How Resource Limits Are Enforced

Before running a query, Vertica determines how much memory it requires to run. If the query contains a fenced-mode UDx which implements the getPerInstanceResources() function in its factory class, Vertica calls it to determine the amount of memory the UDx needs and adds this to the total required for the query. Based on these requirements, Vertica decides how to handle the query:

  • If the total amount of memory required (including the amount that the UDxs report that they need) is larger than the session's MEMORYCAP or resource pool's MAXMEMORYSIZE setting, Vertica rejects the query. For more information about resource pools, see Resource Pool Architecture in the Administrator's Guide.
  • If the amount of memory is below the limit set by the session and resource pool limits, but there is currently not enough free memory to run the query, Vertica queues it until enough resources become available.
  • If there are enough free resources to run the query, Vertica executes it.

Vertica has no other way to determine the amount of resources a UDx requires other than the values it reports using the getPerInstanceResources() function. A UDx could use more resources than it claims, which could cause performance issues for other queries that are denied resources. You can set an absolute limit on the amount of memory UDxs can allocate. See Setting Memory Limits for Fenced-Mode UDxs for more information.

If the process executing your UDx attempts to allocate more memory than the limit set by the FencedUDxMemoryLimitMB configuration parameter, it receives a bad_alloc exception. For more information about FencedUDxMemoryLimitMB, see Setting Memory Limits for Fenced-Mode UDxs.

Below is the output of loading a UDSF that consumes 500MB of memory, then changing the memory settings to cause out-of-memory errors. The MemoryAllocationExample UDSF in the following example is just the Add2Ints UDSF example altered as shown in Allocating Resources for UDxs and Informing Vertica of Resource Requirements to allocate 500MB of RAM.

=> CREATE LIBRARY mylib AS '/home/dbadmin/MemoryAllocationExample.so';
CREATE LIBRARY
=> CREATE FUNCTION usemem AS NAME 'MemoryAllocationExampleFactory' LIBRARY mylib
-> FENCED;
CREATE FUNCTION
=> SELECT usemem(1,2);
 usemem 
--------
      3
(1 row)

The following statements demonstrate setting the session's MEMORYCAP to lower than the amount of memory that the UDSF reports it uses. This causes Vertica to return an error before it executes the UDSF.

=> SET SESSION MEMORYCAP '100M';
SET
=> SELECT usemem(1,2);
ERROR 3596:  Insufficient resources to execute plan on pool sysquery 
[Request exceeds session memory cap: 520328KB > 102400KB]
=> SET SESSION MEMORYCAP = default;
SET

The resource pool can also prevent a UDx from running if it requires more memory than is available in the pool. The following statements demonstrate the effect of creating and using a resource pool that has too little memory for the UDSF to run. Similar to the session's MAXMEMORYCAP limit, the pool's MAXMEMORYSIZE setting prevents Vertica from executing the query containing the UDSF.

=> CREATE RESOURCE POOL small MEMORYSIZE '100M' MAXMEMORYSIZE '100M';
CREATE RESOURCE POOL
=> SET SESSION RESOURCE POOL small;
SET
=> CREATE TABLE ExampleTable(a int, b int);
CREATE TABLE
=> INSERT /*+direct*/ INTO ExampleTable VALUES (1,2);
 OUTPUT 
--------
      1
(1 row)
=> SELECT usemem(a, b) FROM ExampleTable;
ERROR 3596:  Insufficient resources to execute plan on pool small 
[Request Too Large:Memory(KB) Exceeded: Requested = 523136, Free = 102400 (Limit = 102400, Used = 0)]
=> DROP RESOURCE POOL small; --Dropping the pool resets the session's pool
DROP RESOURCE POOL

Finally, setting the FencedUDxMemoryLimitMB configuration parameter to lower than the UDx actually allocates results in the UDx throwing an exception. This is a different case than either of the previous two examples, since the query actually executes. The UDx's code needs to catch and handle the exception. In this example, it uses the vt_report_error macro to report the error back to Vertica and exit.

=> ALTER DATABASE mydb SET FencedUDxMemoryLimitMB = 300;
=> SELECT usemem(1,2);
	ERROR 3412:  Failure in UDx RPC call InvokeSetup(): Error calling setup() in 	
	User Defined Object [usemem] at [MemoryAllocationExample.cpp:32], error code:
 	1, message: Couldn't allocate memory :[std::bad_alloc]
=> ALTER DATABASE mydb SET FencedUDxMemoryLimitMB = -1;
=> SELECT usemem(1,2);
 usemem 
--------
      3
(1 row)

See Also