On 17.10.22 13:46, 黄杰 wrote:
David Hildenbrand <david@xxxxxxxxxx> 于2022年10月17日周一 19:33写道：
On 17.10.22 11:48, 黄杰 wrote:
David Hildenbrand <david@xxxxxxxxxx> 于2022年10月17日周一 16:44写道：
On 12.10.22 10:15, Albert Huang wrote:
From: "huangjie.albert" <huangjie.albert@xxxxxxxxxxxxx>
implement these two functions so that we can set the mempolicy to
the inode of the hugetlb file. This ensures that the mempolicy of
all processes sharing this huge page file is consistent.
In some scenarios where huge pages are shared:
if we need to limit the memory usage of vm within node0, so I set qemu's
mempilciy bind to node0, but if there is a process (such as virtiofsd)
shared memory with the vm, in this case. If the page fault is triggered
by virtiofsd, the allocated memory may go to node1 which depends on
virtiofsd.
Any VM that uses hugetlb should be preallocating memory. For example,
this is the expected default under QEMU when using huge pages.
Once preallocation does the right thing regarding NUMA policy, there is
no need to worry about it in other sub-processes.
Hi, David
thanks for your reminder
Yes, you are absolutely right, However, the pre-allocation mechanism
does solve this problem.
However, some scenarios do not like to use the pre-allocation mechanism, such as
scenarios that are sensitive to virtual machine startup time, or
scenarios that require
high memory utilization. The on-demand allocation mechanism may be better,
so the key point is to find a way support for shared policy。
Using hugetlb -- with a fixed pool size -- without preallocation is like
playing with fire. Hugetlb reservation makes one believe that on-demand
allocation is going to work, but there are various scenarios where that
can go seriously wrong, and you can run out of huge pages.
If you're using hugetlb as memory backend for a VM without
preallocation, you really have to be very careful. I can only advise
against doing that.
Also: why does another process read/write *first* to a guest physical
memory location before the OS running inside the VM even initialized
that memory? That sounds very wrong. What am I missing?
for example : virtio ring buffer.
For the avial descriptor, the guest kernel only gives an address to
the backend,
and does not actually access the memory.
Okay, thanks. So we're essentially providing uninitialized memory to a
device? Hm, that implies that the device might have access to memory
that was previously used by someone else ... not sure how to feel about
that, but maybe this is just the way of doing things.
The "easy" user-space fix would be to simply similarly mbind() in the
other processes where we mmap(). Has that option been explored?
--
Thanks,
David / dhildenb
