Re: OT: Does Linux have any "Perfect Code"

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Russell Leighton a écrit :

Bryan Cantrill of Sun (ala DTrace) has a notion of perfect code:

    http://blogs.sun.com/bmc/entry/on_i_dreaming_in_code

He also has some examples (from bottom comment section of above):



Can you list a small number of examples of "software perfection"?

Posted by Russell Leighton on November 14, 2007 at 04:02 AM PST #

Russell,

My canonical small example of perfection in Solaris would be Jeff Bonwick's mod-by-a-billion code in hrt2ts():

http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/timers.c#875

Solaris of course has lots of bigger, more complicated examples. Now on the one hand, one wants to refrain from pointing to thousands of lines of code and saying that there are no bugs therein, but on the other, there are many subsystems that have been in place and in heavy use for years without defect or modification. At the risk of being egocentric, the cyclic subsystem (which is executed at least 100 times per second on every Solaris system) had its last substantial fix over six years ago, and its last fix of any flavor over three years ago:

http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/cyclic.c

Modesty (and the lack, of course, of a proof of its correctness) prevents me from calling the cyclic subsystem perfect -- but such as unknown defects remain, there are damn few of them, and we can say that they must be a result of highly usual (or at least, heretofore unseen) circumstances.

A non-Solaris example -- and one that I've been known to use as the canonical example of the persistence of software -- is Super Mario Kart. This is a game that was developed (to its completion) fifteen years ago for the Super Nintendo console. Source code, to the best of my knowledge, is not publicly available and may indeed be lost -- but the binaries persist and (if my coworkers are any indication) remain in active use. Given the longevity of, say, Homer's Odyssey, there is reason to believe that Super Mario Kart will survive in perpetuity -- that thousands of years from now, twenty-somethings somewhere will be using the software exactly as it is used today. Is this perfection? Perhaps not -- but it also might not be discernible from perfection...

Posted by Bryan Cantrill on November 14, 2007 at 07:51 AM PST #

Does Linux have any such examples true software perfection?


I dont know, (what a strange idea is it anyway ?) but reading two Solaris functions just gave me the example of non true software perfection.

http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/timers.c#1106

I would say this code was OK 10 years ago.

Now that a processor (say an Opteron in 64 bits mode, used on SUN hardware), can perform a multiply in few cycles, ts2hrt() could use a normal multiply and an addition.

Processors are improving, compilers are improving, memory sizes are increasing, source code (and algorithms) should be changed accordingly.

(gcc for example already knows the reciprocal division trick and so can compile this :

hrt2ts_div(hrtime_t hrt, timestruc_t *tsp)
{
    tsp->tv_nsec = do_div(hrt, NANOSEC);
    tsp->tv_sec = hrt;
}

to :

        movq    %rdi, %rdx
        movabsq $19342813113834067, %rax
        shrq    $9, %rdx
        mulq    %rdx
        shrq    $11, %rdx
        imulq   $1000000000, %rdx, %rax
        movq    %rdx, (%rsi)
        subq    %rax, %rdi
        movl    %edi, 8(%rsi)

while

hrtime_t
ts2hrt(const timestruc_t *tsp)
{
return tsp->tv_sec * NANOSEC + tsp->tv_nsec;
}

can be inlined as it is trivial (and much faster than Solaris version)

        movq    (%rdi), %rdx
        mov     8(%rdi), %eax
        imulq   $1000000000, %rdx, %rdx
        addq    %rdx, %rax

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