Re: aarch64 pmap tweaks for review

[Andrew Doran <ad%netbsd.org@localhost>]

On Tue, Jun 02, 2020 at 05:03:39PM +0900, Ryo Shimizu wrote:

> That's excelent! And thank you.
> At a cursory glance, I think there is no problem.
> 
> it's okay to remove the pv_pa (#ifdef XXXAD) part :)

vtophys() would be nice to use there but it looks quite tricky.

Thanks,
Andrew

> 
> 
> >I made some tweaks to the aarch64 pmap based on lessons learned in the x86
> >pmap recently.  They reduce memory consumption and speed up things like
> >fork/exec/exit/UBC a little:
> >
> >	http://www.netbsd.org/~ad/2020/aarch64.diff
> >
> >Approximate times for kernel build on RK3399 with all 6 cores running at
> >600MHz:
> >
> >	before	1354.07s real  6092.55s user  1591.35s system
> >	after	1307.90s real  6026.60s user  1432.83s system
> >
> >Description below.  Comments welcome.
> >
> >Thanks,
> >Andrew
> >
> >- Fix a lock order reversal via pmap_page_protect().
> >
> >- Align struct pmap to a cache line boundary.
> >
> >- Move wired/resident count update out from PMAPCOUNTERS ifdef in one place.
> >  It shouldn't depend on it.
> >
> >- Make sure pmap is always locked when updating stats.  Then atomics are no
> >  longer needed to update stats.
> >
> >- Remove unneeded traversal of PV lists in pmap_enter_pv().
> >
> >- Shrink struct vm_page from 136 to 128 bytes (cache line sized - reduce
> >  cache misses).
> >
> >- Shrink struct pv_entry from 48 to 32 bytes (power of 2 sized - reduce cache
> >  misses).
> >
> >- Embed a pv_entry in each vm_page.  That means PV entries don't need to be
> >  allocated for pages that are mapped only once, for example private
> >  anonymous memory / COW pages / most UBC mappings.  Dynamic PV entries are
> >  then used only for stuff like shared libraries and shared memory.
> >
> >- Comment out PMAPCOUNTERS option because global counters are costly on MP
> >  due to cache coherency overhead.  The problem gets exponentially worse as
> >  more CPUs are added.
> >
> >- Use the pmap as a source of pre-zeroed pages for the VM system.
> >
> >- Do unlocked checks in pmap_page_protect() and pmap_clear_modify(): avoid
> >  taking the lock if the page has no mappings.
> 
> -- 
> ryo shimizu