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

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

On 01/05/2011 02:00 PM, Alexander Graf wrote:
> On 05.01.2011, at 20:54, Andrea Arcangeli wrote:
>
>    
>> Hello everyone,
>>
>> On Wed, Jan 05, 2011 at 08:44:38PM +0100, 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.
>>>        
>> I'm neutral... so feel free to decide what I should implement ;).
>>
>> One comment on combining -m ksm=off (or -mem_nomerge) with
>> -mem-path. It seems unnecessary because ksm can't be turned on on
>> VM_HUGETLB vmas (MADV_MERGEABLE will return -EINVAL) and mem-path only
>> makes sense if used in combination with hugetlbfs (which sets
>> VM_HUGETLB of course).
>>      
> Sure, not all combinations make sense. But "-mem size=1G,path=/dev/shm/vm1.ram,populate=on" would make sense, no? TPH should go along the same lines here too. It'd just be a flag "tph" that defaults to on if available.
>
> That way we could also do all the sanity checks in a single place. I really like the idea of combining memory management command line parameters into a single option :). In the end I'd assume it's Anthony's call though.
>    

Where it's really helpful is in the ever elusive configuration file 
format.  -mem becomes:

[mem]
size=1G
path=/dev/shm/vm1.ram
populate=on

Which is nice from a grouping perspective.

Regards,

Anthony Liguori

> Alex
>
>