Resource management is a feature within Kinetica designed to meet three goals:
- Expand the operating capacity of a cluster by backing the in-memory database with several layers of disk-based storage
- Allow the prioritization of data within those layers
- Allow the prioritization of user requests
Three components are involved in managing resources:
- Storage Tiers: Data containment layers within the database
- Tier Strategies: Data object eviction priorities within each storage tier
- Resource Groups: Resource fencing--process scheduling priorities and other limits imposed on a specific group of users
Within these components, there are two mechanisms for managing resources:
- Eviction: Moving data from one tier to a lower tier in order to make room for other data to be processed
- Scheduling: Ordering work to be processed
Kinetica in Azure automatically manages resources for optimal performance.
A tier is a container in which data can be stored, either on a temporary or permanent basis. There are five basic types of tier:
Each tier is defined by an overall capacity and an expected level of performance that is generally inversely proportional to that capacity. For example, the VRAM Tier will usually have the smallest total memory, but be the most performant for processing data, while the Cold Storage Tier would be the largest tier, but the least performant.
The VRAM Tier is composed of GPU RAM on each host. It is generally the smallest & fastest tier. Any operations that make use of the GPU require the data on which they are operating to be located in the VRAM Tier.
There is always one VRAM Tier in CUDA-enabled installations. It may comprise the VRAM in multiple GPUs per host, and due to restrictions on cross-GPU VRAM access, data stored in the tier may be temporarily duplicated across GPUs as necessary for processing by GPU-based kernels.
For non-CUDA installations, there is no VRAM Tier, and any configuration related to the VRAM Tier is ignored.
The RAM Tier is composed of the system memory on each host. It is generally larger & slower than the VRAM Tier. Any operation not serviced by the GPUs will be processed by the CPUs, and the corresponding data must be loaded in the RAM Tier.
There is always one RAM Tier. It comprises all of the RAM on each host that has been made available for data storage. The RAM Tier is not used for small, non-data objects or variables that are allocated and deallocated for program flow control or used to store metadata or other similar information. These objects continue to use either the stack or the regular runtime memory allocator. The RAM Tier should be sized on each host such that there is sufficient RAM left over to handle these objects, as well as the needs of other processes running on the same host.
RAM on a given host may be divided between NUMA nodes; in general, Kinetica will attempt to access data stored in the RAM Tier from threads that prefer execution on the same NUMA node, but there is no hard restriction on cross-node access as there is with cross-GPU access to VRAM, so data stored in the RAM Tier will not be duplicated between NUMA nodes.
The Disk Cache Tier is useful as a temporary swap space for data that doesn't fit in RAM or VRAM.
It is also useful as the only disk-based tier that can hold filtered views, join views, & memory-only tables. These objects are often created by the system in servicing user requests for data, but can also be created by users directly. With a supporting Disk Cache Tier, these tables can be cached to disk when RAM is at capacity and reloaded into RAM when needed in a subsequent operation.
It is recommended that the disk be faster than the one backing the Persist Tier, since this tier is used as an intermediary cache between the RAM Tier and Persist Tier. However, given that the Disk Cache Tier, unlike the Persist Tier, can hold non-persistable data objects, it is recommended to make use of one irrespective of relative performance.
If the disk backing the Disk Cache Tier is not faster than the disk backing the Persist Tier, the caching of persistent objects can be disabled to improve performance. Since an object loaded from the Persist Tier into the RAM Tier will already exist in the Persist Tier in its original form, it will not need to be written to the Disk Cache Tier or re-written to the Persist Tier when evicted from the RAM Tier. Given disks of equivalent performance backing these two tiers, there would be no benefit in reading/writing persistent objects from/to the Disk Cache; thus, the performance benefit in disabling the caching of these objects.
Multiple Disk Cache Tier instances can be defined on different disks with different capacities and performance parameters, though no instances are required.
The Persist Tier is a single tier that contains data that survives between database restarts. Although, like the Disk Cache Tier, it is disk-based, its behavior is different: data for persistent objects is always present in the Persist Tier (or off-loaded to the Cold Storage Tier, if configured), and will be synchronized periodically with copies of persisted objects in the RAM Tier as updates are made to the data.
The Persist Tier is the primary permanent storage tier, and should be sized appropriately to contain all operational data being processed by the database.
In general, the size limit on the Persist Tier should only be set if one or more Cold Storage Tiers are configured. Without a supporting Cold Storage Tier to evict objects in the Persist Tier to, operations requiring space in the Persist Tier will fail when the limit is reached.
Cold Storage Tiers can be used to extend the storage capacity of the Persist Tier for infrequently accessed data and for spill-over when the Persist Tier is at capacity.
The Cold Storage Tier is typically a much larger capacity physical disk or a cloud-based storage system which may not be as performant as the Persist Tier storage.
Eviction is the forced movement of data from a higher tier to a lower tier in order to make room for other data to be moved into that higher tier.
Each object in the system has a level of evictability that is dependent on the type of object it is and the available tiers below it into which it could be evicted.
Tables can be evicted from any tier to the tier below.
Any object that exists at its lowest possible tier will be considered unevictable by the system, regardless of eviction priority.
Tier strategy is also considered when determining the lowest possible tier. For example, a memory-only table with a tier strategy that contains no Disk Cache Tier will have the RAM Tier as its lowest possible tier and be unevictable from it.
For each user request, some amount of memory, potentially in multiple tiers, will be required to fulfill it. The system may need to load source data into memory, allocate memory for processing the request, and allot memory for holding the response data. During the life of the request, the system will continually assess memory/disk needs at each tier level to complete processing successfully.
If, at some point, the system determines that not enough space within a given tier exists to meet the needs of the request being processed, request-based eviction will commence. The system will make an assessment of all the evictable objects in that tier, based on their eviction priority. If enough objects can be evicted to provide the capacity for the current operation to succeed, they will be removed. If not, the request will fail.
Since a request is handled in smaller chunks and may have many associated operations that need to be performed, the request-based eviction process may be invoked several times for a request received when the system is near capacity usage in one or more tiers.
As request-based eviction can result in a great deal of data movement and processing overhead for a user request, watermark-based eviction can be employed to make room for new user requests before they arrive and improve the overall response time for users.
Watermark-based eviction is a feature designed to preemptively make space for new requests when a system is near capacity in a given tier. The goal is to keep enough space in each tier to avoid having a request wait for space to be cleared via request-based eviction. Watermark-based eviction occurs in the background, independent of user requests.
High & low watermarks are used to govern when watermark-based evictions take place. When the high watermark usage percentage is crossed within a tier, the system will begin evicting objects from that tier based on their eviction priority. The system will continue to evict objects until the low watermark usage percentage is crossed.
Watermark-based eviction can be disabled for any tier or for any of the configured Disk Cache Tier instances.
As storage tiers become saturated with data objects, some objects will need to be evicted from the tier in which they currently reside to a lower-level tier capable of holding them, to allow other data objects take their place.
An eviction priority is the priority level of a data object within a given tier that determines the likelihood of that object's eviction relative to other data objects in the same tier. Objects with the same eviction priority will be considered in least recently used fashion for eviction; e.g., given two objects of eviction priority 5, the one that was used more recently will remain, while the one that was used less recently will be evicted.
The priority levels range from 1 to 10, where:
- 1: the object will be among the first to be evicted
- 9: the object will be among the last to be evicted
- 10: the object will not be able to be evicted
An eviction priority of 1 within the VRAM Tier will cause a data object to be evicted immediately from VRAM after use. This can have a performance benefit, as most objects processed on the GPU will not be immediately reprocessed and make use of the cached object. It is often more performant to keep the VRAM Tier free for new data to process than to keep already-processed data in the tier to be evicted subsequently, when needed.
The eviction policy between request-based eviction & watermark-based eviction differs in that the former will only evict if the target capacity can be reached, while the latter will continuously evict until the target capacity is reached.
Consider a tier with a capacity of 100GB containing four objects having the following priorities & sizes and a remaining capacity of 10GB:
Request-Based Eviction Processing
If a request for 50GB of capacity is received, the system would determine that the remaining capacity of 10% would be insufficient to fulfill the request and request-based eviction would be triggered.
The object with eviction priority 1 (A) will be considered first for eviction. If removed, a total capacity of 40GB would be free. Since that doesn't meet the required 50GB, the least recently used object of eviction priority 4 would be evicted. In this case, the removal of either B or C would result in enough capacity for the request, but whichever was used less recently would be added to the eviction set. At that point, the system would have an eviction set of the necessary size, so all objects in the set would be evicted and the request would proceed.
If, instead, a request for 80GB of capacity is received, the accumulation of evictable objects into an eviction set would take place in the same way. However, once A, B, & C were added to the eviction set, there would only be 70GB free. At that point, since the only remaining object, D, has an eviction priority of 10 (and is therefore unevictable), the system would be unable to find enough space to handle the request. The request would be rejected immediately, and no evictions would take place.
Watermark-Based Eviction Processing
Consider the same scenario with objects A, B, C, & D totaling 90GB of the 100GB available, and with configured high & low watermarks of 95% and 50%, respectively.
If a request for 7GB of capacity is received with an eviction priority of 9, it would be processed successfully and leave the total usage at 97% of capacity:
Since the 97% usage would exceed the 95% high watermark, watermark-based eviction would be triggered, and evictions would commence immediately until the 50% low watermark is reached.
In similar fashion to request-based eviction, the object with eviction priority 1 (A) will be considered first. However, in this scheme, it would be evicted immediately, lowering the total usage to 67%. Next, the less recently used object of the two with eviction priority 4 would be considered.
If object B were less recently used than C, it would be evicted and lower usage to 57%. Since the low watermark would still not have been crossed, object C would also be evicted and lower total usage to 37%. At this point, watermark-based eviction would cease.
If object C were less recently used of the two, it would be evicted and lower usage to 47%. The low watermark would have been crossed, and watermark-based eviction would cease.
Data is loaded from each tier below the RAM Tier directly into the RAM Tier as the need arises in servicing user requests. For operations requiring GPU processing, data is moved from the RAM Tier into the VRAM Tier.
Regardless of tier, eviction will always cause an evictable object to be removed from its present tier or to be moved to a lower tier. However, whether the object is moved to a lower tier or simply removed from the existing one is dependent on the tier configuration as well as some additional circumstances outlined below:
- VRAM Tier: objects with eviction priority 1 are removed from the tier immediately after use; objects with eviction priorities greater than 1 are removed as they are evicted
- RAM Tier: by default, only
persistable objects are evictable from the RAM Tier and are removed when
evicted, since they will already exist in the Persist Tier; if a
Disk Cache Tier is configured and is part of an object's
tier strategy, the movement rules are
amended as follows, based on the object's ability to be persisted:
- Persistable objects (tables, result tables marked as persistent, etc.) are moved to the Disk Cache Tier, if caching of persistable objects in the Disk Cache Tier is enabled
- Non-persistable objects (joins, projections, memory-only tables, etc.) are moved to the Disk Cache Tier
- Disk Cache Tier: only persistable objects are evictable from the Disk Cache Tier; they are removed when evicted
- Persist Tier: if a Cold Storage Tier is configured, objects will be moved to it
- Cold Storage Tier: data objects are not evictable from the Cold Storage Tier
Any time an object expires (see TTL) or is deleted, all associated data objects in all tiers will be deleted.
When the database is started, table data are loaded from the Persist Tier into the RAM Tier. Table data with higher eviction priorities in the RAM Tier are loaded before those with lower eviction priorities.
Loading continues until the midpoint (average) of the high & low watermarks of the RAM Tier is reached. Any high & low watermarks defined for individual ranks within the RAM Tier will be used for data contained within those ranks. If watermark-based eviction is disabled, loading will continue until the capacity of the RAM Tier is reached.
A tier strategy is the association of data object priorities to a set of tiers of distinct types. Any data object assigned to a given tier strategy will take on the priority level defined by the tier strategy for the tier in which it resides.
The strategy is formed by chaining together the tier types and their respective eviction priorities. Any given tier may appear no more than once in the chain. This means that for a system with multiple Disk Cache Tier instances and/or Cold Storage Tier instances, only one of each may appear in a given tier strategy.
For instance, a table can be assigned the following tier strategy, given an installation with a Disk Cache Tier instance DISK2 and a Cold Storage Tier instance COLD5:
This results in the following strategy for tiering:
|VRAM||1||Lowest priority within VRAM; automatically evicted after use|
|RAM||5||Average priority within RAM; evicted when RAM size limit is exceeded and no objects with priority 1 - 4 exist to be evicted first|
|DISK2||3||Below-average priority within Disk Cache Tier DISK2; no effect in relation to other Disk Cache Tier instances|
|PERSIST||9||Highest evictable priority within persistent storage; among the last to be evicted to Cold Storage Tier COLD5|
|COLD5||Uses COLD5 as final backing tier; no priority needs to be set, as no eviction from this tier can take place|
Each table created by the user can have a tier strategy applied to it, which, in turn, will apply that tier strategy to all of its columns. A user can override that table-level tier strategy for any of the table's columns, independently of each other.
A default tier strategy is applied to all tables and columns not explicitly assigned a strategy. It is also applied to any other data objects created by the user or on behalf of a user's request. For instance filters, joins, & projections (among others) will all have this default tier strategy. For Kinetica in Azure, this strategy is:
Tier Strategy Predicates
A tier strategy predicate offers conditional tiering for a table and for its columns, individually, if needed. It is an expression, associated with a tier strategy, that when evaluated to true for a given set of data, will apply the associated tier strategy to the matching data set.
Several tier strategy predicates and their associated tier strategies can be chained together to form a tier strategy clause, which will be evaluated in sequence to determine the appropriate tier strategy to apply. The last tier strategy in the clause can have the tier strategy predicate omitted in order to make it the default if no other expressions are matched. If no default or tier strategy clause is specified for a column, the table's tier strategy is used. If no default is specified for a table, the default tier strategy is used.
For instance, consider the following tier strategy, applied to a table with a timestamp column named last_seen, a positive integer column named id, and a string column named name:
The overall goal of this setup would be to elevate the VRAM & RAM priorities of record data that has been recently updated, while providing a disk cache for the remaining data. The id & name column data for records with updates in the last minute will have their eviction priorities elevated to allow caching in the VRAM Tier and to prevent eviction entirely from the RAM Tier.
Specifically, the following tier strategies would be applied:
|Last Seen||Column||Tier Strategy|
|< 1 minute ago||id||VRAM 2, RAM 10|
|name||VRAM 2, RAM 10|
|<other columns>||VRAM 1, RAM 9|
|> 1 minute ago but < 3 days ago||id||VRAM 1, RAM 9|
|name||VRAM 1, RAM 9|
|<other columns>||VRAM 1, RAM 9|
|> 3 days ago||id||VRAM 1, RAM 8, DISK2 7|
|name||VRAM 1, RAM 8, DISK2 7|
|<other columns>||VRAM 1, RAM 8, DISK2 7|
Resource groups are used to enforce simultaneous memory, thread, & priority limits for one or more users. A user can have a resource group assigned directly, or can be assigned a role that has been assigned a resource group.
The configurable limits for users within each resource group are:
- VRAM Tier maximum memory consumption
- RAM Tier maximum memory consumption
- Maximum CPU concurrency that can be requested for the execution of jobs
- Maximum scheduling priority that can be requested for the execution of jobs
- Maximum eviction priority that can be assigned to objects
Ranking and Effective Resource Groups
Since a user may have multiple roles and each role may have an associated resource group, a user may ultimately be assigned multiple resource groups. In this case, an effective resource group is determined by the system, depending on whether the group is assigned directly or via role, and by the relative ranking of the indirectly-assigned resource groups.
The ranking of a resource group is assigned during its creation and can be altered at any time. The ranking applies system-wide; i.e., there is no per-user ranking of resource groups. The resource group with the highest ranking will have a rank of 1.
The following method is used to determine a user's effective resource group:
- If a user has a direct resource group assignment, this will be the effective resource group
- If a user has no direct resource group assignment, but does have one or more roles with associated resource groups, the relative rankings of these resource groups will be assessed, and the one with the highest ranking will be the effective resource group
- If a user has no direct or indirect (via role) resource groups assigned, a default resource group will be assigned as the effective resource group
A user with more than one resource group assigned cannot choose which resource group to use, nor will the user be given a composite of the assigned resource groups; e.g., the maximum value of all resource limits across the user's assigned groups.
Each user request is broken down into one or more component tasks, which the database must complete to return an appropriate response. These tasks are scheduled by the system for execution in the order they were received, but may be executed out-of-order, depending on resource availability and each task's scheduling priority.
A scheduling priority is assigned to each user request and is used by the system to determine the order in which it services those requests. The higher the priority, the sooner the request will be serviced, relative to other requests.
The scheduling priority of a request is dictated by the requesting user's resource group. Once a request is made, the scheduling priority of the user's resource group is assigned to it. As the request is broken down into tasks over the course of its processing, scheduling priorities will be assigned to those tasks, as they are created. Because of this delayed assignment of priority, any change to the scheduling priority of a user's resource group may impact the effective scheduling priority of the user's currently running requests as well as those of that user's future requests.
There is no guarantee of ordering of requests being serviced for requests of the same scheduling priority.