Re: [PATCH 03/15] qom: introduce reclaimer to release obj

[Avi Kivity <avi@redhat.com>]

On 08/09/2012 10:49 AM, Paolo Bonzini wrote:
> Il 09/08/2012 09:33, liu ping fan ha scritto:
>> Yes, it is to defer destructors.
>> See 0009-memory-prepare-flatview-and-radix-tree-for-rcu-style.patch
>> When MemoryRegion is _del_subregion from mem in updater, it may be
>> still in use by reader -- radix or flatview, so defer its destructors
>> to the reclaimer --phys_map_release(PhysMap *map)
> 
> How are you sure that the reader is already out of its critical section
> by the time the reclaimer runs?
> 
>> If we have rcu, it could be elegant to do this.
> 
> Yeah, I think inventing primitives is dangerous and difficult to review;
> and it may be difficult to replace it with proper call_rcu.
> 
> You should probably make a proof-of-concept using liburcu.  Then we can
> decide how to implement RCU in a way that is portable enough for QEMU's
> needs.

IMO we should start with a simple mutex (which will cover only the
lookup and map rebuild).  This should reduce the contention to basically
nothing (still leaving a cache line bounce).  If a profile shows the
cache line bounce hurting us, or perhaps contention in ultralarge
guests, then we should switch to rcu.


-- 
error compiling committee.c: too many arguments to function