Re: [PATCH v14 04/10] mm/demotion/dax/kmem: Set node's abstract distance to MEMTIER_DEFAULT_DAX_ADISTANCE

[huang ying <huang.ying.caritas@gmail.com>]

On Tue, Aug 16, 2022 at 1:10 PM Aneesh Kumar K V
<aneesh.kumar@linux.ibm.com> wrote:
>
> On 8/15/22 8:09 AM, Huang, Ying wrote:
> > "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
> >

[snip]

> >>
> >> +/*
> >> + * Default abstract distance assigned to the NUMA node onlined
> >> + * by DAX/kmem if the low level platform driver didn't initialize
> >> + * one for this NUMA node.
> >> + */
> >> +#define MEMTIER_DEFAULT_DAX_ADISTANCE       (MEMTIER_ADISTANCE_DRAM * 2)
> >
> > If my understanding were correct, this is targeting Optane DCPMM for
> > now.  The measured results in the following paper is,
> >
> > https://arxiv.org/pdf/2002.06018.pdf
> >
> > Section: 2.1 Read/Write Latencies
> >
> > "
> > For read access, the latency of DCPMM was 400.1% higher than that of
> > DRAM. For write access, it was 407.1% higher.
> > "
> >
> > Section: 2.2 Read/Write Bandwidths
> >
> > "
> > For read access, the throughput of DCPMM was 37.1% of DRAM. For write
> > access, it was 7.8%
> > "
> >
> > According to the above data, I think the MEMTIER_DEFAULT_DAX_ADISTANCE
> > can be "5 * MEMTIER_ADISTANCE_DRAM".
> >
>
> If we look at mapping every 100% increase in latency as a memory tier, we essentially
> will have 4 memory tier here. Each memory tier is covering a range of abstract distance 128.
> which makes a total adistance increase from MEMTIER_ADISTANCE_DRAM by 512. This puts
> DEFAULT_DAX_DISTANCE at 1024 or  MEMTIER_ADISTANCE_DRAM * 2

If my understanding were correct, you are suggesting to use a kind of
logarithmic mapping from latency to abstract distance?  That is,

  abstract_distance = log2(latency)

While I am suggesting to use a kind of linear mapping from latency to
abstract distance.  That is,

  abstract_distance = C * latency

I think that linear mapping is easy to understand.

Are there some good reasons to use logarithmic mapping?

Best Regards,
Huang, Ying