Re: [External] Re: [PATCH v1 0/1] Improved Memory Tier Creation for CPUless NUMA Nodes

[Ho-Ren (Jack) Chuang]

On Mon, Mar 4, 2024 at 10:36 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> "Ho-Ren (Jack) Chuang" <horenchuang@bytedance.com> writes:
>
> > On Sun, Mar 3, 2024 at 6:47 PM Huang, Ying <ying.huang@intel.com> wrote:
> >>
> >> "Ho-Ren (Jack) Chuang" <horenchuang@bytedance.com> writes:
> >>
> >> > The memory tiering component in the kernel is functionally useless for
> >> > CPUless memory/non-DRAM devices like CXL1.1 type3 memory because the 
> >> > nodes
> >> > are lumped together in the DRAM tier.
> >> > https://lore.kernel.org/linux-mm/PH0PR08MB7955E9F08CCB64F23963B5C3A860A@PH0PR08MB7955.namprd08.prod.outlook.com/T/
> >>
> >> I think that it's unfair to call it "useless".  Yes, it doesn't work if
> >> the CXL memory device are not enumerate via drivers/dax/kmem.c.  So,
> >> please be specific about in which cases it doesn't work instead of too
> >> general "useless".
> >>
> >
> > Thank you and I didn't mean anything specific. I simply reused phrases
> > we discussed
> > earlier in the previous patchset. I will change them to the following in v2:
> > "At boot time, current memory tiering assigns all detected memory nodes
> > to the same DRAM tier. This results in CPUless memory/non-DRAM devices,
> > such as CXL1.1 type3 memory, being unable to be assigned to the
> > correct memory tier,
> > leading to the inability to migrate pages between different types of 
> > memory."
> >
> > Please see if this looks more specific.
>
> I don't think that the description above is accurate.  In fact, there
> are 2 ways to enumerate the memory device,
>
> 1. Mark it as reserved memory (E820_TYPE_SOFT_RESERVED, etc.) in E820
>    table or something similar.
>
> 2. Mark it as normal memory (E820_TYPE_RAM) in E820 table or something
>    similar
>
> For 1, the memory device (including CXL memory) is onlined via
> drivers/dax/kmem.c, so will be put in proper memory tiers.  For 2, the
> memory device is indistinguishable with normal DRAM with current
> implementation.  And this is what this patch is working on.
>
> Right?

Good point! How about this?:
"
When a memory device, such as CXL1.1 type3 memory, is emulated as
normal memory (E820_TYPE_RAM), the memory device is indistinguishable
from normal DRAM in terms of memory tiering with the current implementation.
The current memory tiering assigns all detected normal memory nodes
to the same DRAM tier. This results in normal memory devices with
different attributions being unable to be assigned to the correct memory tier,
leading to the inability to migrate pages between different types of memory.
"

--
Best regards,
Ho-Ren (Jack) Chuang
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