The Universe of Discourse

Fri, 03 Jan 2020

Benchmarking shell pipelines and the Unix “tools” philosophy

Sometimes I look through the HTTP referrer logs to see if anyone is talking about my blog. I use the f 11 command to extract the referrer field from the log files, count up the number of occurrences of each referring URL, then discard the ones that are internal referrers from elsewhere on my blog. It looks like this:

    f 11 access.2020-01-0* | count | grep -v plover

(I've discussed f before. The f 11 just prints the eleventh field of each line. It is essentially shorthand for awk '{print $11}' or perl -lane 'print $F[10]'. The count utility is even simpler; it counts the number of occurrences of each distinct line in its input, and emits a report sorted from least to most frequent, essentially a trivial wrapper around sort | uniq -c | sort -n. Civilization advances by extending the number of important operations which we can perform without thinking about them.)

This has obvious defects, but it works well enough. But every time I used it, I wondered: is it faster to do the grep before the count, or after? I didn't ever notice a difference. But I still wanted to know.

After years of idly wondering this, I have finally looked into it. The point of this article is that the investigation produced the following pipeline, which I think is a great example of the Unix “tools” philosophy:

        for i in $(seq 20); do 
          TIME="%U+%S" time \
             sh -c f 11 access.2020-01-0* | grep -v plover | count > /dev/null' \
               2>&1 | bc -l ;
        done | addup

I typed this on the command line, with no backslashes or newlines, so it actually looked like this:

        for i in $(seq 20); do TIME="%U+%S" time sh -c 'f 11 access.2020-01-0* | grep -v plover |count > /dev/null' 2>&1 | bc -l ; done | addup

Okay, what's going on here? The pipeline I actually want to analyze, with f | grep| count, is there in the middle, and I've already explained it, so let's elide it:

        for i in $(seq 20); do 
          TIME="%U+%S" time \
             sh -c '¿SOMETHING? > /dev/null' 2>&1 | bc -l ;
        done | addup

Continuing to work from inside to out, we're going to use time to actually do the timings. The time command is standard. It runs a program, asks the kernel how long the program took, then prints a report.

The time command will only time a single process (plus its subprocesses, a crucial fact that is inexplicably omitted from the man page). The ¿SOMETHING? includes a pipeline, which must be set up by the shell, so we're actually timing a shell command sh -c '...' which tells time to run the shell and instruct it to run the pipeline we're interested in. We tell the shell to throw away the output of the pipeline, with > /dev/null, so that the output doesn't get mixed up with time's own report.

The default format for the report printed by time is intended for human consumption. We can supply an alternative format in the $TIME variable. The format I'm using here is %U+%S, which comes out as something like 0.25+0.37, where 0.25 is the user CPU time and 0.37 is the system CPU time. I didn't see a format specifier that would emit the sum of these directly. So instead I had it emit them with a + in between, and then piped the result through the bc command, which performs the requested arithmetic and emits the result. We need the -l flag on bc because otherwise it stupidly does integer arithmetic. The time command emits its report to standard error, so I use 2>&1 to redirect the standard error into the pipe.

[ Addendum 20200108: We don't actually need -l here; I was mistaken. ]

Collapsing the details I just discussed, we have:

        for i in $(seq 20); do 
          (run once and emit the total CPU time)
        done | addup

seq is a utility I invented no later than 1993 which has since become standard in most Unix systems. (As with netcat, I am not claiming to be the first or only person to have invented this, only to have invented it independently.) There are many variations of seq, but the main use case is that seq 20 prints


Here we don't actually care about the output (we never actually use $i) but it's a convenient way to get the for loop to run twenty times. The output of the for loop is the twenty total CPU times that were emitted by the twenty invocations of bc. (Did you know that you can pipe the output of a loop?) These twenty lines of output are passed into addup, which I wrote no later than 2011. (Why did it take me so long to do this?) It reads a list of numbers and prints the sum.

All together, the command runs and prints a single number like 5.17, indicating that the twenty runs of the pipeline took 5.17 CPU-seconds total. I can do this a few times for the original pipeline, with count before grep, get times between 4.77 and 5.78, and then try again with the grep before the count, producing times between 4.32 and 5.14. The difference is large enough to detect but too small to notice.

(To do this right we also need to test a null command, say

    sh -c 'sleep 0.1 < /dev/null'

because we might learn that 95% of the reported time is spent in running the shell, so the actual difference between the two pipelines is twenty times as large as we thought. I did this; it turns out that the time spent to run the shell is insignificant.)

What to learn from all this? On the one hand, Unix wins: it's supposed to be quick and easy to assemble small tools to do whatever it is you're trying to do. When time wouldn't do the arithmetic I needed it to, I sent its output to a generic arithmetic-doing utility. When I needed to count to twenty, I had a utility for doing that; if I hadn't there are any number of easy workarounds. The shell provided the I/O redirection and control flow I needed.

On the other hand, gosh, what a weird mishmash of stuff I had to remember or look up. The -l flag for bc. The fact that I needed bc at all because time won't report total CPU time. The $TIME variable that controls its report format. The bizarro 2>&1 syntax for redirecting standard error into a pipe. The sh -c trick to get time to execute a pipeline. The missing documentation of the core functionality of time.

Was it a win overall? What if Unix had less compositionality but I could use it with less memorized trivia? Would that be an improvement?

I don't know. I rather suspect that there's no way to actually reach that hypothetical universe. The bizarre mishmash of weirdness exists because so many different people invented so many tools over such a long period. And they wouldn't have done any of that inventing if the compositionality hadn't been there. I think we don't actually get to make a choice between an incoherent mess of composable paraphernalia and a coherent, well-designed but noncompositional system. Rather, we get a choice between a incoherent but useful mess and an incomplete, limited noncompositional system.

(Notes to self: (1) In connection with Parse::RecDescent, you once wrote about open versus closed systems. This is another point in that discussion. (2) Open systems tend to evolve into messes. But closed systems tend not to evolve at all, and die. (3) Closed systems are centralized and hierarchical; open systems, when they succeed, are decentralized and organic. (4) If you are looking for another example of a successful incoherent mess of composable paraphernalia, consider Git.)

[ Addendum: Add this to the list of “weird mishmash of trivia”: There are two time commands. One, which I discussed above, is a separate executable, usually in /usr/bin/time. The other is built into the shell. They are incompatible. Which was I actually using? I would have been pretty confused if I had accidentally gotten the built-in one, which ignores $TIME and uses a $TIMEFORMAT that is interpreted in a completely different way. I was fortunate, and got the one I intended to get. But it took me quite a while to understand why I had! The appearance of the TIME=… assignment at the start of the shell command disabled the shell's special builtin treatment of the keyword time, so it really did use /usr/bin/time. This computer stuff is amazingly complicated. I don't know how anyone gets anything done. ]

[ Addenda 20200104: (1) Perl's module ecosystem is another example of a successful incoherent mess of composable paraphernalia. (2) Of the seven trivia I included in my “weird mishmash”, five were related to the time command. Is this a reflection on time, or is it just because time was central to this particular example? ]

[ Addendum 20200104: And, of course, this is exactly what Richard Gabriel was thinking about in Worse is Better. Like Gabriel, I'm not sure. ]

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