buffer_manager_cached_memory

SingleStore keeps unused Buffer Manager memory mapped instead of returning every freed buffer to the operating system so that later allocations can use it. Buffer_manager_cached_memory tracks this cached memory.

While Total_server_memory is at or below 90% of maximum_memory or at least 10 GB of free memory remains, Buffer_manager_cached_memory is capped at 25% of maximum_memory on each node. When the cache size reaches the capped limit, additional freed memory blocks (or buffers) are returned to the operating system. Buffers that are already cached are still mapped and left in place, therefore crossing the capped limit does not trim the cache. The cache shrinks when a subsequent allocation uses the cached buffer, and it can get unmapped if another allocation may otherwise exceed maximum_memory.

When Total_server_memory is above 90% of maximum_memory and less than 10 GB of free memory remains, newly freed buffers are cached only until the cache size reaches approximately 1% of maximum_memory (vs. the default 25%). Existing cached buffers are not trimmed when this limit applies.

As cached memory stays mapped, Total_server_memory includes Buffer_manager_cached_memory. After a large rowstore DELETE, Alloc_table_memory can fall while Total_server_memory stays high because the freed memory blocks are still held as unrecycled or cached memory.

The cache is not released on a fixed schedule. Run TRIGGER GC FLUSH to explicitly release unused Buffer Manager cached memory.

View the size of cached memory reserved for each node in the output of the SHOW STATUS EXTENDED command. For example:

SHOW STATUS EXTENDED LIKE 'buffer_manager_cached%';

+------------------------------+---------+
| Variable_name                |  Value  |
+------------------------------+---------+
| Buffer_manager_cached_memory | 80.0 MB |
+------------------------------+---------+

Refer to Identifying and Reducing Memory Usage for more information on Buffer_manager_cached_memory and Total_server_memory.