Re: [PATCH 4/4] NUMA: realize NUMA memory pinning

Anthony Liguori
2010-08-31T22:03:26Z

On 08/31/2010 03:54 PM, Andrew Theurer wrote:
> On Mon, 2010-08-23 at 16:27 -0500, Anthony Liguori wrote:
>
>> On 08/23/2010 04:16 PM, Andre Przywara wrote:
>>
>>> Anthony Liguori wrote:
>>>
>>>> On 08/23/2010 01:59 PM, Marcelo Tosatti wrote:
>>>>
>>>>> On Wed, Aug 11, 2010 at 03:52:18PM +0200, Andre Przywara wrote:
>>>>>
>>>>>> According to the user-provided assignment bind the respective part
>>>>>> of the guest's memory to the given host node. This uses Linux'
>>>>>> mbind syscall (which is wrapped only in libnuma) to realize the
>>>>>> pinning right after the allocation.
>>>>>> Failures are not fatal, but produce a warning.
>>>>>>
>>>>>> Signed-off-by: Andre Przywara<andre.przywara@amd.com>
>>>>>> ...
>>>>>>
>>>>> Why is it not possible (or perhaps not desired) to change the binding
>>>>> after the guest is started?
>>>>>
>>>>> Sounds unflexible.
>>>>>
>>> The solution is to introduce a monitor interface to later adjust the
>>> pinning, allowing both changing the affinity only (only valid for
>>> future fault-ins) and actually copying the memory (more costly).
>>>
>> This is just duplicating numactl.
>>
>>
>>> Actually this is the next item on my list, but I wanted to bring up
>>> the basics first to avoid recoding parts afterwards. Also I am not
>>> (yet) familiar with the QMP protocol.
>>>
>>>> We really need a solution that lets a user use a tool like numactl
>>>> outside of the QEMU instance.
>>>>
>>> I fear that is not how it's meant to work with the Linux' NUMA API. In
>>> opposite to the VCPU threads, which are externally visible entities
>>> (PIDs), the memory should be private to the QEMU process. While you
>>> can change the NUMA allocation policy of the _whole_ process, there is
>>> no way to externally distinguish parts of the process' memory.
>>> Although you could later (and externally) migrate already faulted
>>> pages (via move_pages(2) and by looking in /proc/$$/numa_maps), you
>>> would let an external tool interfere with QEMUs internal memory
>>> management. Take for instance the change of the allocation policy
>>> regarding the 1MB and 3.5-4GB holes. An external tool would have to
>>> either track such changes or you simply could not change such things
>>> in QEMU.
>>>
>> It's extremely likely that if you're doing NUMA pinning, you're also
>> doing large pages via hugetlbfs. numactl can already set policies for
>> files in hugetlbfs so all you need to do is have a separate hugetlbfs
>> file for each numa node.
>>
> Why would we resort to hugetlbfs when we have transparent hugepages?
>

If you care about NUMA pinning, I can't believe you don't want
guaranteed large page allocation which THP does not provide.

The general point though is that we should find a way to partition
memory in qemu such that an external process can control the actual NUMA
placement. This gives us maximum flexibility.

Otherwise, what do we implement in QEMU? Direct pinning of memory to
nodes? Can we migrate memory between nodes? Should we support
interleaving memory between two virtual nodes? Why pick and choose when
we can have it all.

> FWIW, large apps like databases have set a precedent for managing their
> own NUMA policies.

Of course because they know what their NUMA policy should be. They live
in a simple world where they assume they're the only application in the
system, they read the distance tables, figure they'll use XX% of all
physical memory, and then pin how they see fit.

But an individual QEMU process lives in a complex world. It's almost
never the only thing on the system and it's only allowed to use a subset
of resources. It's not sure what set of resources it can and can't use
and that's often times changing. The topology chosen for a guest is
static but it's host topology may be dynamic due to thinks like live
migration.

In short, QEMU absolutely cannot implement a NUMA policy in a vacuum.
Instead, it needs to let something with a larger view of the system
determine a NUMA policy that makes sense overall.

There are two ways we can do this. We can implement monitor commands
that attempt to expose every single NUMA tunable possible. Or, we can
tie into the existing commands which guarantee that we support every
possible tunable and that as NUMA support in Linux evolves, we get all
the new features for free.

And, since numactl already supports setting policies on files in
hugetlbfs, all we need is a simple change to qemu to allow -mem-path to
work per-node instead of globally. And it's useful to implement other
types of things like having one node be guaranteed large pages and
another node THP or some other fanciness.

Sounds awfully appealing to me.

> I don't see why qemu should be any different.
> Numactl is great for small apps that need to be pinned in one node, or
> spread evenly on all nodes. Having to get hugetlbfs involved just to
> workaround a shortcoming of numactl just seems like a bad idea.
>

You seem to be asserting that we should implement a full NUMA policy in
QEMU. What should it be when we don't (in QEMU) know what else is
running on the system?

Regards,

Anthony Liguori