Memory trimming

Under certain workloads, glibc can accumulate a significant amount of free memory in its allocation arena.

Under certain workloads, glibc can accumulate a significant amount of free memory in its allocation arena. This memory consumes physical memory as indicated by its usage of resident set size (RSS), which glibc does not always return to the operating system. High retention of physical memory by glibc—glibc memory bloat—can adversely affect other processes, and, under high workloads, can sometimes cause Vertica to run out of memory.

Vertica provides two configuration parameters that let you control how frequently Vertica detects and consolidates much of the glibc-allocated free memory, and then returns it to the operating system:

MemoryPollerTrimThreshold: Sets the threshold for the memory poller to start checking whether to trim glibc-allocated memory.
The memory poller compares MemoryPollerTrimThreshold—by default, set to 0.83— with the following expression:
```
RSS / available-memory
```
If this expression evaluates to a value higher than MemoryPollerTrimThreshold, then the memory poller starts checking the next threshold—set in MemoryPollerMallocBloatThreshold—for glibc memory bloat.

Note
On high-memory machines where very large Vertica RSS values are atypical, consider a higher setting for MemoryPollerTrimThreshold. To turn off auto-trimming, set this parameter to 0.
MemoryPollerMallocBloatThreshold: Sets the threshold of glibc memory bloat.
The memory poller calls glibc function malloc_info() to obtain the amount of free memory in malloc. It then compares MemoryPollerMallocBloatThreshold—by default, set to 0.3—with the following expression:
```
free-memory-in-malloc / RSS
```
If this expression evaluates to a value higher than MemoryPollerMallocBloatThreshold, the memory poller calls glibc function malloc_trim(). This function reclaims free memory from malloc and returns it to the operating system. Details on calls to malloc_trim() are written to system table MEMORY_EVENTS.

For example, the memory poller calls malloc_trim() when the following conditions are true:
- MemoryPollerMallocBloatThreshold is set to 0.5.
- malloc_info() returns 15GB memory in malloc free.
- RSS is 30GB.
Note
This parameter is ignored if MemoryPollerTrimThreshold is set to 0 (disabled).

Trimming memory manually

If auto-trimming is disabled, you can manually reduce glibc-allocated memory by calling Vertica function MEMORY_TRIM. This function calls malloc_trim().