Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support for qemu

[Anthony Liguori <aliguori@us.ibm.com>]

Avi Kivity wrote:
> On 11/06/2009 02:29 AM, Anthony Liguori wrote:
>>> Helpers are really bad.  On launch, I find the fragile and hard to 
>>> do proper error handling with (but that's probably just me).  But 
>>> the real problem is at runtime, if you have a 16GB guest then you 
>>> have to write-protect 4M ptes and then kvm has to tear down or write 
>>> protect (not sure which mmu notifier is called) 4M shadow ptes.  
>>> Once that's done, the guest will have to fault its way back; that's 
>>> at least 4M exits, around 10 seconds worth of cpu time to execute a 
>>> couple of syscalls.
>>
>>
>> FWIW, with large pages, it's only 8k exits which comes out to about 
>> 16ms.  If nothing else, it's an argument for the important of 
>> transparent large pages.
>>
>
> No, it's an argument against fork() of large programs.

After putting together a work around, I'm starting to have my doubts 
about how real of a problem this is.

You're only write protecting memory, correct?  So it's equivalent to 
enabling dirty tracking during live migration.  In my mind, if the cost 
associated with hot plug is a fraction of the cost of live migration, 
we're in good shape.

It's not likely that a 16GB guest is going to write-fault in it's 
entirely memory range immediately.  In fact, it's likely to be amortized 
over a very long period of time so I have a hard time believing this is 
really an issue in practice.

Arguably, it's a much bigger problem for live migration.

-- 
Regards,

Anthony Liguori