Re: [patch 00/13] RFC: split the global mutex

[Marcelo Tosatti <mtosatti@redhat.com>]

On Sun, Apr 20, 2008 at 02:16:52PM +0300, Avi Kivity wrote:
> >The iperf numbers are pretty good. Performance of UP guests increase 
> >slightly but SMP
> >is quite significant.
>
> I expect you're seeing contention induced by memcpy()s and inefficient 
> emulation.  With the dma api, I expect the benefit will drop.

You still have to memcpy() with the dma api. Even with vringfd the
kernel->user copy has to be performed under the global mutex protection,
difference being that several packets can be copied per-syscall instead
of only one.

> >Note that workloads with multiple busy devices (such as databases, web 
> >servers) should
> >be the real winners.
> >
> >What is the feeling on this? Its not _that_ intrusive and can be easily 
> >NOP'ed out for
> >QEMU.
> >
> >  
> 
> I think many parts are missing (or maybe, I missed them).  You need to 
> lock the qemu internals (there are many read-mostly qemu caches 
> scattered around the code), lock against hotplug, etc.  

Yes, there are some parts missing, such as the bh list and hotplug as
you mention.

> For pure cpu emulation, there is a ton of work to be done: protecting
> the translator as well as making the translated code smp safe.

I now believe there is a lot of work (which was not clear before).
Not particularly interested in getting real emulation to be
multithreaded.

Anyways, the lack of multithreading in qemu emulation should not be a
blocker for these patches to get in, since these are infrastructural
changes.

> I think that QemuDevice makes sense, and that we want this long term, 
> but that we first need to improve efficiency (which reduces cpu 
> utilization _and_ improves scalability) rather than look at scalability 
> alone (which is much harder in addition to the drawback of not reducing 
> cpu utilization).

Will complete the QEMUDevice+splitlock patchset, keep it uptodated, and
test it under a wider variety of workloads.

Thanks.


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