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).