Multithreading 为什么在模拟ubuntu中使用theora编码时，两个线程比一个线程慢？

我曾尝试使用一个线程，但似乎在使用virtualbox的模拟ubuntu时，当尝试使用2个线程而不是1个线程时，多线程测试实际上要慢一些。我使用了theora编码器，如中所述。我的硬件是intel i7 haswell，有2个内核，我已经为2个CPU配置了VirtualBox。为什么结果不如预期？我希望多线程编码速度更快，但速度要慢得多

developer@developer-VirtualBox:~/theora-multithread/examples$ lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                2
On-line CPU(s) list:   0,1
Thread(s) per core:    1
Core(s) per socket:    2
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 69
Stepping:              1
CPU MHz:               0.000
BogoMIPS:              1687.55
L1d cache:             32K
L1d cache:             32K
L2d cache:             6144K
NUMA node0 CPU(s):     0,1
developer@developer-VirtualBox:~/theora-multithread/examples$ time ./encoder_example --number-of-threads 1 wavesound.wav tmp.yuv -o TEST-1-thread.ogg
File wavesound.wav is 16 bit 2 channel 44100 Hz RIFF WAV audio.
File tmp.yuv is 48x48 25.00 fps YUV12 video.
Number of Threads: 1
Compressing....
      0:46:32.08 audio: 66kbps video: 3kbps                 
done.


real    0m23.907s
user    0m12.319s
sys 0m1.623s
developer@developer-VirtualBox:~/theora-multithread/examples$ time ./encoder_example --number-of-threads 2 wavesound.wav tmp.yuv -o TEST-2-thread.ogg
File wavesound.wav is 16 bit 2 channel 44100 Hz RIFF WAV audio.
File tmp.yuv is 48x48 25.00 fps YUV12 video.
Number of Threads: 2
Compressing....
      0:46:32.08 audio: 66kbps video: 3kbps                 
done.


real    1m7.882s
user    0m22.370s
sys 0m33.304s
developer@developer-VirtualBox:~/theora-multithread/examples$ 

主机操作系统（Win 8.1）中的CPU-Z报告了以下有关硬件的信息

Processor 1         ID = 0
    Number of cores     2 (max 8)
    Number of threads   4 (max 16)
    Name            Intel Core i3/i5/i7 4xxx
    Codename        Haswell ULT
    Specification       Intel(R) Core(TM) i7-4558U CPU @ 2.80GHz
    Package (platform ID)   Socket 1168 BGA (0x6)
    CPUID           6.5.1
    Extended CPUID      6.45
    Core Stepping       C0
    Technology      22 nm
    TDP Limit       28 Watts
    Tjmax           100.0 °C
    Core Speed      798.4 MHz
    Multiplier x Bus Speed  8.0 x 99.8 MHz
    Stock frequency     2800 MHz
    Instructions sets   MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, EM64T, VT-x, AES, AVX, AVX2, FMA3
    L1 Data cache       2 x 32 KBytes, 8-way set associative, 64-byte line size
    L1 Instruction cache    2 x 32 KBytes, 8-way set associative, 64-byte line size
    L2 cache        2 x 256 KBytes, 8-way set associative, 64-byte line size
    L3 cache        4 MBytes, 16-way set associative, 64-byte line size
    FID/VID Control     yes

测试2 使用更大的音频文件（视频只是从静态png创建的虚拟视频）进行测试，则差异并没有那么大（？）

测试2（仅视频） 只有在测试视频时，我才能使用线程数再现加速：

developer@developer-VirtualBox:~/theora-multithread$ time ./examples/encoder_example -v 1 -a 1 --number-of-threads 1 stream.yuv > theora_testfile_1.ogg
File stream.yuv is 320x240 15.00 fps YUV12 video.
Number of Threads: 1
Compressing....
      0:00:07.60 audio: 0kbps video: 138kbps                 
done.


real    0m2.136s
user    0m1.920s
sys 0m0.083s
developer@developer-VirtualBox:~/theora-multithread$ time ./examples/encoder_example -v 1 -a 1 --number-of-threads 2 stream.yuv > theora_testfile_2.ogg
File stream.yuv is 320x240 15.00 fps YUV12 video.
Number of Threads: 2
Compressing....
      0:00:07.60 audio: 0kbps video: 139kbps                 
done.


real    0m2.043s
user    0m1.994s
sys 0m0.175s

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如果我不得不猜测的话，那是因为创建线程和上下文切换的开销比进程本身更昂贵

请记住，内核线程要比用户线程昂贵得多。如果可以，请避免内核级线程

为了获得更好的性能，请尝试同时执行较大的任务，并避免触发上下文切换的操作（如等待资源或阻塞）

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另外，重用线程资源。为每个任务创建新线程可能会影响应用程序的性能。池线程有助于避免创建它们的开销。

我可以使用更大的音频文件进行测试和/或同时启动两个作业，并选择先完成的单线程或多线程。视频文件只是因为查看输入所需的视频。我也可以直接用本地安装的ubuntu进行测试，如果这能起到作用的话。代码的作者确实报告了一个加速，如果可以的话，我想复制这个增益。一个更大的音频文件可能会有所帮助。这实际上取决于开发人员决定如何管理线程和分段任务。感谢您提供的信息。现在我可以得到一个加速，但只有视频。我认为该程序是多线程的，仅用于视频压缩，音频压缩可能是相同的。

developer@developer-VirtualBox:~/theora-multithread$ time ./examples/encoder_example -v 1 -a 1 --number-of-threads 1 stream.yuv > theora_testfile_1.ogg
File stream.yuv is 320x240 15.00 fps YUV12 video.
Number of Threads: 1
Compressing....
      0:00:07.60 audio: 0kbps video: 138kbps                 
done.


real    0m2.136s
user    0m1.920s
sys 0m0.083s
developer@developer-VirtualBox:~/theora-multithread$ time ./examples/encoder_example -v 1 -a 1 --number-of-threads 2 stream.yuv > theora_testfile_2.ogg
File stream.yuv is 320x240 15.00 fps YUV12 video.
Number of Threads: 2
Compressing....
      0:00:07.60 audio: 0kbps video: 139kbps                 
done.


real    0m2.043s
user    0m1.994s
sys 0m0.175s