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Re: [Qemu-devel] SMP support

From: Fabrice Bellard
Subject: Re: [Qemu-devel] SMP support
Date: Tue, 22 Nov 2005 20:23:05 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.3) Gecko/20040913

Joe Batt wrote:

        qemu -smp 2 -cdrom 

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.


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