On Sat, Aug 9, 2008 at 2:31 PM, Anthony Liguori <[EMAIL PROTECTED]> wrote: > Sukanto Ghosh wrote: >> >> I understand the idea behind ballooning as " it effectively increases >> or decreases the amount of physical memory given to the guest, with >> the help of the guest's native memory management algorithms". The >> benefit is obvious in cases where the hypervisor does a hard >> partitioning of the memory between the guests. The guest vm (virtual >> memory manager) will indirectly say that these are my least used >> pages, or the safest candidates for eviction. >> >> But in case of kvm, the guest memory is itself allocated and managed >> by the host linux vm. So, suppose if the guest evicts a lru page >> (which might even not actually reside in host physical memory at the >> time) and even the host chooses a lru page for eviction, the host >> decision is better in a sense that this page is definitely residing in >> the physical memory. So, are we gaining from using a balloon driver in >> the guest. >> > > What ballooning is useful for in KVM, is reducing the amount of memory a > guest uses by a large amount. It's not terribly useful for shaving a few > dozen MBs from a guest, but it is useful for dropping a 1GB guest down to > 512GB. > > Since the guest tells the host what portions of it's memory it won't be > using, we can evict it completely from memory (it's not even swapped, it's > just deleted). >
But for deleting these pages shouldn't it be ensured that the guest remains under the illusion that these pages are least recently used (may be, by modifying the guest pte for those pages, so the reference bits are always set for them) and the guest's page eviction algorithm won't touch these pages. Later on when it is possible to give back the guest those pages, you remove this illusion. Another alternative will be, not to have the guest under any sort of illusion and give it back the pages when it wants to evict them. But this will defeat the purpose of having the balloon. Which way is it done in kvm/linux ? -- Regards, Sukanto Ghosh -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
