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

Mon Feb 8 21:51:11 UTC 2021

On 2021-Feb-8, at 13:42, Mark Millard <marklmi at yahoo.com> wrote:

> 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 at hamclock:~$ uname -a
>> Linux hamclock 5.4.83-v7l+ #1379 SMP Mon Dec 14 13:11:54 GMT 2020 armv7l
>> GNU/Linux
>> pi at 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 at hamclock:~$ g++ -Wall -o pthread_bench{,.cpp} -lpthread -lm
>> pi at 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 at freebsdpi ~]$ uname -a
>> FreeBSD freebsdpi 13.0-CURRENT FreeBSD 13.0-CURRENT #0
>> main-c255641-gf2b794e1e90: Thu Jan  7 08:00:13 UTC 2021
>> root at releng1.nyi.freebsd.org:/usr/obj/usr/src/arm64.aarch64/sys/GENERIC
>> arm64
>> [ecdowney at 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 at freebsdpi ~]$ g++ -Wall -o pthread_bench{,.cpp} -lpthread -lm
>> [ecdowney at freebsdpi ~]$ sysctl dev.cpu.0.freq
>> dev.cpu.0.freq: 1500
>> [ecdowney at 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>_______________________________________________
>> freebsd-arm at freebsd.org mailing list
>> https://lists.freebsd.org/mailman/listinfo/freebsd-arm
>> To unsubscribe, send any mail to "freebsd-arm-unsubscribe at freebsd.org"
>> 
> 
> 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
> 

I forgot to note that the RPi4B has heatsinks and
a fan and has a good 5.1A 3.5A power supply.

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