On 26.09.23 20:57, David Hildenbrand wrote:
Quoting from patch #16:
Having large virtio-mem devices that only expose little memory to a VM
is currently a problem: we map the whole sparse memory region into the
guest using a single memslot, resulting in one gigantic memslot in KVM.
KVM allocates metadata for the whole memslot, which can result in quite
some memory waste.
Assuming we have a 1 TiB virtio-mem device and only expose little (e.g.,
1 GiB) memory, we would create a single 1 TiB memslot and KVM has to
allocate metadata for that 1 TiB memslot: on x86, this implies allocating
a significant amount of memory for metadata:
(1) RMAP: 8 bytes per 4 KiB, 8 bytes per 2 MiB, 8 bytes per 1 GiB
-> For 1 TiB: 2147483648 + 4194304 + 8192 = ~ 2 GiB (0.2 %)
With the TDP MMU (cat /sys/module/kvm/parameters/tdp_mmu) this gets
allocated lazily when required for nested VMs
(2) gfn_track: 2 bytes per 4 KiB
-> For 1 TiB: 536870912 = ~512 MiB (0.05 %)
(3) lpage_info: 4 bytes per 2 MiB, 4 bytes per 1 GiB
-> For 1 TiB: 2097152 + 4096 = ~2 MiB (0.0002 %)
(4) 2x dirty bitmaps for tracking: 2x 1 bit per 4 KiB page
-> For 1 TiB: 536870912 = 64 MiB (0.006 %)
So we primarily care about (1) and (2). The bad thing is, that the
memory consumption doubles once SMM is enabled, because we create the
memslot once for !SMM and once for SMM.
Having a 1 TiB memslot without the TDP MMU consumes around:
* With SMM: 5 GiB
* Without SMM: 2.5 GiB
Having a 1 TiB memslot with the TDP MMU consumes around:
* With SMM: 1 GiB
* Without SMM: 512 MiB
... and that's really something we want to optimize, to be able to just
start a VM with small boot memory (e.g., 4 GiB) and a virtio-mem device
that can grow very large (e.g., 1 TiB).
Consequently, using multiple memslots and only mapping the memslots we
really need can significantly reduce memory waste and speed up
memslot-related operations. Let's expose the sparse RAM memory region using
multiple memslots, mapping only the memslots we currently need into our
device memory region container.
The hyper-v balloon driver has similar demands [1].
For virtio-mem, this has to be turned manually on ("dynamic-memslots=on"),
due to the interaction with vhost (below).
If we have less than 509 memslots available, we always default to a single
memslot. Otherwise, we automatically decide how many memslots to use
based on a simple heuristic (see patch #12), and try not to use more than
256 memslots across all memory devices: our historical DIMM limit.
As soon as any memory devices automatically decided on using more than
one memslot, vhost devices that support less than 509 memslots (e.g.,
currently most vhost-user devices like with virtiofsd) can no longer be
plugged as a precaution.
Quoting from patch #12:
Plugging vhost devices with less than 509 memslots available while we
have memory devices plugged that consume multiple memslots due to
automatic decisions can be problematic. Most configurations might just fail
due to "limit < used + reserved", however, it can also happen that these
memory devices would suddenly consume memslots that would actually be
required by other memslot consumers (boot, PCI BARs) later. Note that this
has always been sketchy with vhost devices that support only a small number
of memslots; but we don't want to make it any worse.So let's keep it simple
and simply reject plugging such vhost devices in such a configuration.
Eventually, all vhost devices that want to be fully compatible with such
memory devices should support a decent number of memslots (>= 509).
The recommendation is to plug such vhost devices before the virtio-mem
decides, or to not set "dynamic-memslots=on". As soon as these devices
support a reasonable number of memslots (>= 509), this will start working
automatically.
I run some tests on x86_64, now also including vfio and migration tests.
Seems to work as expected, even when multiple memslots are used.
Patch #1 -- #3 are from [2] that were not picked up yet.
Patch #4 -- #12 add handling of multiple memslots to memory devices
Patch #13 -- #16 add "dynamic-memslots=on" support to virtio-mem
Patch #15 -- #16 make sure that virtio-mem memslots can be enabled/disable
atomically
If there is no further feedback until the end of the week, I'll queue
this to mem-next.
--
Cheers,
David / dhildenb
