On Tue, 2012-12-04 at 17:30 +0200, Gleb Natapov wrote: > 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). Ah, of course. That means for 512 slots we're wasting a full page just for the pointers, whereas we can fit 56 slots in the same space. Given that most users get by just fine w/ 32 slots, I don't think that's a win in the typical case. Maybe if we want to support sparse arrays, but a tree would probably be better at that point. A drawback of the growable array is that userspace can subvert any savings by using slot N-1 first, but that's why we put a limit at a reasonable size. Thanks, Alex -- 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
