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

[Alexander Graf]

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.


Alex