Re: [RFC PATCH 0/5] Removal of AioContext lock, bs->parents and ->children: proof of concept

[Paolo Bonzini]

On 3/30/22 12:53, Hanna Reitz wrote:
> > Seems a good compromise between drains and rwlock. What do you think?
>
> Well, sounds complicated.  So I’m asking myself whether this would be 
> noticeably better than just an RwLock for graph modifications, like the 
> global lock Vladimir has proposed.

A global lock would have to be taken by all iothreads on every I/O 
operation, even a read-write lock would scale because it has a global 
CoMutex inside and rdlock/unlock both take it.  Even if the critical 
section is small, the cacheline bumping would be pretty bad.

Perhaps we could reuse the code in cpus-common.c, which relies on a list 
of possible readers and is quite cheap (two memory barriers basically) 
for writers.  Here we would have a list of AioContexts as the possible 
readers.

The slow path uses a QemuMutex, a QemuCond for the readers and a 
QemuCond for the writers.  The reader QemuCond can be replaced by a 
CoQueue, I think.

Paolo