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递到我编写的某些Perl中。这是一个长期运行的过程，有时甚至是几天，所以我想找出瓶颈所在，并尝试打开它们。我想知道是否将数据管道传输到脚本中的速度快于脚本处理数据的速度。如果是这种情况，我将尝试调整我的脚本，但如果不需要，则不会。我看到有人谈论在缓冲区已满时设置标志，以防止对其进行更多写操作，但是我如何检查该标志是否被设置或设置频率？有任何想法吗？

我将使用系统调用跟踪工具跟踪您的Perl脚本：strace（Linux），dtruss（OS X），ktrace（FreeBSD），truss（Solaris）等。目标是查看您的Perl脚本花了多少时间等待从其脚本读取信息。 stdin以及其他程序在写入其stdout上花费的时间。

在这里，我以作家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，而最后一个数字是在系统调用中花费的时间。因此，我们可以对Perl进行一些后期处理以对其进行汇总... [*]

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
...

您可能更喜欢制作SystemTap或DTrace脚本，该脚本可以一次跟踪双方，仅跟踪正确的文件描述符，并每秒钟左右打印一次良好的状态更新，每秒钟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，则我的粗略聚合是不正确的；在这种情况下，它会低估工作时间。

dtrace版本实际上很整洁。

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

和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
...

您可以pv -TC在管道中插入命令：

cmd1 | pv -TC | cmd2

pv使用其自己的缓冲区，并-T使其报告平均在1秒钟内的存储量（默认情况下）。

如果始终为100％，则意味着cmd1产出产出要比cmd2消耗产出更快。如果没有，那就是相反的方式。注意管道本身可以容纳64kB。

另请参阅-B以指定pv的缓冲区大小。您可以使用多个，pv例如：

$ 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]