Submodule box.slab¶

The box.slab submodule provides access to slab allocator statistics. The slab allocator is the main allocator used to store tuples. This can be used to monitor the total memory usage and memory fragmentation.

box.runtime.info()¶

Show a memory usage report (in bytes) for the Lua runtime.

Return:	lua is the heap size of the Lua garbage collector; maxalloc is the maximal memory quota that can be allocated for Lua; used is the current memory size used by Lua.
Rtype:	table

Example:

tarantool> box.runtime.info()
---
- lua: 913710
  maxalloc: 4398046510080
  used: 12582912
...
tarantool> box.runtime.info().used
---
- used: 12582912
...

box.slab.info()¶

Show an aggregated memory usage report (in bytes) for the slab allocator.

This report is useful for assessing out-of-memory risks: the risks are high if both arena_used_ratio and quota_used_ratio are high (90-95%).

If quota_used_ratio is low, then high arena_used_ratio and/or items_used_ratio indicate that the memory fragmentation is low (i.e. the memory is used efficiently).

If quota_used_ratio is high (approaching 100%), then low arena_used_ratio (50-60%) indicates that the memory is heavily fragmentized. Most probably, there is no immediate out-of-memory risk in this case, but generally this is an issue to consider. For example, probable risks are that the entire memory quota is used for tuples, and there is are no slabs left for a piece of an index. Or that all slabs are allocated for storing tuples, but in fact all the slabs are half-empty.

Return:

items_used_ratio = items_used / slab_count * slab_size (these are slabs used only for tuples, no indexes); quota_size is the maximum amount of memory that the slab allocator can use for both tuples and indexes (as configured in slab_alloc_arena parameter, e.g. the default is 1 gigabyte = 2^30 bytes = 1,073,741,824 bytes); arena_used_ratio = arena_used / arena_size; quota_used is the amount of memory that is already distributed to the slab allocator; arena_size is the total memory used for tuples and indexes together (including allocated, but currently free slabs); arena_used is the efficient memory used for storing tuples and indexes together (omitting allocated, but currently free slabs).

Rtype:

table

Example:

tarantool> box.slab.info()
---
- items_used_ratio: 1.8%
  quota_size: 1073741824
  items_used: 4208
  quota_used: 8388608
  arena_size: 2325176
  arena_used: 1003632
...

tarantool> box.slab.info().arena_used
---
- 1003632
...

box.slab.stats()¶

Show a detailed memory usage report (in bytes) for the slab allocator. The report is broken down into groups by data item size as well as by slab size (64-byte, 136-byte, etc). The report includes the memory allocated for storing both tuples and indexes.

return:	mem_free is the allocated, but currently unused memory; mem_used is the memory used for storing data items (tuples and indexes); item_count is the number of stored items; item_size is the size of each data item; slab_count is the number of slabs allocated; slab_size is the size of each allocated slab.
rtype:	table

Example:

Here is a sample report for the first group:

tarantool> box.slab.stats()[1]
---
- mem_free: 16232
  mem_used: 48
  item_count: 2
  item_size: 24
  slab_count: 1
  slab_size: 16384
...

This report is saying that there are 2 data items (item_count = 2) stored in one (slab_count = 1) 24-byte slab (item_size = 24), so mem_used = 2 * 24 = 48 bytes. Also, slab_size is 16384 bytes, of which 16384 - 48 = 16232 bytes are free (mem_free).

A complete report would show memory usage statistics for all groups:

tarantool> box.slab.stats()
---
- - mem_free: 16232
    mem_used: 48
    item_count: 2
    item_size: 24
    slab_count: 1
    slab_size: 16384
  - mem_free: 15720
    mem_used: 560
    item_count: 14
    item_size: 40
    slab_count: 1
    slab_size: 16384
  <...>
  - mem_free: 32472
    mem_used: 192
    item_count: 1
    item_size: 192
    slab_count: 1
    slab_size: 32768
  - mem_free: 1097624
    mem_used: 999424
    item_count: 61
    item_size: 16384
    slab_count: 1
    slab_size: 2097152
  ...

The total mem_used for all groups in this report equals arena_used in box.slab.info() report.