>
> As all this is trying to avoid bus turnarounds (i.e. switching from
> reading to writing), wouldn't it be fastest to just trust that the CPU
> has at least 4k worth of cache? (and hope for the best that we don't
> get interrupted in the meanwhile).
>
> void copy_page (char *dest, char *source)
> {
> long *dst = (long *)dest,
> *src=(long *)source,
> *end= (long *)(source+PAGE_SIZE);
> #if 1
> register int i;
> long t=0;
> static long tt;
>
> for (i=0;i<PAGE_SIZE/sizeof (long);i += cache_line_size()/sizeof(long))
> /* Actually the innards of this loop should be:
> (void) from[i];
> however, the compiler will probably optimize that away. */
> t += src[i];
>
> tt = t;
> #endif
> while (src < end)
> *dst++ = *src++;
>
> }
>
> So, this is 15 lines of C, and it'd be interesting to benchmark this
> against the assembly.
>
Well you asked for it :) :
clear_page by 'normal_clear_page' took 12196 cycles (318.1 MB/s)
clear_page by 'slow_zero_page' took 12207 cycles (317.9 MB/s)
clear_page by 'fast_clear_page' took 29272 cycles (132.6 MB/s)
clear_page by 'faster_clear_page' took 4831 cycles (803.1 MB/s)
copy_page by 'normal_copy_page' took 12607 cycles (307.8 MB/s)
copy_page by 'slow_copy_page' took 13617 cycles (285.0 MB/s)
copy_page by 'fast_copy_page' took 9531 cycles (407.1 MB/s)
copy_page by 'faster_copy' took 5585 cycles (694.7 MB/s)
copy_page by 'even_faster' took 5621 cycles (690.3 MB/s)
copy_page by 'even_faster_nopre' took 5837 cycles (664.8 MB/s)
copy_page by 'c_source' took 17296 cycles (224.3
MB/s)
The last one is yours :). I'd assume this is because the compiler is
not
using mmx instructions for this. (the nopre is a routine I added to
check
the speed with only a single prefetch instruction. When I tried adding
the routing with the single prefetch instruction to mmx.c and
recompiling
and rebooted, the system stayed up a lot longer, but it still crashed (I
was in Xwindows and the crash was partially written to the log file)
after around 3 minutes of work in X.
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