Re: Changing the default configuration (was Re:

Christopher Kings-Lynne wrote:
> > Seriously, I know Linux can change these on the fly, and I'm pretty sure
> > Solaris can too. I haven't played with BSD for a while so can't speak
> > about that. Anyone else know?
>
> You cannot change SHMMAX on the fly on FreeBSD.

And part of the reason is because some/most BSD's map the page tables
into physical RAM (kernel space) rather than use some shared page table
mechanism. This is good because it prevents the shared memory from
being swapped out (performance disaster).

It doesn't actually allocate RAM unless someone needs it, but it does
lock the shared memory into a specific fixed location for all processes.

The more flexible approach is to make shared memory act just like the
memory of a user process, and have other user processes share those page
tables, but that adds extra overhead and can cause the memory to behave
just like user memory (swapable).

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