Re: [Qemu-devel] [PATCH 13/15] exec: use mmap for PhysPageMap->nodes

[Peter Lieven]

Am 13.07.2016 um 12:27 schrieb Paolo Bonzini:
> On 12/07/2016 16:34, Peter Lieven wrote:
> > Am 11.07.2016 um 12:37 schrieb Paolo Bonzini:
> > > On 11/07/2016 11:31, Peter Lieven wrote:
> > > > Am 28.06.2016 um 12:43 schrieb Paolo Bonzini:
> > > > > On 28/06/2016 11:01, Peter Lieven wrote:
> > > > > > this was causing serious framentation in conjunction with the
> > > > > > subpages since RCU was introduced. The node space was allocated
> > > > > > at approx 32kB then reallocted to approx 75kB and this a few hundred
> > > > > > times at startup. And thanks to RCU the freeing was delayed.
> > > > > >
> > > > > > Signed-off-by: Peter Lieven <address@hidden>
> > > > > The size of the node from the previous as->dispatch could be used as a
> > > > > hint for the new one perhaps, avoiding the reallocation?
> > > > This here seems also to work:
> > > >
> > > > diff --git a/exec.c b/exec.c
> > > > index 0122ef7..2691c0a 100644
> > > > --- a/exec.c
> > > > +++ b/exec.c
> > > > @@ -187,10 +187,12 @@ struct CPUAddressSpace {
> > > >
> > > >   static void phys_map_node_reserve(PhysPageMap *map, unsigned nodes)
> > > >   {
> > > > +    static unsigned alloc_hint = 16;
> > > >       if (map->nodes_nb + nodes > map->nodes_nb_alloc) {
> > > > -        map->nodes_nb_alloc = MAX(map->nodes_nb_alloc * 2, 16);
> > > > +        map->nodes_nb_alloc = MAX(map->nodes_nb_alloc, alloc_hint);
> > > >           map->nodes_nb_alloc = MAX(map->nodes_nb_alloc, map->nodes_nb +
> > > > nodes);
> > > >           map->nodes = g_renew(Node, map->nodes, map->nodes_nb_alloc);
> > > > +        alloc_hint = map->nodes_nb_alloc;
> > > >       }
> > > >   }
> > > >
> > > >
> > > > Question is still, mmap for this?
> > > Nice!  Can you submit a patch for this?
> > Of course, but please see my other comment. We still should consider mmap for 
> > this cause we have close to 500 Physmaps about 70KB each which
> > are all allocted at the same time - I think due to RCU.
> That I think is not material for 2.7, and also I want to take a look at
> creating nodes lazily.

Okay, then I will send a patch to avoid the realloc and we look at this later.

Peter


> Paolo


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Mit freundlichen Grüßen

Peter Lieven

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