memtier - Man Page

interposer library which enables the memkind memory tiering


In order to use the memkind memory tiering, pass the following environment variables along with the command: MEMKIND_MEM_TIERS="..." command {arguments ...}


This library enables the memkind memory tiering mechanism. With this funcionality, allocations will be split between different types of memory automatically. The library allows making allocations with the usage of multiple kinds keeping a specified ratio between them. This ratio determines how much of the total allocated memory should be allocated with each kind. Both LD_PRELOAD and MEMKIND_MEM_TIERS environment variables are mandatory for enabling the memkind memory tiering.



A semicolon-separated list of tier configurations ended with a policy parameter. Each of the tier configurations is a comma-separated list of tier parameters in the format: param_name:value,(...) For the list of available tier parameters, please see the Tier Parameters section below.


Semicolon-separated list of threshold configurations. Each configuration consists of a comma-separated list of threshold parameters in the same format as MEMKIND_MEM_TIERS , where the param_name is one of the INIT_VAL, MIN_VAL and MAX_VAL. Each parameter can be set only once. When any of the parameters are missing, they will be set to a default value. If threshold configuration is missing, threshold with default configuration will be used. If any of the parameters is set more than once, the application will be aborted. Order of provided configurations is important - configuration of Nth threshold defines threshold between Nth and (N+1)th tier. Note that for N+1 tiers there should be at most N thresholds defined. Set this variable only when DYNAMIC_THRESHOLD policy is used. For more details about param_name see the Threshold Parameters section below. See also Examples section for the usage of MEMKIND_MEM_THRESHOLDS environment variable.

Tier Parameters


(required) - kind of memory used in memory tier. Allowed kind names are 'DRAM', 'KMEM_DAX' and 'FS_DAX'. There can be multiple different kind names provided in the MEMKIND_MEM_TIERS environment variable - one in each tier configuration. Additionally, in this variable more than one 'FS_DAX' kind name is allowed. More information about available kinds can be found in the memkind(3) manual, where the 'DRAM' kind name allocates memory with the usage of the MEMKIND_DEFAULT kind, the 'KMEM_DAX' with the MEMKIND_DAX_KMEM kind and the 'FS_DAX' with a file-backed kind of memory created with the memkind_create_pmem() function.


(required only for the FS_DAX kind) - the path to the location where pmem file will be created. The path has to exist. For other kinds, if set, will cause an error.


(optional, only for the FS_DAX kind) - if set, it limits the size of pmem file and the maximum size of total allocations from persistent memory. By default, no limit is introduced. Pass this option only with FS_DAX kind. For other kinds, if set, will cause an error. The accepted formats are: 1, 1K, 1M, 1G. See the memkind(3) manual for information about limitations to this value which are the same as for the max_size value of the memkind_create_pmem() function.


(required) - the part of the ratio tied to the given kind. It's an unsigned type in a range from 1 to UINT_MAX. See Examples section.


(required, only one in the whole environment variable) - determines the algorithm used to distribute allocations between provided memory kinds. This parameter has to be the last parameter in MEMKIND_MEM_TIERS configuration string. Currently only STATIC_RATIO and DYNAMIC_THRESHOLD policies are valid. See the Policies section.

NOTE: The application will fail when provided environment variable string is not in the correct format.



All allocations are made in such a way that the ratio between memory tiers is constant. No threshold is used.


The ratio between memory tiers is kept with a help of a threshold between kinds which value changes in time. Minimum two tiers are required for this policy, otherwise the application will be aborted. The threshold value can change in time to keep the desired ratio between tiers, but it will not be lesser than MIN_VAL and it will not be greater than MAX_VAL. For every allocation, if its size is greater than or equal to INIT_VAL, it will come from the (N+1)th tier.

Default values for a threshold between first two tiers in the MEMKIND_MEM_TIERS environment variable are:

INIT_VAL = 1024, MIN_VAL = 513, MAX_VAL = 1536.

If there are more tiers defined, each next undefined threshold will have all parameters increased by 1024, so the next undefined threshold between the next two tiers will have:

INIT_VAL = 2048, MIN_VAL = 1537, MAX_VAL = 2560.

Threshold Parameters


(optional) - the initial value of the threshold between two adjacent Nth and (N+1)th tiers. It must be greater than or equal to MIN_VAL and less than or equal to MAX_VAL.


(optional) - the minimum value of the threshold.


(optional) - the maximum value of the threshold.

NOTE: Because setting the above parameters is optional, they will be set to default values in case they are not defined.

Dram Fallback Policy

With the usage of both DRAM and KMEM_DAX tiers, if there is not enough memory to satisfy the DRAM tier memory allocation request, the allocation will fall back to PMEM memory.

With the usage of both DRAM and FS_DAX tiers, if there is not enough memory to satisfy the DRAM tier memory allocation request, the allocation will fail.

If there is not enough memory to satisfy the FS_DAX or KMEM_DAX tier memory allocation request, the allocation will fail.


The following example will run ls with the memkind memory tiering library. Make sure that paths to both and are included in LD_LIBRARY_PATH. During the application run, 20% of the allocated memory will come from PMEM memory and 80% will come from DRAM ( FS_DAX:DRAM ratio is 1:4): MEMKIND_MEM_TIERS="KIND:FS_DAX,PATH:/mnt/pmem0,PMEM_SIZE_LIMIT:10G,RATIO:1;KIND:DRAM,RATIO:4;POLICY:STATIC_RATIO" /bin/ls -l

The example value of MEMKIND_MEM_TIERS environment variable where all allocations will come from PMEM memory with filesystem created with the path /mnt/pmem0 (PMEM file size is limited only by the specified filesystem): MEMKIND_MEM_TIERS="KIND:FS_DAX,PATH:/mnt/pmem0,RATIO:1;POLICY:STATIC_RATIO"

The example value of MEMKIND_MEM_THRESHOLDS environment variable. With INIT_VAL=64, on the application start all allocations lower than 64 bytes threshold will come from DRAM and equal to or greater than this value will come from PMEM memory NUMA nodes. The threshold value changes during the runtime in order to maintain the ratio. MIN_VAL=1 and MAX_VAL=10000 set the lower and upper limits of the threshold value. Note that the DYNAMIC_THRESHOLD policy has to be set in MEMKIND_MEM_TIERS environment variable: MEMKIND_MEM_TIERS="KIND:DRAM,RATIO:1;KIND:KMEM_DAX,RATIO:4;POLICY:DYNAMIC_THRESHOLD" MEMKIND_MEM_THRESHOLDS="INIT_VAL=64,MIN_VAL=1,MAX_VAL=10000"


libmemtier works for applications that do not statically link a malloc implementation.

See Also

memkind(3), malloc(3)


2021-03-01 Intel Corporation MEMTIER