Re: nmbclusters on amd64

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

On Wed, 24 Aug 2016, Michael van Elst wrote:
> netbsd%precedence.co.uk@localhost (Stephen Borrill) writes:
>
> > # gdb /netbsd
> > (gdb) print nmbclusters
> > $1 = 32768
>
> This is the initial value. It is usually clamped to 25% of all
> memory (for direct mapping) or 25% of kernel memory depending on
> the machine. However, some systems (arm and i386) limit it further.
>
> #ifndef NMBCLUSTERS_MAX
> #define NMBCLUSTERS_MAX (0x2000000 / MCLBYTES)  /* Limit to 64MB for clusters */
> #endif
>
> The value is probably wrong as the comment and the commit message
> talk about a 64MB limit but the value corresponds to 32MB.

Machine in question has 16GB RAM and is running amd64. Reading further, it 
looks like setting NMBCLUSTERS in netbsd-7 as a compile-time 
option is only capable of limiting the number which is generally 
counterproductive:

#ifdef NMBCLUSTERS_MAX
        max_size = MIN(max_size, NMBCLUSTERS_MAX);
#endif

#ifdef NMBCLUSTERS
        return MIN(max_size, NMBCLUSTERS);
#else
        return max_size;
#endif

Perhaps this should have been MAX(max_size, NMBCLUSTERS) which makes more 
sense to me.

--
Stephen