second +(smaller) operand size. For example to show speeds for 3x3 up to 20x3 in +cycles, + + ./speed -s 3-20 -c mpn_mul_basecase.3 + +A negative ".<-r>" parameter fixes the size of the product to the absolute +value r. For example to show speeds for 10x10 up to 19x1 in cycles, + + ./speed -s 10-19 -c mpn_mul_basecase.-20 + +mul_basecase with no parameter does an NxN multiply, so for example to show +speeds in cycles for 1x1, 2x2, 3x3, etc, up to 20x20, in cycles, + + ./speed -s 1-20 -c mpn_mul_basecase + +sqr_basecase is implemented by a "triangular" method on most CPUs, making it +up to twice as fast as mul_basecase. In practice loop overheads and the +products on the diagonal mean it falls short of this. Here's an example +running the two and showing by what factor an NxN mul_basecase is slower +than an NxN sqr_basecase. (Some versions of sqr_basecase only allow sizes +below SQR_TOOM2_THRESHOLD, so if it crashes at that point don't worry.) + + ./speed -s 1-20 -r mpn_sqr_basecase mpn_mul_basecase + +The technique described above with -CD for showing the time difference in +cycles per limb between two size operations can be done on an NxN +mul_basecase using -E to change the basis for the size increment to N*N. +For instance a 20x20 operation is taken to be doing 400 limbs, and a 16x16 +doing 256 limbs. The following therefore shows the per crossproduct speed +of mul_basecase and sqr_basecase at around 20x20 limbs. + + ./speed -s 16-20 -t 4 -CDE mpn_mul_basecase mpn_sqr_basecase + +Of course sqr_basecase isn't really doing NxN crossproducts, but it can be +interesting to compare it to mul_basecase as if it was. For sqr_basecase +the -F option can be used to base the deltas on N*(N+1)/2 operations, which +is the triangular products sqr_basecase does. For example, + + ./speed -s 16-20 -t 4 -CDF mpn_sqr_basecase + +Both -E and -F are preliminary and might change. A consistent approach to +using them when claiming certain per crossproduct or per triangularproduct +speeds hasn't really been established, but the increment between speeds in +the range karatsuba will call seems sensible, that being k to k/2. For +instance, if the karatsuba threshold was 20 for the multiply and 30 for the +square, + + ./speed -s 10-20 -t 10 -CDE mpn_mul_basecase + ./speed -s 15-30 -t 15 -CDF mpn_sqr_basecase + + + +EXAMPLE COMPARISONS - MALLOC + +The gmp manual recommends application programs avoid excessive initializing +and clearing of mpz_t variables (and mpq_t and mpf_t too). Every new +variable will at a minimum go through an init, a realloc for its first +store, and finally a clear. Quite how long that takes depends on the C +library. The following compares an mpz_init/realloc/clear to a 10 limb +mpz_add. Don't be surprised if the mallocing is quite slow. + + ./speed -s 10 -c mpz_init_realloc_clear mpz_add + +On some systems malloc and free are much slower when dynamic linked. The +speed-dynamic program can be used to see this. For example the following +measures malloc/free, first static then dynamic. + + ./speed -s 10 -c malloc_free + ./speed-dynamic -s 10 -c malloc_free + +Of course a real world program has big problems if it's doing so many +mallocs and frees that it gets slowed down by a dynamic linked malloc. + + + + + +EXAMPLE COMPARISONS - STRING CONVERSIONS + +mpn_get_str does a binary to string conversion. The base is specified with +a ".<r>" parameter, or decimal by default. Power of 2 bases are much faster +than general bases. The following compares decimal and hex for instance. + + ./speed -s 1-20 -c mpn_get_str mpn_get_str.16 + +Smaller bases need more divisions to split a given size number, and so are +slower. The following compares base 3 and base 9. On small operands 9 will +be nearly twice as fast, though at bigger sizes this reduces since in the +current implementation both divide repeatedly by 3^20 (or 3^40 for 64 bit +limbs) and those divisions come to dominate. + + ./speed -s 1-20 -cr mpn_get_str.3 mpn_get_str.9 + +mpn_set_str does a string to binary conversion. The base is specified with +a ".<r>" parameter, or decimal by default. Power of 2 bases are faster than +general bases on large conversions. + + ./speed -s 1-512 -f 2 -c mpn_set_str.8 mpn_set_str.10 + +mpn_set_str also has some special case code for decimal which is a bit +faster than the general case, basically by giving the compiler a chance to +optimize some multiplications by 10. + + ./speed -s 20-40 -c mpn_set_str.9 mpn_set_str.10 mpn_set_str.11 + + + + +EXAMPLE COMPARISONS - GCDs + +mpn_gcd_1 has a threshold for when to reduce using an initial x%y when both +x and y are single limbs. This isn't tuned currently, but a value can be +established by a measurement like + + ./speed -s 10-32 mpn_gcd_1.10 + +This runs src[0] from 10 to 32 bits, and y fixed at 10 bits. If the div +threshold is high, say 31 so it's effectively disabled then a 32x10 bit gcd +is done by nibbling away at the 32-bit operands bit-by-bit. When the +threshold is small, say 1 bit, then an initial x%y is done to reduce it to a +10x10 bit operation. + +The threshold in mpn/generic/gcd_1.c or the various assembler +implementations can be tweaked up or down until there's no more speedups on +interesting combinations of sizes. Note that this affects only a 1x1 limb +operation and so isn't very important. (An Nx1 limb operation always does +an initial modular reduction, using mpn_mod_1 or mpn_modexact_1_odd.) + + + + +SPEED PROGRAM EXTENSIONS + +Potentially lots of things could be made available in the program, but it's +been left at only the things that have actually been wanted and are likely +to be reasonably useful in the future. + +Extensions should be fairly easy to make though. speed-ext.c is an example, +in a style that should suit one-off tests, or new code fragments under +development. + +many.pl is a script for generating a new speed program supplemented with +alternate versions of the standard routines. It can be used for measuring +experimental code, or for comparing different implementations that exist +within a CPU family. + + + + +THRESHOLD EXAMINING + +The speed program can be used to examine the speeds of different algorithms +to check the tune program has done the right thing. For example to examine +the karatsuba multiply threshold, + + ./speed -s 5-40 mpn_mul_basecase mpn_kara_mul_n + +When examining the toom3 threshold, remember it depends on the karatsuba +threshold, so the right karatsuba threshold needs to be compiled into the +library first. The tune program uses specially recompiled versions of +mpn/mul_n.c etc for this reason, but the speed program simply uses the +normal libgmp.la. + +Note further that the various routines may recurse into themselves on sizes +far enough above applicable thresholds. For example, mpn_kara_mul_n will +recurse into itself on sizes greater than twice the compiled-in +MUL_TOOM22_THRESHOLD. + +When doing the above comparison between mul_basecase and kara_mul_n what's +probably of interest is mul_basecase versus a kara_mul_n that does one level +of Karatsuba then calls to mul_basecase, but this only happens on sizes less +than twice the compiled MUL_TOOM22_THRESHOLD. A larger value for that +setting can be compiled-in to avoid the problem if necessary. The same +applies to toom3 and DC, though in a trickier fashion. + +There are some upper limits on some of the thresholds, arising from arrays +dimensioned according to a threshold (mpn_mul_n), or asm code with certain +sized displacements (some x86 versions of sqr_basecase). So putting huge +values for the thresholds, even just for testing, may fail. + + + + +FUTURE + +Make a program to check the time base is working properly, for small and +large measurements. Make it able to test each available method, including +perhaps the apparent resolution of each. + +Make a general mechanism for specifying operand overlap, and a syntax like +maybe "mpn_add_n.dst=src2" to select it. Some measuring routines do this +sort of thing with the "r" parameter currently. + + + +---------------- +Local variables: +mode: text +fill-column: 76 +End: |