Re: [libvirt] [PATCH] qemu: Don't use -mem-prealloc among with .prealloc=yes
On 11/5/18 3:49 PM, Michal Privoznik wrote:
https://bugzilla.redhat.com/show_bug.cgi?id=1624223
There are two ways to request memory preallocation on cmd line:
-mem-prealloc and .prealloc attribute to memory-backend-file.
However, as it turns out it's not safe to use both at the same
time. Prefer -mem-prealloc as it is more backward compatible
compared to switching to "-numa node,memdev= + -object
memory-backend-file".
Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com>
---
src/qemu/qemu_command.c | 37 +++++++++++++------
src/qemu/qemu_command.h | 1 +
src/qemu/qemu_domain.c | 2 +
src/qemu/qemu_domain.h | 3 ++
src/qemu/qemu_hotplug.c | 3 +-
.../hugepages-numa-default-dimm.args | 2 +-
6 files changed, 35 insertions(+), 13 deletions(-)
Ping?
After some discussion with QEMU devels, I found out that this might be
harmful/suboptimal. Thing is, if -mem-prealloc is used then qemu will
fully allocate the memory (this is done by actually touching every page
that has been allocated). Then, if .prealloc=yes is specified,
mbind(flags = MPOL_MF_STRICT | MPOL_MF_MOVE) is called which:
a) has to (possibly) move the memory to a different NUMA node,
b) can have no effect when hugepages are in play (thus ignoring user
request to place memory on desired NUMA nodes).
While I could live with a), I couldn't with b). The patch is still valid.
Michal