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. Regards, Orit > > Peter > >