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Subject: Re: [Qemu-devel] Re: KQEMU code organization
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To: qemu-devel@nongnu.org, Fabrice Bellard <fabrice@bellard.org>

Fabrice Bellard wrote:
> Anthony Liguori wrote:
>   
>> [...]
>> FWIW, the l1_phys_map table is a current hurdle in getting performance. 
>> When we use proper accessors to access the virtio_ring, we end up taking
>> a significant performance hit (around 20% on iperf).  I have some simple
>> patches that implement a page_desc cache that cache the RAM regions in a
>> linear array.  That helps get most of it back.
>>
>> I'd really like to remove the l1_phys_map entirely and replace it with a
>> sorted list of regions.  I think this would have an overall performance
>> improvement since its much more cache friendly.  One thing keeping this
>> from happening is the fact that the data structure is passed up to the
>> kernel for kqemu.  Eliminating that dependency would be a very good thing!
>>     
>
> If the l1_phys_map is a performance bottleneck it means that the
> internals of QEMU are not properly used. In QEMU/kqemu, it is not
> accessed to do I/Os : a cache is used thru tlb_table[]. I don't see why
> KVM cannot use a similar system.
>
>   

In that case, replacing l1_phys_map by a region list is a good thing.  
l1_phys_map consumes a large amount of memory.

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