Re: freebsd 6x slower than Raspbian 10 buster on RPi 4b

On 2021-Feb-8, at 12:12, Elwood Downey <elwood.downey at gmail.com> wrote:

> Hello all!
> 
> Just wanted to share a comparison I did between freebsd and raspbian on the
> same RPi 4b with 1 GB RAM. I wrote a tiny C++ program that creates
> pthreads, each of which mallocs an array and spins filling it with sqrtf of
> the array index. Setting it to 3 threads (the hw has 4 cores), I found
> freebsd takes consistently 6.5x wall-clock time longer than with raspbian.
> Below are the sessions for each showing pertinent details. Attached is the
> program itself (if it doesn't make it through the newsgroup, mail me direct
> for a copy). One good news is the thread overhead for freebsd is about 100x
> smaller so kudos to the scheduler.
> 
> This is surprising and disappointing. Any comments welcome, especially what
> I'm doing wrong here. Thank you for your time.
> 
> Elwood Downey
> Tucson AZ
> 
> 
> 
> *Raspbian:*
> 
> pi@hamclock:~$ uname -a
> Linux hamclock 5.4.83-v7l+ #1379 SMP Mon Dec 14 13:11:54 GMT 2020 armv7l
> GNU/Linux
> pi@hamclock:~$ g++ --version
> g++ (Raspbian 8.3.0-6+rpi1) 8.3.0
> Copyright (C) 2018 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> pi@hamclock:~$ g++ -Wall -o pthread_bench{,.cpp} -lpthread -lm
> pi@hamclock:~$ ./pthread_bench 10000 3
> tot thr :   4.917360
> mean thr:   1.639120
> tot wall:   1.726206
> thr gain:   2.84865
> overhead:   5.04494 %
> 
> 
> *Freebsd:*
> 
> [ecdowney@freebsdpi ~]$ uname -a
> FreeBSD freebsdpi 13.0-CURRENT FreeBSD 13.0-CURRENT #0
> main-c255641-gf2b794e1e90: Thu Jan  7 08:00:13 UTC 2021
> [email protected]:/usr/obj/usr/src/arm64.aarch64/sys/GENERIC
> arm64
> [ecdowney@freebsdpi ~]$ g++ --version
> g++ (FreeBSD Ports Collection) 10.2.0
> Copyright (C) 2020 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> [ecdowney@freebsdpi ~]$ g++ -Wall -o pthread_bench{,.cpp} -lpthread -lm
> [ecdowney@freebsdpi ~]$ sysctl dev.cpu.0.freq
> dev.cpu.0.freq: 1500
> [ecdowney@freebsdpi ~]$ ./pthread_bench 10000 3
> tot thr :  33.810808
> mean thr:  11.270269
> tot wall:  11.277030
> thr gain:    2.9982
> overhead: 0.0599537 %
> <pthread_bench.cpp>_______________________________________________
> [email protected] mailing list
> https://lists.freebsd.org/mailman/listinfo/freebsd-arm
> To unsubscribe, send any mail to "[email protected]"
> 

One issue is default optimization level vs. using a
specific controlled level:

# g++10 -Wall -o pthread_bench pthread_bench.cpp -lpthread -lm
# ./pthread_bench 10000 3
tot thr :  25.900658
mean thr:   8.633552
tot wall:   8.633356
thr gain:   3.00007
overhead: -0.00227026 %

# g++10 -Wall -O2 -o pthread_bench pthread_bench.cpp -lpthread -lm
# ./pthread_bench 10000 3
tot thr :   1.133682
mean thr:   0.377894
tot wall:   0.376152
thr gain:   3.01389
overhead: -0.463111 %

(I'm not certain that the gcc port and the linux have the
same configuration for how g++10 was built or the default
optimizations used.)

# g++10 -v
Using built-in specs.
COLLECT_GCC=g++10
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc10/gcc/aarch64-portbld-freebsd14.0/10.2.0/lto-wrapper
Target: aarch64-portbld-freebsd14.0
Configured with: /wrkdirs/usr/ports/lang/gcc10/work/gcc-10.2.0/configure --disable-multilib --disable-bootstrap --disable-nls --enable-gnu-indirect-function --enable-plugin --libdir=/usr/local/lib/gcc10 --libexecdir=/usr/local/libexec/gcc10 --program-suffix=10 --with-as=/usr/local/bin/as --with-gmp=/usr/local --with-gxx-include-dir=/usr/local/lib/gcc10/include/c++/ --with-ld=/usr/local/bin/ld --with-pkgversion='FreeBSD Ports Collection' --with-system-zlib --enable-languages=c,c++,objc,fortran --prefix=/usr/local --localstatedir=/var --mandir=/usr/local/man --infodir=/usr/local/share/info/gcc10 --build=aarch64-portbld-freebsd14.0
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.2.0 (FreeBSD Ports Collection) 


Another issue is g++ and libstdc++ vs. clang++ (system c++)
and (system) libc++. So trying system clang and libc++:

# c++ -Wall -o pthread_bench pthread_bench.cpp -lpthread -lm
# ./pthread_bench 10000 3
tot thr :   2.525239
mean thr:   0.841746
tot wall:   0.849135
thr gain:    2.9739
overhead:   0.87018 %

# c++ -Wall -O2 -o pthread_bench pthread_bench.cpp -lpthread -lm
# ./pthread_bench 10000 3
tot thr :   0.000000
mean thr:   0.000000
tot wall:   0.000369
thr gain:         0
overhead:       100 %

That last is because the compiler optimized run(. . .) down
to just:

0000000000400a24 <_Z3runPv> mov x0, xzr
0000000000400a28 <_Z3runPv+0x4> ret

The source code needs to do something to prevent
the compiler from optimizing out currently unused
computations.

Having the compilers check more material also
produces notices like:

g++:
pthread_bench.cpp: In function 'void* run(void*)':
pthread_bench.cpp:18:18: warning: unused parameter 'dummy' [-Wunused-parameter]
   18 | void *run (void *dummy)
      |            ~~~~~~^~~~~
pthread_bench.cpp: In function 'int main(int, char**)':
pthread_bench.cpp:41:19: warning: ISO C++ forbids variable length array 'tid' [-Wvla]
   41 |         pthread_t tid[n_th];
      |                   ^~~

clang++:
pthread_bench.cpp:18:18: warning: unused parameter 'dummy' [-Wunused-parameter]
void *run (void *dummy)
                 ^
pthread_bench.cpp:41:22: warning: variable length arrays are a C99 feature [-Wvla-extension]
        pthread_t tid[n_th];
      
               ^


FYI: Here is a mix of using g++10 but with the FreeBSD
system libc++ instead of gcc's libstdc++ :

# g++10 -Wno-psabi -nostdinc -nostdinc++ -I/usr/include/c++/v1 -I/usr/include -mno-outline-atomics -flto -Wall -O2 -o pthread_bench pthread_bench.cpp -lpthread -lm
# ./pthread_bench 10000 3
tot thr :   1.126198
mean thr:   0.375399
tot wall:   0.376237
thr gain:   2.99332
overhead:  0.222732 %



My FreeBSD context on the RPi4B is based on non-debug
builds of main (14-CURRENT at this point):

# ~/fbsd-based-on-what-freebsd-main.sh 
merge-base: 847dfd2803f6c8b077e3ebc68e35adff2c79a65f
merge-base: CommitDate: 2021-02-03 21:24:22 +0000
325d7069b027 (HEAD -> mm-src) mm-src snapshot for mm's patched build in git context.
847dfd2803f6 (freebsd/main, freebsd/HEAD, pure-src, main) readelf: do not trucate section name with -W
FreeBSD RPi4B 14.0-CURRENT FreeBSD 14.0-CURRENT mm-src-n244624-325d7069b027 GENERIC-NODBG  arm64 aarch64 1400003 1400003

It is a tailored build for cortex-a72 via -mcpu=
use. The RPi4B's config.txt has:

over_voltage=6
arm_freq=2000
arm_freq_min=2000
sdram_freq_min=3200

FYI:

# sysctl hw.physmem
hw.physmem: 8465969152




===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)

_______________________________________________
[email protected] mailing list
https://lists.freebsd.org/mailman/listinfo/freebsd-arm
To unsubscribe, send any mail to "[email protected]"