On Jan 31, 2013 4:03 PM, "Peter Lieven" <p...@dlhnet.de> wrote: > > > 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. >
Nobody uses them, now that transparent huge pages are available.
