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

[Michael Roth <mdroth@linux.vnet.ibm.com>]

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.