Re: [RFC] memory tiering: use small chunk size and more tiers

[Aneesh Kumar K V]

On 10/27/22 12:29 PM, Huang Ying wrote:
> We need some way to override the system default memory tiers.  For
> the example system as follows,
> 
> type		abstract distance
> ----		-----------------
> HBM		300
> DRAM		1000
> CXL_MEM		5000
> PMEM		5100
> 
> Given the memory tier chunk size is 100, the default memory tiers
> could be,
> 
> tier		abstract distance	types
>                 range
> ----		-----------------       -----
> 3		300-400			HBM
> 10		1000-1100		DRAM
> 50		5000-5100		CXL_MEM
> 51		5100-5200		PMEM
> 
> If we want to group CXL MEM and PMEM into one tier, we have 2 choices.
> 
> 1) Override the abstract distance of CXL_MEM or PMEM.  For example, if
> we change the abstract distance of PMEM to 5050, the memory tiers
> become,
> 
> tier		abstract distance	types
>                 range
> ----		-----------------       -----
> 3		300-400			HBM
> 10		1000-1100		DRAM
> 50		5000-5100		CXL_MEM, PMEM
> 
> 2) Override the memory tier chunk size.  For example, if we change the
> memory tier chunk size to 200, the memory tiers become,
> 
> tier		abstract distance	types
>                 range
> ----		-----------------       -----
> 1		200-400			HBM
> 5		1000-1200		DRAM
> 25		5000-5200		CXL_MEM, PMEM
> 
> But after some thoughts, I think choice 2) may be not good.  The
> problem is that even if 2 abstract distances are almost same, they may
> be put in 2 tier if they sit in the different sides of the tier
> boundary.  For example, if the abstract distance of CXL_MEM is 4990,
> while the abstract distance of PMEM is 5010.  Although the difference
> of the abstract distances is only 20, CXL_MEM and PMEM will put in
> different tiers if the tier chunk size is 50, 100, 200, 250, 500, ....
> This makes choice 2) hard to be used, it may become tricky to find out
> the appropriate tier chunk size that satisfying all requirements.
> 

Shouldn't we wait for gaining experience w.r.t how we would end up
mapping devices with different latencies and bandwidth before tuning these values? 

> So I suggest to abandon choice 2) and use choice 1) only.  This makes
> the overall design and user space interface to be simpler and easier
> to be used.  The overall design of the abstract distance could be,
> 
> 1. Use decimal for abstract distance and its chunk size.  This makes
>    them more user friendly.
> 
> 2. Make the tier chunk size as small as possible.  For example, 10.
>    This will put different memory types in one memory tier only if their
>    performance is almost same by default.  And we will not provide the
>    interface to override the chunk size.
> 

this could also mean we can end up with lots of memory tiers with relative
smaller performance difference between them. Again it depends how HMAT
attributes will be used to map to abstract distance.



> 3. Make the abstract distance of normal DRAM large enough.  For
>    example, 1000, then 100 tiers can be defined below DRAM, this is
>    more than enough in practice.

Why 100? Will we really have that many tiers below/faster than DRAM? As of now 
I see only HBM below it.

> 
> 4. If we want to override the default memory tiers, just override the
>    abstract distances of some memory types with a per memory type
>    interface.
> 
> This patch is to apply the design choices above in the existing code.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> Cc: Bharata B Rao <bharata@xxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Cc: Hesham Almatary <hesham.almatary@xxxxxxxxxx>
> Cc: Jagdish Gediya <jvgediya.oss@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxx>
> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> ---
>  include/linux/memory-tiers.h | 7 +++----
>  mm/memory-tiers.c            | 7 +++----
>  2 files changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
> index 965009aa01d7..2e39d9a6c8ce 100644
> --- a/include/linux/memory-tiers.h
> +++ b/include/linux/memory-tiers.h
> @@ -7,17 +7,16 @@
>  #include <linux/kref.h>
>  #include <linux/mmzone.h>
>  /*
> - * Each tier cover a abstrace distance chunk size of 128
> + * Each tier cover a abstrace distance chunk size of 10
>   */
> -#define MEMTIER_CHUNK_BITS	7
> -#define MEMTIER_CHUNK_SIZE	(1 << MEMTIER_CHUNK_BITS)
> +#define MEMTIER_CHUNK_SIZE	10
>  /*
>   * Smaller abstract distance values imply faster (higher) memory tiers. Offset
>   * the DRAM adistance so that we can accommodate devices with a slightly lower
>   * adistance value (slightly faster) than default DRAM adistance to be part of
>   * the same memory tier.
>   */
> -#define MEMTIER_ADISTANCE_DRAM	((4 * MEMTIER_CHUNK_SIZE) + (MEMTIER_CHUNK_SIZE >> 1))
> +#define MEMTIER_ADISTANCE_DRAM	((100 * MEMTIER_CHUNK_SIZE) + (MEMTIER_CHUNK_SIZE / 2))
>  #define MEMTIER_HOTPLUG_PRIO	100
>  
>  struct memory_tier;
> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> index fa8c9d07f9ce..e03011428fa5 100644
> --- a/mm/memory-tiers.c
> +++ b/mm/memory-tiers.c
> @@ -165,11 +165,10 @@ static struct memory_tier *find_create_memory_tier(struct memory_dev_type *memty
>  	bool found_slot = false;
>  	struct memory_tier *memtier, *new_memtier;
>  	int adistance = memtype->adistance;
> -	unsigned int memtier_adistance_chunk_size = MEMTIER_CHUNK_SIZE;
>  
>  	lockdep_assert_held_once(&memory_tier_lock);
>  
> -	adistance = round_down(adistance, memtier_adistance_chunk_size);
> +	adistance = rounddown(adistance, MEMTIER_CHUNK_SIZE);
>  	/*
>  	 * If the memtype is already part of a memory tier,
>  	 * just return that.
> @@ -204,7 +203,7 @@ static struct memory_tier *find_create_memory_tier(struct memory_dev_type *memty
>  	else
>  		list_add_tail(&new_memtier->list, &memory_tiers);
>  
> -	new_memtier->dev.id = adistance >> MEMTIER_CHUNK_BITS;
> +	new_memtier->dev.id = adistance / MEMTIER_CHUNK_SIZE;
>  	new_memtier->dev.bus = &memory_tier_subsys;
>  	new_memtier->dev.release = memory_tier_device_release;
>  	new_memtier->dev.groups = memtier_dev_groups;
> @@ -641,7 +640,7 @@ static int __init memory_tier_init(void)
>  #endif
>  	mutex_lock(&memory_tier_lock);
>  	/*
> -	 * For now we can have 4 faster memory tiers with smaller adistance
> +	 * For now we can have 100 faster memory tiers with smaller adistance
>  	 * than default DRAM tier.
>  	 */
>  	default_dram_type = alloc_memory_type(MEMTIER_ADISTANCE_DRAM);