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

[Avi Kivity <avi@redhat.com>]

On 03/29/2010 11:42 PM, Anthony Liguori wrote:
>>> 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.
>>
>> Sooner or later nature and the ever more complicated code will force 
>> us towards (3).  As an example, we've observed live migration to 
>> throttle vcpus when sending a large guest's zeroed memory over; the 
>> bandwidth control doesn't kick in since zero pages are compressed, so 
>> the iothread spends large amounts of time reading memory.
>
>
> Making things re-entrant is different than (3) in my mind.
>
> There's no reason that VCPU threads should run in lock-step with live 
> migration during the live phase.  Making device models re-entrant and 
> making live migration depend not depend on the big global lock is a 
> good thing to do.

It's not sufficient.  If you have a single thread that runs both live 
migrations and timers, then timers will be backlogged behind live 
migration, or you'll have to yield often.  This is regardless of the 
locking model (and of course having threads without fixing the locking 
is insufficient as well, live migration accesses guest memory so it 
needs the big qemu lock).

> What I'm skeptical of, is whether converting virtio-9p or qcow2 to 
> handle each request in a separate thread is really going to improve 
> things. 

Currently qcow2 isn't even fullly asynchronous, so it can't fail to 
improve things.

> The VNC server is another area that I think multithreading would be a 
> bad idea.

If the vnc server is stuffing a few megabytes of screen into a socket, 
then timers will be delayed behind it, unless you litter the code with 
calls to bottom halves.  Even worse if it does complicated compression 
and encryption.

>>
>> But if those system calls are blocking, you need a thread?
>
> You can dispatch just the system call to a thread pool.  The advantage 
> of doing that is that you don't need to worry about locking since the 
> system calls are not (usually) handling shared state.

There is always implied shared state.  If you're doing direct guest 
memory access, you need to lock memory against hotunplug, or the syscall 
will end up writing into freed memory.  If the device can be 
hotunplugged, you need to make sure all threads have returned before 
unplugging it.

>> On a philosophical note, threads may be easier to model complex 
>> hardware that includes a processor, for example our scsi card (and 
>> how about using tcg as a jit to boost it :)
>
> Yeah, it's hard to argue that script evaluation shouldn't be done in a 
> thread.  But that doesn't prevent me from being very cautious about 
> how and where we use threading :-)

Caution where threads are involved is a good thing.  They are inevitable 
however, IMO.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.