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

[liu ping fan]

On Thu, Aug 9, 2012 at 4:18 PM, Avi Kivity <address@hidden> wrote:
> 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.
>
Agree, I think this will pin us on the major issue -- mmio perfermance

Regards,
pingfan
>
> --
> error compiling committee.c: too many arguments to function