Re: x86: page enter optimization

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Fri, Oct 14, 2016 at 07:22:50PM +0200, Maxime Villard wrote:
> Le 11/10/2016 à 18:26, Joerg Sonnenberger a écrit :
> > On Tue, Oct 11, 2016 at 02:42:00PM +0200, Maxime Villard wrote:
> > > All this to say that in pmap_enter_ma on x86, an optimization is possible. In
> > > this function, new_pve and new_sparepve are always allocated, but not always
> > > needed. The reason it is done this way is because preemption is disabled in the
> > > critical part, so obviously the allocation needs to be performed earlier.
> > 
> > What happens if we cache a pair in curcpu? Basically, instead of the
> > pool get, check if curcpu already has one and take that out, at the end,
> > if the pair was unused, put it into curcpu.
> > 
> > Joerg
> > 
> 
> I see what you mean, but I believe there is a locking issue. Fetching the
> pointers from curcpu needs to be done with preemption disabled, but if we
> find out the pointers are NULL then we need to allocate them, and it cannot
> be done here anymore.

Worst case, enter a critical section to get them and exit it. After
that, do an allocation if necessary. From your numbers, a good number of
instances will not have to do anything.

Joerg