Re: [Qemu-devel] [PATCH] pc: madvise(MADV_DONTNEED) memory on reset

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/24/2010 02:59 PM, Blue Swirl wrote:
> On 2/24/10, Anthony Liguori<aliguori@us.ibm.com>  wrote:
>    
>> If you compare the RSS of a freshly booted guest and the same guest after a
>>   reboot, it's very likely the freshly booted guest will have an RSS that is
>>   much lower the the rebooted guest because the previous run of the guest faulted
>>   in all available memory.
>>
>>   This patch addresses this issue by using madvise() during reset.  It only
>>   resets RAM areas which means it has to be done in the machine.  I've only done
>>   this for the x86 target because I'm fairly confident that this is allowed
>>   architecturally on x86 although I'm not sure this is universely true.
>>
>>   This does not appear to have an observable cost with a large memory guest and
>>   I can't really think of any down sides.
>>      
> I think it would be much cleaner to make the madvise() calls from
> exec.c, now you are duplicating some of the functionality there. The
> calls could be controlled by a global variable (set only in pc.c) so
> non-PC architectures would not be disturbed.
>    

One thing we could do (that I think has other uses) is to add a context 
parameter to qemu_ram_alloc().  We could start with a simple flag of 
something like QRAM_RAM and QRAM_ROM.  QRAM_RAM would get automatically 
madvise()'d on reset.

But that said, does anyone know of an architecture where this type of 
reset would be a problem?  Would it be a problem on sparc?

Regards,

Anthony Liguori