GCC ARM性能下降 [英] GCC ARM Performance drop
问题描述
我偶然发现了GCC的一个非常奇怪的问题.问题是性能下降了25％.这是故事.
I stumbled upon very strange issue with GCC. The issue is 25% drop in performance. Here is the story.
我有一些软件是fp32计算密集型的(使用TVM编译的神经网络).我为ARM(rk3399设备)编译了它，这是信息:
I have a pice of software which is fp32 compute intensive (neural networks compiled with TVM). I compile it for ARM (rk3399 device), here is info:
gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/arm-linux-gnueabihf/5/lto-wrapper
Target: arm-linux-gnueabihf
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.12' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libitm --disable-libquadmath --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-armhf/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-armhf --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-armhf --with-arch-directory=arm --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --enable-multilib --disable-sjlj-exceptions --with-arch=armv7-a --with-fpu=vfpv3-d16 --with-float=hard --with-mode=thumb --disable-werror --enable-multilib --enable-checking=release --build=arm-linux-gnueabihf --host=arm-linux-gnueabihf --target=arm-linux-gnueabihf
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu/Linaro 5.4.0-6ubuntu1~16.04.12)
uname -a
Linux FriendlyELEC 4.4.143 #1 SMP Tue Nov 20 11:10:11 CST 2018 aarch64 aarch64 aarch64 GNU/Linux
lscpu
Architecture: aarch64
Byte Order: Little Endian
CPU(s): 6
On-line CPU(s) list: 0-5
Thread(s) per core: 1
Core(s) per socket: 3
Socket(s): 2
Model name: ARMv8 Processor rev 2 (v8l)
CPU max MHz: 1800.0000
CPU min MHz: 408.0000
Hypervisor vendor: horizontal
Virtualization type: full
代码最初是慢"的，而cpp11是，我决定尝试cpp17和cpp14.不支持cpp17，但支持cpp14.我切换到cpp14，瞧，我的性能提高了约25％.我真的对其进行了测试，以确保提升实际上是真实的，而不是测量错误.我进行了一周的增强，然后重新启动了设备，性能没有了！
The code was initially "slow" and cpp11, I decided to try cpp17 and cpp14. cpp17 was not supported, but cpp14 was. I switched to cpp14 and voila I got boost around 25% in performance. I really tested it to make sure the boost is in fact real and not a measuring mistake. I had this boost for a week then my device rebooted and the boost in performance was gone!
这听起来很疯狂，但是我非常确定自己的代码和测量结果.在这个头之前，我没有显式的编译标志.现在，我正在尝试找出GCC的编译标志来回收丢失的内容，但是我对GCC的经验不足.这可能是什么问题?哪些标志会严重影响性能?
It may sound crazy, but I'm very sure in my code and measurements I had. I didn't have explicit compile flags prior to this gimmick. Now I'm trying to figure out compile flags for GCC to reclaim what was lost, but I don't have much experience with GCC. What could be the issue here? What flags can affect performance that much?
代码使用.so文件，并使用llvm和gcc进行编译
the code uses .so files, compiled with use of llvm and gcc
llvm -device=arm_cpu -target=armv8l-linux-gnueabihf -mattr=+neon,fp-armv8
推荐答案
这不是GCC错误.这是CPU频率缩放问题.我的设备上装有带有Linux(ubuntu)的ARM的设备，奇怪的行为和不同的基准测试结果是由于操作系统控制的CPU频率奇怪而引起的.
It's not GCC fault. It's CPU frequency scaling problem. I had device with ARM with Linux (ubuntu) on board, strange behavior and different benchmarking results are due to strange cpu frequency governing by OS.
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