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如何判断管道缓冲区是否已满？

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如何判断管道缓冲区是否已满？

我正在将一个程序的输出传输到我编写的一些 Perl 中。这是一个漫长的过程，有时需要几天的时间，所以我想找出我的瓶颈在哪里，并尝试解决它们。我想知道数据通过管道传输到我的脚本中的速度是否比我的脚本处理它的速度快。如果是这种情况，我会尝试调整我的脚本，但如果不需要的话我不会这样做。我看到有关在缓冲区已满时设置一个标志以防止对其进行更多写入的讨论，但如何检查该标志是否设置或设置的频率如何？有任何想法吗？

答案1

我会使用系统调用跟踪工具来跟踪您的 Perl 脚本：strace(Linux)、dtruss(OS X)、ktrace(FreeBSD)、truss(Solaris) 等。目标是查看您的 Perl 脚本花费多少时间等待从其读取stdin 以及其他程序等待写入其 stdout 所花费的时间。

在这里，我以 writer 作为瓶颈来测试这一点：

terminal 1$ gzip -c < /dev/urandom | cat > /dev/null

terminal 2$ ps auxw | egrep 'gzip|cat'
slamb    25311 96.0  0.0  2852  516 pts/0    R+   23:35   3:40 gzip -c
slamb    25312  0.8  0.0  2624  336 pts/0    S+   23:35   0:01 cat

terminal 2$ strace -p 25312 -s 0 -rT -e trace=read
Process 25312 attached - interrupt to quit
     0.000000 read(0, ""..., 4096) = 4096 <0.005207>
     0.005531 read(0, ""..., 4096) = 4096 <0.000051>

这里的第一个数字是自上一个系统调用开始以来的时间，最后一个数字是系统调用所花费的时间。所以我们可以用 Perl 进行一些后处理来聚合它...... [*]

terminal 2$ strace -p 25312 -s 0 -rT -e trace=read 2>&1 | perl -nle 'm{^\s*([\d.]+) read\(0, .*<([\d.]+)>} or next; $total_work += $1 - $last_wait; $total_wait += $2; $last_wait = $2; print "working for $total_work sec, waiting for $total_wait sec"; $last_wait = $2;'
working for 0 sec, waiting for 0.005592 sec
...
working for 0.305356 sec, waiting for 2.28624900000002 sec
...

terminal 2$ strace -p 25311 -s 0 -rT -e trace=write 2>&1 | perl -nle 'm{^\s*([\d.]+) write\(1, .*<([\d.]+)>} or next; $total_work += $1 - $last_wait; $total_wait += $2; $last_wait = $2; print "working for $total_work sec, waiting for $total_wait sec"; $last_wait = $2;'
...
working for 5.15862000000001 sec, waiting for 0.0555740000000007 sec
...

您可以更花哨地制作一个 SystemTap 或 DTrace 脚本，它可以同时跟踪双方，仅跟踪正确的文件描述符，并每秒左右打印一个很好的状态更新，其中双方等待对方的时间百分比。

[*] - 警告：如果在其他文件描述符上调用读/写，我的粗略聚合不太正确；在这种情况下，它会低估工作时间。

dtrace 版本实际上非常简洁。

terminal 1$ gzip -c < /dev/urandom | cat > /dev/null

terminal 2$ ps aux | egrep 'gzip| cat'
slamb    54189  95.8  0.0   591796    584 s006  R+   12:49AM  22:49.55 gzip -c
slamb    54190   0.4  0.0   599828    392 s006  S+   12:49AM   0:06.08 cat

terminal 2$ cat > pipe.d <<'EOF'
#!/usr/sbin/dtrace -qs

BEGIN
{
  start = timestamp;
  writer_blocked = 0;
  reader_blocked = 0;
}

tick-1s, END
{
  this->elapsed = timestamp - start;
  printf("since startup, writer blocked %3d%% of time, reader %3d%% of time\n",
         100 * writer_blocked / this->elapsed,
         100 * reader_blocked / this->elapsed);
}

syscall::write:entry
/pid == $1 && arg0 == 1/
{
  self->entry = timestamp;
}

syscall::write:return
/pid == $1 && self->entry != 0/
{
  writer_blocked += timestamp - self->entry;
  self->entry = 0;
}

syscall::read:entry
/pid == $2 && arg0 == 0/
{
  self->entry = timestamp;
}

syscall::read:return
/pid == $2 && self->entry != 0/
{
  reader_blocked += timestamp - self->entry;
  self->entry = 0;
}
EOF

terminal 2$ chmod u+x pipe.d
terminal 2$ sudo ./pipe.d 54189 54190
since startup, writer blocked   0% of time, reader  98% of time
since startup, writer blocked   0% of time, reader  99% of time
since startup, writer blocked   0% of time, reader  99% of time
since startup, writer blocked   0% of time, reader  99% of time
since startup, writer blocked   0% of time, reader  99% of time
^C
since startup, writer blocked   0% of time, reader  99% of time

SystemTap 版本：

terminal 1$ gzip -c /dev/urandom | cat > /dev/null

terminal 2$ ps auxw | egrep 'gzip| cat'
slamb     3405  109  0.0   4356   584 pts/1    R+   02:57   0:04 gzip -c /dev/urandom
slamb     3406  0.2  0.0  10848   588 pts/1    S+   02:57   0:00 cat

terminal 2$ cat > probes.stp <<'EOF'
#!/usr/bin/env stap

global start
global writer_pid
global writes
global reader_pid
global reads

probe begin {
  start = gettimeofday_us()
  writer_pid = strtol(argv[1], 10)
  reader_pid = strtol(argv[2], 10)
}

probe timer.s(1), end {
  elapsed = gettimeofday_us() - start
  printf("since startup, writer blocked %3d%% of time, reader %3d%% of time\n",
         100 * @sum(writes) / elapsed,
         100 * @sum(reads) / elapsed)
}

probe syscall.write.return {
  if (pid() == writer_pid && $fd == 1)
    writes <<< gettimeofday_us() - @entry(gettimeofday_us())
}

probe syscall.read.return {
  if (pid() == reader_pid && $fd == 0)
    reads <<< gettimeofday_us() - @entry(gettimeofday_us())
}
EOF

terminal 2$ chmod a+x probes.stp
terminal 2$ sudo ./pipe.stp 3405 3406
since startup, writer blocked   0% of time, reader  99% of time
...

答案2

您可以pv -TC在管道中插入命令：

cmd1 | pv -TC | cmd2

pv使用自己的缓冲区并-T使其报告 1 秒周期内的平均填充情况（默认情况下）。

如果它始终是 100%，则意味着生产产出的速度比消耗产出的cmd1速度更快。cmd2如果不是，那就反过来了。请注意，管道本身可以容纳 64kB。

另请参阅-B指定pv的缓冲区大小。您可以使用多个pvs，如下所示：

$ cmd1 | pv -cCTrbN 'cmd1 -> cmd2' | cmd2 | pv -cCTrbN 'cmd2 -> cmd3' | cmd3
cmd1 -> cmd2: 1.92GiB { 53%} [ 387MiB/s]
cmd2 -> cmd3: 1.92GiB {  0%} [ 387MiB/s]

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