* Orit Wasserman (owass...@redhat.com) wrote: > On 01/30/2014 08:23 PM, Dr. David Alan Gilbert wrote: > >* Orit Wasserman (owass...@redhat.com) wrote: > >>Signed-off-by: Orit Wasserman <owass...@redhat.com> > >>--- > >> migration.c | 7 +++++++ > >> 1 file changed, 7 insertions(+) > >> > >>diff --git a/migration.c b/migration.c > >>index 46a7305..25add6f 100644 > >>--- a/migration.c > >>+++ b/migration.c > >>@@ -479,6 +479,13 @@ void qmp_migrate_set_cache_size(int64_t value, Error > >>**errp) > >> return; > >> } > >> > >>+ /* Cache should not be larger than guest ram size */ > > > >Why? (It's admittedly odd, but does it actually break something if it's > >larger?) > > > > Because how XBZRLE works, the idea is that for workload that changes the same > pages > frequently, we can reduce the amount of transferred data sent by sending only > the diff. > We also compress the diff itself. > > The cache is used to store the previous page so we can calculate the diff, so > at most it will > contain all the guest pages.
It's a hash based cache though isn't it - so there will be some contention for a cache size==ram size case? Also this does mean that you have to be a little careful to pick a sane XBZRLE cache size, since one that's too large will now fail; I can only see that being a problem on a machine with a mix of huge and tiny VMs. ( I sent the reviewd-by tag separately.) Dave -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
