Re: [Qemu-devel] SMP support

[Fabrice Bellard <fabrice@bellard.org>]

Joe Batt wrote:
> This
> 
> 	qemu -smp 2 -cdrom KNOPPIX_V3.8-2005-02-28-CeBIT_Edition-qemu-0.6.1-2.iso
> 
> fails, but work fine without the -smp flag.  The KNOPPIX has fairly
> random seg faults.  I had similar results with 4 virutal cpus.
> 
> The host is a dual Xeon with hyperthreading turned on (so 4 total
> "CPUs") running Debian testing, and a custom 2.6.13 kernel.  The
> compiler is gcc 3.3.5.
> 
> You didn't offer up any description.  Will this take advantage of host
> SMP?  How?  (Each of us could read the code, or you could save the
> collective a bunch of time...)

I will add some documentation soon. To be short: QEMU is still single 
threaded which means that an SMP host does not increase the speed of an 
SMP target. Each target CPU uses slots of 1 ms on the host. Currently 
the performance is not optimal if "HLT" instructions are used on the 
target CPUs (i.e. if some target CPUs are idle) but it will be improved 
very soon. The QEMU accelerator is also not supported for SMP targets 
(although it should work fine on SMP hosts).

For the future it could be possible to use host threads and to make the 
QEMU accelerator target SMP aware but it is not my priority. I think it 
is better to improve the correctness of the SMP target and to improve 
the speed of the single threaded QEMU accelerator.

Fabrice.