On Mon, May 18, 2015 at 07:22:34PM +0300, Andrey Korolyov wrote: > On Wed, Feb 18, 2015 at 7:27 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Tue, Feb 17, 2015 at 04:53:45PM -0800, Eric Northup wrote: > >> On Tue, Feb 17, 2015 at 4:32 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> 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. > >> > > >> >> > How does kvm handle this issue? > >> >> > >> >> It doesn't. > >> >> > >> >> Paolo > >> > > >> > I'm guessing kvm doesn't do memory scans on data path, > >> > vhost does. > >> > > >> > qemu is just doing things that kernel didn't expect it to need. > >> > > >> > Instead, I suggest reducing number of GPA<->HVA mappings: > >> > > >> > you have GPA 1,5,7 > >> > map them at HVA 11,15,17 > >> > then you can have 1 slot: 1->11 > >> > > >> > To avoid libc reusing the memory holes, reserve them with MAP_NORESERVE > >> > or something like this. > >> > >> This works beautifully when host virtual address bits are more > >> plentiful than guest physical address bits. Not all architectures > >> have that property, though. > > > > AFAIK this is pretty much a requirement for both kvm and vhost, > > as we require each guest page to also be mapped in qemu memory. > > > >> > We can discuss smarter lookup algorithms but I'd rather > >> > userspace didn't do things that we then have to > >> > work around in kernel. > >> > > >> > > >> > -- > >> > 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 > > -- > > To unsubscribe from this list: send the line "unsubscribe netdev" in > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > Hello, > > any chance of getting the proposed patch in the mainline? Though it > seems that most users will not suffer from relatively slot number > ceiling (they can decrease slot 'granularity' for larger VMs and > vice-versa), fine slot size, 256M or even 128M, with the large number > of slots can be useful for a certain kind of tasks for an > orchestration systems. I`ve made a backport series of all seemingly > interesting memslot-related improvements to a 3.10 branch, is it worth > to be tested with straighforward patch like one from above, with > simulated fragmentation of allocations in host? I'd rather people worked on the 1:1 mapping, it will also speed up lookups. I'm concerned if I merge this one, motivation for people to work on the right fix will disappear. -- 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
