SAP Hana - Memory

About

When SAP HANA starts up, the OS reserves memory for

• the program code (sometimes called the text),
• the program stack,
• and static data.

It then dynamically reserves additional data memory upon requests from the SAP HANA memory manager.

• SAP Hana - Used memory
• SAP Hana - Memory Pool (global_allocation_limit). The licensed amount of memory.
• Virtual Memory - Virtual memory

It is theoretically possible that the memory manager will still face a need for more memory that it cannot satisfy. For instance, when too many concurrent transactions use up all memory, or when a particularly complex query performs a cross-join on very large tables, creating a huge intermediate result that exceed the available memory. Such situations can even lead to an Out Of Memory failure.

On most SAP HANA hosts, physical memory ranges from 256 Gigabytes (GB) to multiple Terabytes (TB).

Articles Related

Indicators

It is important to understand the behavior of used memory over time and under peak loads. Most important indicators are used memory and peak used memory

Over time / Memory relationship

Since SAP HANA contains its own memory manager and memory pool, external indicators such as:

• the size of resident memory at host level
• and the size of virtual and resident memory at process level

can be misleading when you are estimating the real memory requirements of an SAP HANA deployment.

Once a temporary computation completes or a table is dropped, the freed memory is returned to the memory manager, who recycles it to its pool, usually without informing the OS. Thus, from SAP HANA’s perspective, the amount of Used Memory shrinks, but the process’ virtual and resident sizes are not affected.

Monitoring

See:

• Service Monitoring

select 
	  service_name as "service_name"
	, round(logical_memory_size/1024/1024/1024,2) as "logical_memory_size Gb"
	, round(physical_memory_size/1024/1024/1024,2) as "physical_memory_size Gb"
	, round(code_size/1024/1024/1024,2) as "code_size Gb"
	, round(stack_size/1024/1024/1024,2) as "stack_size Gb"
	, round(heap_memory_allocated_size/1024/1024/1024,2) as "heap_memory_allocated_size Gb"
	, round(heap_memory_used_size/1024/1024/1024,2) as "heap_memory_used_size Gb"
	, round(shared_memory_allocated_size/1024/1024/1024,2) as "shared_memory_allocated_size Gb"
	, round(shared_memory_used_size/1024/1024/1024,2) as "shared_memory_used_size Gb"
	, round(compactors_allocated_size/1024/1024/1024,2) as "compactors_allocated_size Gb"
	, round(compactors_freeable_size/1024/1024/1024,2) as "compactors_freeable_size Gb"
	, round(allocation_limit/1024/1024/1024,2) as "allocation_limit Gb"
	, round(effective_allocation_limit/1024/1024/1024,2) as "effective_allocation_limit Gb"
	, round(total_memory_used_size/1024/1024/1024,2) as "total_memory_used_size Gb"
from M_SERVICE_MEMORY

Output example:

Documentation / Reference

• Monitoring Memory Usage
• SAP Note 1704499
• SAP Hana Memory Explained