On Tue, Feb 17, 2015 at 02:11:37PM +0100, Paolo Bonzini wrote: > > > On 17/02/2015 13:32, Michael S. Tsirkin wrote: > > On Tue, Feb 17, 2015 at 11:59:48AM +0100, Paolo Bonzini wrote: > >> > >> > >> On 17/02/2015 10:02, Michael S. Tsirkin wrote: > >>>> Increasing VHOST_MEMORY_MAX_NREGIONS from 65 to 509 > >>>> to match KVM_USER_MEM_SLOTS fixes issue for vhost-net. > >>>> > >>>> Signed-off-by: Igor Mammedov <imammedo@xxxxxxxxxx> > >>> > >>> This scares me a bit: each region is 32byte, we are talking > >>> a 16K allocation that userspace can trigger. > >> > >> What's bad with a 16K allocation? > > > > It fails when memory is fragmented. > > If memory is _that_ fragmented I think you have much bigger problems > than vhost. > > > I'm guessing kvm doesn't do memory scans on data path, vhost does. > > It does for MMIO memory-to-memory writes, but that's not a particularly > fast path. > > KVM doesn't access the memory map on fast paths, but QEMU does, so I > don't think it's beyond the expectations of the kernel. QEMU has an elaborate data structure to deal with that. > For example you > can use a radix tree (not lib/radix-tree.c unfortunately), and cache > GVA->HPA translations if it turns out that lookup has become a hot path. All vhost lookups are hot path. > The addressing space of x86 is in practice 44 bits or fewer, and each > slot will typically be at least 1 GiB, so you only have 14 bits to > dispatch on. It's probably possible to only have two or three levels > in the radix tree in the common case, and beat the linear scan real quick. Not if there are about 6 regions, I think. > The radix tree can be tuned to use order-0 allocations, and then your > worries about fragmentation go away too. > > Paolo Increasing the number might be reasonable for workloads such as nested virt. But depending on this in userspace when you don't have to is not a good idea IMHO. -- MST -- 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
