Am 31.01.2013 um 14:59 schrieb Avi Kivity <avi.kiv...@gmail.com>: > > On Jan 31, 2013 12:29 PM, "Orit Wasserman" <owass...@redhat.com> wrote: > > > > On 01/31/2013 11:48 AM, Gleb Natapov wrote: > > > On Thu, Jan 31, 2013 at 09:47:24AM +0200, Orit Wasserman wrote: > > >> On 01/31/2013 08:57 AM, Peter Lieven wrote: > > >>> Hi, > > >>> > > >>> I just came across an idea and would like to have feedback if it makes > > >>> sence or not. > > >>> > > >>> If a VM is started without preallocated memory all memory that has not > > >>> been written to > > >>> reads as zeros, right? > > >> Hi, > > >> No the memory will be unmapped (we allocate on demand). > > >>> If a VM with a lot of unwritten memory is migrated or if the memory > > >>> contains a lot > > >>> of zeroed out memory (e.g. Windows or Linux guest with page > > >>> sanitization) all this memory > > >>> is allocated on the target during live migration. Especially with KSM > > >>> this leads > > >>> to the problem that this memory is allocated and might be not available > > >>> completely as > > >>> merging of the pages will happen async. > > >>> > > >>> Wouldn't it make sense to not send zero pages in the first round where > > >>> the complete > > >>> ram is sent (if it is detectable that we are in this stage)? > > >> We send one byte per zero page at the moment (see is_dup_page) we can > > >> further optimizing it > > >> by not sending it. > > >> I have to point out that this is a very idle guest and we need to work > > >> on a loaded guest > > >> which is the more hard problem in migration. > > >> > > >> Also I notice that the bottle neck in migrating unmapped pages is the > > >> detection of those pages > > >> because we map the pages in order to check them, for a large guest this > > >> is very expensive as mapping a page > > >> results in a page fault in the host. > > >> So what will be very helpful is actually locating those pages without > > >> mapping them > > >> which looks very complicated. > > >> > > > What is wrong with mincore()? > > > > Avi/Michael do you remember why mincore can't be used to check if a guest > > page is unmapped? > > A page may be not in core, but also nonzero (for example swap). What about hugetables? Afaik, there are not swappable - at least they where somewhen in the past. Maybe it would be possible to have a performance improvement if they are used.
Peter
