Re: [Qemu-devel] [PATCH -V3 09/32] virtio-9p: Implement P9_TWRITE/ Thread model in QEMU

[Anthony Liguori <aliguori@linux.vnet.ibm.com>]

On 03/29/2010 01:36 AM, jvrao wrote:
> Aneesh Kumar K.V wrote:
>    
>> From: Anthony Liguori<aliguori@us.ibm.com>
>>      
> We have implemented all the vfs calls in state machine model so that we are prepared
> for the model where the VCPU thread(s) does the initial work until it needs to block then it
> submits that work (via a function pointer) to a thread pool.  A thread in that thread pool
> picks up the work, and completes the blocking call, when blocking call returns a callback is
> invoked in the IO thread.  Then the IO thread runs until the next blocking function, and goto start.
>
> Basically the VCPU/IO threads does all the non-blocking work, and let the threads in the
> thread pool work on the blocking calls like mkdir() stat() etc.
>
> My question is, why not let the whole work done by the thread in the thread pool?
> VCPU thread receives the PDU and hands over the entire job to worker thread.
> When all work is completed, either the worker thread or the IO thread(we can switch back at this point if needed) marks the request as completed in the virtqueue and injects an
> interrupt to notify the guest.
>
> We can still keep the same number of threads in the thread pool.
> This way, we are not increasing #of threads employed by QEMU...also it makes code lot
> more easy to read/maintain.
>
> I may be missing something..but would like to know more on the advantages of this model.
>    

In qemu, we tend to prefer state-machine based code.

There are a few different models that we have discussed at different points:

1) state machines with a thread pool to make blocking functions 
asynchronous (what we have today)

2) co-operative threading

3) pre-emptive threading

All three models are independent of making device models re-entrant.  In 
order to allow VCPU threads to run in qemu simultaneously, we need each 
device to carry a lock and for that lock to be acquired upon any of the 
public functions for the device model.

For individual device models or host services, I think (3) is probably 
the worst model overall.  I personally think that (1) is better in the 
long run but ultimately would need an existence proof to compare against 
(2).  (2) looks appealing until you actually try to have the device 
handle multiple requests at a time.

For virtio-9p, I don't think (1) is much of a burden to be honest.  In 
particular, when the 9p2000.L dialect is used, there should be a 1-1 
correlation between protocol operations and system calls which means 
that for the most part, there's really no advantage to threading from a 
complexity point of view.

Regards,

Anthony Liguori

> Thanks,
> JV
>