| Message ID | 1415289167-24661-1-git-send-email-imammedo@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 06/11/2014 16:52, Igor Mammedov wrote: > With the 3 private slots, this gives us 512 slots total. > Motivation for this is in addition to assigned devices > support more memory hotplug slots, where 1 slot is > used by a hotplugged memory stick. > It will allow to support upto 256 hotplug memory > slots and leave 253 slots for assigned devices and > other devices that use them. > > Signed-off-by: Igor Mammedov <imammedo@redhat.com> It would use more memory, and some loops are now becoming more expensive. In general adding a memory slot to a VM is not cheap, and I question the wisdom of having 256 hotplug memory slots. But the slowdown mostly would only happen if you actually _use_ those memory slots, so it is not a blocker for this patch. We probably should change the kmemdup + heap sort of __kvm_set_memory_region + update_memslots to copy the array and insert the new item at the right place, at the same time. Using a heap sort is overkill and unnecessarily goes from O(n^2) to O(n^2 log n). With a bigger constant in front as well. If you want to do it, I'd be grateful. Otherwise I can look at it as time permits. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, 06 Nov 2014 17:23:58 +0100 Paolo Bonzini <pbonzini@redhat.com> wrote: > > > On 06/11/2014 16:52, Igor Mammedov wrote: > > With the 3 private slots, this gives us 512 slots total. > > Motivation for this is in addition to assigned devices > > support more memory hotplug slots, where 1 slot is > > used by a hotplugged memory stick. > > It will allow to support upto 256 hotplug memory > > slots and leave 253 slots for assigned devices and > > other devices that use them. > > > > Signed-off-by: Igor Mammedov <imammedo@redhat.com> > > It would use more memory, and some loops are now becoming more > expensive. In general adding a memory slot to a VM is not cheap, and > I question the wisdom of having 256 hotplug memory slots. But the > slowdown mostly would only happen if you actually _use_ those memory > slots, so it is not a blocker for this patch. It might be useful to have a big amount of slots for big guests and although linux works with minimum section 128Mb but Windows memory hotplug works just fine even with page-sized slots so when unplug in QEMU is implemented it would be possible to drop balooning driver at least there. And providing that memslots could be allocated during runtime when guest programs devices or maps roms (i.e. no fail path), I don't see a way to fix it in QEMU (i.e. avoid abort when limit is reached). Hence an attempt to bump memslots limit to 512, where current 125 are reserved for initial memory mappings and passthrough devices 256 goes to hotplug memory slots and leaves us 128 free slots for future expansion. To see what would be affected by large amount of slots I played with perf a bit and the biggest hotspot offender with large amount of memslots was: gfn_to_memslot() -> ... -> search_memslots() I'll try to make it faster for this case so 512 memslots wouldn't affect guest performance. So please consider applying this patch. > > Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 14/11/2014 15:10, Igor Mammedov wrote: > On Thu, 06 Nov 2014 17:23:58 +0100 Paolo Bonzini <pbonzini@redhat.com> wrote: >> It would use more memory, and some loops are now becoming more >> expensive. In general adding a memory slot to a VM is not cheap, and >> I question the wisdom of having 256 hotplug memory slots. But the >> slowdown mostly would only happen if you actually _use_ those memory >> slots, so it is not a blocker for this patch. > It might be useful to have a big amount of slots for big guests > and although linux works with minimum section 128Mb but Windows memory > hotplug works just fine even with page-sized slots so when unplug in > QEMU is implemented it would be possible to drop balooning driver at > least there. I think for a big (64G?) guest it doesn't make much sense anyway to balloon at a granularity that is less than 1G or even more. So I like the idea of dropping ballooning in favor of memory hotplug for big guests. > And providing that memslots could be allocated during runtime when guest > programs devices or maps roms (i.e. no fail path), I don't see a way > to fix it in QEMU (i.e. avoid abort when limit is reached). > Hence an attempt to bump memslots limit to 512, where current 125 > are reserved for initial memory mappings and passthrough devices > 256 goes to hotplug memory slots and leaves us 128 free slots for > future expansion. > > To see what would be affected by large amount of slots I played with > perf a bit and the biggest hotspot offender with large amount of > memslots was: > > gfn_to_memslot() -> ... -> search_memslots() > > I'll try to make it faster for this case so 512 memslots wouldn't > affect guest performance. > > So please consider applying this patch. Yes, sorry for the delay---I am definitely going to apply it. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 6ed0c30..cfd60e3 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -33,7 +33,7 @@ #define KVM_MAX_VCPUS 255 #define KVM_SOFT_MAX_VCPUS 160 -#define KVM_USER_MEM_SLOTS 125 +#define KVM_USER_MEM_SLOTS 509 /* memory slots that are not exposed to userspace */ #define KVM_PRIVATE_MEM_SLOTS 3 #define KVM_MEM_SLOTS_NUM (KVM_USER_MEM_SLOTS + KVM_PRIVATE_MEM_SLOTS)
With the 3 private slots, this gives us 512 slots total. Motivation for this is in addition to assigned devices support more memory hotplug slots, where 1 slot is used by a hotplugged memory stick. It will allow to support upto 256 hotplug memory slots and leave 253 slots for assigned devices and other devices that use them. Signed-off-by: Igor Mammedov <imammedo@redhat.com> --- previously increased to 125 slots for assigned devices by 0f888f5acd --- arch/x86/include/asm/kvm_host.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)