superpages for UMA

[Alan Cox]

On Mon, Aug 18, 2014 at 2:45 PM, Adrian Chadd <adrian at freebsd.org> wrote:

> Hi!
>
> I dug into this a little bit last year. I saw a lot of time spent just
> walking TLBs for VM pages when doing a lot of VM page -> network
> pushing.
>
> On the sandy bridge boxes with 1G page entries, the TLB only has 4 entries.
>
> The high area of memory isn't 1G aligned, so we don't use 1G pages for
> all the stuff that's allocated initially. That includes, among other
> things, all the VM memory that you need.
>
> The other thing that crept up was that we don't try to reserve memory
> in any way - we'll just fragment stuff quickly from the pmap and
> allocate where we can when we can. So there's currently no attempt to
> allocate small kernel structures from the same underlying 1G page.
>
>
For uma_small_alloc(), there is VM_FREEPOOL_DIRECT.  However, this is still
tuned for 2 MB pages.


> That'd be an interesting experiment - allocating VM entries and other
> small things like rtentry and mbuf UMA entries from one or two 1GB
> regions of memory. It may make better use of the 1G (or 2M) TLB
> entries and keep things hot.
>
>
>
> -a
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