Re: [PATCH] seq: speed up the common case by 25x

[Jim Meyering]

Pádraig Brady wrote:

> On 01/04/11 10:20, Jim Meyering wrote:
>> Pádraig Brady wrote:
>>>     seq: speed up the common case by 560%
>>>
>>>     * seq.c (print_range): Print a range of numbers
>>>     without using if statements for speed.
>>>     (print_long_range): Print a range of numbers
>>>     using the unix paradigm of using other utils
>>>     that do it better.
>>
>> Great idea.
>
>> I get this:
>>
>>   $ env time ./seq 10000000 > /dev/null
>>   0.50user 0.06system 0:00.20elapsed 273%CPU (0avgtext+0avgdata 
>> 4800maxresident)k
>>   ...
>>
>> Compare with the old version's time:
>>
>>   5.06user 0.00system 0:05.08elapsed 99%CPU (0avgtext+0avgdata 
>> 2608maxresident)k
>>
>> So here I benefit from parallelism, and end up with not just 560% (5.6x)
>> but a 25x(!) speed-up (on an i7-970).
>
> My April fool's joke has spectacularly backfired.

You choose too well.
seq's performance is a personal pain point,
since I use it regularly, and when it doesn't
scale well, I end up waiting an extra second or two
for larger numbers.  "waiting for seq" is annoying
when I know how much faster it can be.

Yeah, yeah, I should just replace it with a function
that runs the pipeline when there's a single number....
Ok.  Now I'll use this:

seq()
{
  case $#:$1 in 1:[0-9]*) yes | head -n"$1" | cat -n | tr -cd '[0-9\n]';;
    *) env seq "$@";; esac
}

It can generate half a billion numbers in 10 seconds:

    $ time seq 500000000 |tail -3
    500000000
    seq 500000000  25.98s user 5.08s system 305% cpu 10.174 total
    tail  4.92s user 1.62s system 64% cpu 10.174 total