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Submodule box.slab | Tarantool
 

Submodule box.slab

Submodule box.slab

Overview

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.

Index

Below is a list of all box.slab functions.

Name Use
box.runtime.info() Show a memory usage report for Lua runtime
box.slab.info() Show an aggregated memory usage report for slab allocator
box.slab.stats() Show a detailed memory usage report for slab allocator
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.

box.slab.info gives a few ratios:

  • items_used_ratio
  • arena_used_ratio
  • quota_used_ratio

Here are two possible cases for monitoring memtx memory usage:

Case 1: 0.5 < items_used_ratio < 0.9

../../../_images/items_used_ratio1.svg

Apparently your memory is highly fragmented. Check how many slab classes you have by looking at box.slab.stats() and counting the number of different classes. If there are many slab classes (more than a few dozens), you may run out of memory even though memory utilization is not high. While each slab may have few items used, whenever a tuple of a size different from any existing slab class size is allocated, Tarantool may need to get a new slab from the slab arena, and since the arena has few empty slabs left, it will attempt to increase its quota usage, which, in turn, may end up with an out-of-memory error due to the low remaining quota.

Case 2: items_used_ratio > 0.9

../../../_images/items_used_ratio2.svg

You are running out of memory. All memory utilization indicators are high. Your memory is not fragmented, but there are few reserves left on each slab allocator level. You should consider increasing Tarantool’s memory limit (box.cfg.memtx_memory).

To sum up: your main out-of-memory indicator is quota_used_ratio. However, there are lots of perfectly stable setups with a high quota_used_ratio, so you only need to pay attention to it when both arena and item used ratio are also high.

Return:
  • items_size is the total amount of memory (including allocated, but currently free slabs) used only for tuples, no indexes;
  • items_used_ratio = items_used / items_size, where items_size = 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 the memtx_memory parameter, the default is 2^28 bytes = 268,435,456 bytes);
  • quota_used_ratio = quota_used / quota_size;
  • arena_used_ratio = arena_used / arena_size;
  • items_used is the efficient amount of memory (omitting allocated, but currently free slabs) used only for tuples, no indexes;
  • 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_size: 228128
  items_used_ratio: 1.8%
  quota_size: 1073741824
  quota_used_ratio: 0.8%
  arena_used_ratio: 43.2%
  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.