On Tue, Dec 04, 2012 at 08:21:55AM -0700, Alex Williamson wrote: > On Tue, 2012-12-04 at 13:48 +0200, Gleb Natapov wrote: > > On Mon, Dec 03, 2012 at 04:39:05PM -0700, Alex Williamson wrote: > > > Memory slots are currently a fixed resource with a relatively small > > > limit. When using PCI device assignment in a qemu guest it's fairly > > > easy to exhaust the number of available slots. I posted patches > > > exploring growing the number of memory slots a while ago, but it was > > > prior to caching memory slot array misses and thefore had potentially > > > poor performance. Now that we do that, Avi seemed receptive to > > > increasing the memory slot array to arbitrary lengths. I think we > > > still don't want to impose unnecessary kernel memory consumptions on > > > guests not making use of this, so I present again a growable memory > > > slot array. > > > > > > A couple notes/questions; in the previous version we had a > > > kvm_arch_flush_shadow() call when we increased the number of slots. > > > I'm not sure if this is still necessary. I had also made the x86 > > > specific slot_bitmap dynamically grow as well and switch between a > > > direct bitmap and indirect pointer to a bitmap. That may have > > > contributed to needing the flush. I haven't done that yet here > > > because it seems like an unnecessary complication if we have a max > > > on the order of 512 or 1024 entries. A bit per slot isn't a lot of > > > overhead. If we want to go more, maybe we should make it switch. > > > That leads to the final question, we need an upper bound since this > > > does allow consumption of extra kernel memory, what should it be? A > > This is the most important question :) If we want to have 1000s of > > them or 100 is enough? > > We can certainly hit respectable numbers of assigned devices in the > hundreds. Worst case is 8 slots per assigned device, typical case is 4 > or less. So 512 slots would more or less guarantee 64 devices (we do > need some slots for actual memory), and more typically allow at least > 128 devices. Philosophically, supporting a full PCI bus, 256 functions, > 2048 slots, is an attractive target, but it's probably no practical. > > I think on x86 a slot is 72 bytes w/ alignment padding, so a maximum of > 36k @512 slots. > > > Also what about changing kvm_memslots->memslots[] > > array to be "struct kvm_memory_slot *memslots[KVM_MEM_SLOTS_NUM]"? It > > will save us good amount of memory for unused slots. > > I'm not following where that results in memory savings. Can you > clarify. Thanks, > We will waste sizeof(void*) for each unused slot instead of sizeof(struct kvm_memory_slot). -- Gleb. -- 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
