为什么‘`perf’显示0上下文开关？

Question

我在perf stat下运行了一个shell管道，使用taskset 0x1将整个管道固定在一个CPU上。我知道taskset 0x1起了作用，因为它使管道的吞吐量增加了一倍多。但是，perf stat显示管道的不同进程之间的0上下文切换。

那么，perf stat所说的上下文切换究竟是什么意思呢？

我想我对上下文切换到或从管道中的各个任务切换的数量感兴趣。有什么更好的方法来衡量吗？

这是在将dd bs=1M 与dd bs=1M /dev/null进行比较时使用的。如果我能够按需要测量上下文切换，我假设它将有助于量化为什么第一个版本比第二个版本“高效”几倍。

$ rpm -q perf
perf-4.15.0-300.fc27.x86_64
$ uname -r
4.15.17-300.fc27.x86_64

$ perf stat taskset 0x1 sh -c 'dd bs=1M /dev/null'
^C18366+0 records in
18366+0 records out
19258146816 bytes (19 GB, 18 GiB) copied, 5.0566 s, 3.8 GB/s

 Performance counter stats for 'taskset 0x1 sh -c dd if=/dev/zero bs=1M | dd bs=1M of=/dev/null':

       5059.273255      task-clock:u (msec)       #    1.000 CPUs utilized          
                 0      context-switches:u        #    0.000 K/sec                  
                 0      cpu-migrations:u          #    0.000 K/sec                  
               414      page-faults:u             #    0.082 K/sec                  
        36,915,934      cycles:u                  #    0.007 GHz                    
         9,511,905      instructions:u            #    0.26  insn per cycle         
         2,480,746      branches:u                #    0.490 M/sec                  
           188,295      branch-misses:u           #    7.59% of all branches        

       5.061473119 seconds time elapsed

$ perf stat sh -c 'dd bs=1M /dev/null'
^C6637+0 records in
6636+0 records out
6958350336 bytes (7.0 GB, 6.5 GiB) copied, 4.04907 s, 1.7 GB/s
6636+0 records in
6636+0 records out
6958350336 bytes (7.0 GB, 6.5 GiB) copied, 4.0492 s, 1.7 GB/s
sh: Interrupt

 Performance counter stats for 'sh -c dd if=/dev/zero bs=1M | dd bs=1M of=/dev/null':

       3560.269345      task-clock:u (msec)       #    0.878 CPUs utilized          
                 0      context-switches:u        #    0.000 K/sec                  
                 0      cpu-migrations:u          #    0.000 K/sec                  
               355      page-faults:u             #    0.100 K/sec                  
        32,302,387      cycles:u                  #    0.009 GHz                    
         4,823,855      instructions:u            #    0.15  insn per cycle         
         1,167,126      branches:u                #    0.328 M/sec                  
            88,982      branch-misses:u           #    7.62% of all branches        

       4.052844128 seconds time elapsed

Answer 1

perf没有计算上下文开关，因为您不是root用户。

(Linux有64k管道缓冲区。在这两种情况下，您可以看到非常接近于每64k传输2个上下文开关。不太清楚它是如何工作的，但我怀疑它只是在计算上下文开关，而不是从dd切换到另一个dd，或者计算到那个cpu的空闲任务)。

$ sudo perf stat taskset 0x1 sh -c 'dd bs=1M /dev/null'
^C14508+0 records in
14507+0 records out
15211692032 bytes (15 GB, 14 GiB) copied, 3.87098 s, 3.9 GB/s
14508+0 records in
14508+0 records out
15212740608 bytes (15 GB, 14 GiB) copied, 3.87044 s, 3.9 GB/s
taskset: Interrupt

 Performance counter stats for 'taskset 0x1 sh -c dd bs=1M /dev/null':

       3872.597645      task-clock (msec)         #    1.000 CPUs utilized          
           464,325      context-switches          #    0.120 M/sec                  
                 0      cpu-migrations            #    0.000 K/sec                  
               928      page-faults               #    0.240 K/sec                  
    11,099,016,844      cycles                    #    2.866 GHz                    
    13,765,220,898      instructions              #    1.24  insn per cycle         
     3,053,464,009      branches                  #  788.480 M/sec                  
        15,462,959      branch-misses             #    0.51% of all branches        

       3.874121023 seconds time elapsed

$ echo $((15212740608 / 464325))
32763

$ sudo perf stat sh -c 'dd bs=1M /dev/null'
^C7031+0 records in
7031+0 records out
7032+0 records in
7031+0 records out
7372537856 bytes (7.4 GB, 6.9 GiB) copied, 4.27436 s, 1.7 GB/s7372537856 bytes (7.4 GB, 6.9 GiB) copied, 4.27414 s, 1.7 GB/s

sh: Interrupt

 Performance counter stats for 'sh -c dd bs=1M /dev/null':

       3736.056509      task-clock (msec)         #    0.873 CPUs utilized          
           218,047      context-switches          #    0.058 M/sec                  
               206      cpu-migrations            #    0.055 K/sec                  
               877      page-faults               #    0.235 K/sec                  
     8,328,413,541      cycles                    #    2.229 GHz                    
     7,617,859,285      instructions              #    0.91  insn per cycle         
     1,671,904,009      branches                  #  447.505 M/sec                  
        13,827,669      branch-misses             #    0.83% of all branches        

       4.277591869 seconds time elapsed

$ echo $((7372537856 / 218047))
33811