> -----Original Message----- > From: Gleb Natapov [mailto:gleb@xxxxxxxxxx] > Sent: Sunday, August 25, 2013 1:52 AM > To: Chris Leduc > Cc: kvm@xxxxxxxxxxxxxxx > Subject: Re: Partial huge page backing with KVM/qemu > > On Sat, Aug 24, 2013 at 12:32:07AM +0000, Chris Leduc wrote: > > Hi - In a KVM/qemu environment is it possible for the host to back only a > portion of the guests memory with huge pages? In some situations it may > not be desirable to back the entirety of a guest's memory with huge pages > (as can be done via libvirt memoryBacking option). > What are those situations? For example to limit a guest with 64GB of total memory to use 4GB of huge pages for fast lookup memory. This takes advantage of the 4 TLB entries for 1G pages on a Sandy/Ivy Bridge processor to ensure a page walk is never necessary for this fast memory. An example is a high performance data plane application. The remainder of the less frequently accessed memory can be in normal pages. > > What would be very useful is to request huge pages in the guest, either at > boot time or dynamically, and have the host back them with physical huge > pages, but not back the rest of the normal page guest memory with huge > pages from the host. > > > > The equivalent in Xen is setting allowsuperpage=1 on the hypervisor boot > line. > > > As far as I can tell this disables/enables use of huge pages by XEN vm, not > something you say you want. The Xen documentation is not clear on this, but in practice this flag allows the host to back up guest huge page requests with physical huge pages. So the guest could for example add hugepages=N to its boot line and these pages would be backed in the host with corresponding physical huge pages. >From experimentation with KVM, requesting hugepages at guest boot time (without memory backing enabled) will result in guest hugepages backed by host normal pages. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
