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

[Peter Lieven <pl@kamp.de>]

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 <pl@kamp.de>
>>> 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.

Peter