Re: [9fans] etherigbe.c using _xinc?

[erik quanstrom]

> do you have numbers to back up this claim?
> 
> you are claiming that the locked XCHGL
> in tas (pc/l.s) called from lock (port/taslock.c)
> called from incref (port/chan.c) is "much faster"
> than the locked INCL in _xinc (pc/l.s).
> it seems to me that a locked memory bus
> is a locked memory bus.

yes, i do.  xinc on most modern intel is a real
loss.  and a moderate loss on amd.  my atom 330
is an exception.

intel core i7 2.4ghz
loop    0 nsec/call
loopxinc        20 nsec/call
looplock        11 nsec/call

intel 5000 1.6ghz
loop    0 nsec/call
loopxinc        44 nsec/call
looplock        25 nsec/call

intel atom 330 1.6ghz (exception!)
loop    2 nsec/call
loopxinc        14 nsec/call
looplock        22 nsec/call

amd k10 2.0ghz
loop    2 nsec/call
loopxinc        30 nsec/call
looplock        20 nsec/call

intel p4 xeon 3.0ghz

loop    1 nsec/call
loopxinc        76 nsec/call
looplock        42 nsec/call

- erik
TEXT _xinc(SB), 1, $0                           /* void _xinc(long*); */
        MOVL    l+0(FP), AX
        LOCK;   INCL 0(AX)
        RET

TEXT _xdec(SB), 1, $0                           /* long _xdec(long*); */
        MOVL    l+0(FP), BX
        XORL    AX, AX
        LOCK;   DECL 0(BX)
        JLT     _xdeclt
        JGT     _xdecgt
        RET
_xdecgt:
        INCL    AX
        RET
_xdeclt:
        DECL    AX
        RET

#include <u.h>
#include <libc.h>

void    _xinc(uint*);
void    _xdec(uint*);

enum {
        N       = 1<<30,
};

void
loop(void)
{
        uint i;

        for(i = 0; i < N; i++)
                ;
}

void
loopxinc(void)
{
        uint i, x;

        for(i = 0; i < N; i++){
                _xinc(&x);
                _xdec(&x);
        }
}

void
looplock(void)
{
        uint i;
        static Lock l;

        for(i = 0; i < N; i++){
                lock(&l);
                unlock(&l);
        }
}

void
timing(char *s, void (*f)(void))
{
        uvlong t[2];

        t[0] = nsec();
        f();
        t[1] = nsec();
        fprint(2, "%s\t%llud nsec/call\n", s, (t[1] - t[0])/(uvlong)N);
}

void
main(void)
{
        nsec();
        timing("loop", loop);
        timing("loopxinc", loopxinc);
        timing("looplock", looplock);
        exits("");
}