Re: [Qemu-devel] [PATCH] add MADV_DONTFORK to guest physical memory

[Anthony Liguori <aliguori@linux.vnet.ibm.com>]

On 01/05/2011 01:44 PM, Alexander Graf wrote:
> On 05.01.2011, at 19:02, Michael Roth wrote:
>
>    
>> On 01/05/2011 09:10 AM, Andrea Arcangeli wrote:
>>      
>>> The bug is still there so I rediffed the old patch against current
>>> code.
>>>
>>> On a related topic: could somebody give me advice on how to implement
>>> a command line (command line seems enough, the other option would be
>>> monitor command) to make the MADV_MERGEABLE conditional? I got KSM on
>>> THP working fine but KSM may decrease performance by increasing the
>>> number of copy on write and by splitting hugepages, so we'd like to be
>>> able to turn off KSM on a per-VM basis (not on the whole host, which
>>> of course we already can by setting /sys/kernel/mm/ksm/run to 0) so
>>> that high perf VMs will keep running at maximum speed with KSM off but
>>> others may still benefit from KSM. For that I need to make the below
>>> MADV_MERGEABLE madvise conditional to something and the code itself
>>> will be trivial, we've just to converge on a command line option
>>> (hopefully quickly ;).
>>>        
>> There was a -mem_prealloc option added a while back to set MAP_POPULATE on memory mapped in via the -mem-path option. So an analogous -mem_nomerge option or something along that line seems reasonable for conditionally unsetting QEMU_MADV_MERGEABLE.
>>
>> And for consistency you should probably make both your proposed changes for -mem-path'd memory as well.
>>      
> Why not clean up all that mess and introduce a new -mem option that would just take all of the several options as parameters?
>
> -mem size=512,populate=on,ksm=off
>
> and default -m to something reasonable with the new syntax.
>    

Yeah, that does make sense.  Maybe we should consider folding it in with 
the -numa option too?

Regards,

Anthony Liguori

>
> Alex
>
>