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Re: [Qemu-devel] [PATCH 2/2] Add monitor command mem-nodes

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 2/2] Add monitor command mem-nodes
Date: Thu, 13 Jun 2013 18:32:49 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 13/06/2013 08:50, Eduardo Habkost ha scritto:
> I believe an interface based on guest physical memory addresses is more
> flexible (and even simpler!) than one that only allows binding of whole
> virtual NUMA nodes.

And "-numa node" is already one, what about just adding "mem-path=/foo"
or "host_node=NN" suboptions?  Then "-mem-path /foo" would be a shortcut
for "-numa node,mem-path=/foo".

I even had patches to convert -numa to QemuOpts, I can dig them out if
your interested.


> (And I still don't understand why you are exposing QEMU virtual memory
> addresses in the new command, if they are useless).
>>>>>  * The correspondence between guest physical address ranges and ranges
>>>>>    inside the mapped files (so external tools could set the policy on
>>>>>    those files instead of requiring QEMU to set it directly)
>>>>> I understand that your use case may require additional information and
>>>>> additional interfaces. But if we provide the information above we will
>>>>> allow external components set the policy on the hugetlbfs files before
>>>>> we add new interfaces required for your use case.
>>>> But the file backed memory is not good for the host which has many
>>>> virtual machines, in this situation, we can't handle anon THP yet.
>>> I don't understand what you mean, here. What prevents someone from using
>>> file-backed memory with multiple virtual machines?
>> While if we use hugetlbfs backed memory, we should know how many virtual 
>> machines,
>> how much memory each vm will use, then reserve these pages for them. And even
>> should reserve more pages for external tools(numactl) to set memory polices.
>> Even the memory reservation also has it's own memory policies. It's very hard
>> to control it to what we want to set.
> Well, it's hard because we don't even have tools to help on that, yet.
> Anyway, I understand that you want to make it work with THP as well. But
> if THP works with tmpfs (does it?), people then could use exactly the
> same file-based mechanisms with tmpfs and keep THP working.
> (Right now I am doing some experiments to understand how the system
> behaves when using numactl on hugetlbfs and tmpfs, before and after
> getting the files mapped).
>>>> And as I mentioned, the cross numa node access performance regression
>>>> is caused by pci-passthrough, it's a very long time bug, we should
>>>> back port the host memory pinning patch to old QEMU to resolve this 
>>>> performance
>>>> problem, too.
>>> If it's a regression, what's the last version of QEMU where the bug
>>> wasn't present?
>>  As QEMU doesn't support host memory binding, I think
>> this was present since we support guest NUMA, and the pci-passthrough made
>> it even worse.
> If the problem was always present, it is not a regression, is it?

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