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.
Below is a list of all
|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|
Show a memory usage report (in bytes) for the Lua runtime.
luais the heap size of the Lua garbage collector;
maxallocis the maximal memory quota that can be allocated for Lua;
usedis the current memory size used by Lua.
tarantool> box.runtime.info() --- - lua: 913710 maxalloc: 4398046510080 used: 12582912 ... tarantool> box.runtime.info().used --- - used: 12582912 ...
Show an aggregated memory usage report (in bytes) for the slab allocator. This report is useful for assessing out-of-memory risks.
box.slab.infogives a few ratios:
Here are two possible cases for monitoring memtx memory usage:
Case 1: 0.5 <
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.
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 (
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.
items_sizeis the total amount of memory (including allocated, but currently free slabs) used only for tuples, no indexes;
slab_size(these are slabs used only for tuples, no indexes);
quota_sizeis 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);
items_usedis the efficient amount of memory (omitting allocated, but currently free slabs) used only for tuples, no indexes;
quota_usedis the amount of memory that is already distributed to the slab allocator;
arena_sizeis the total memory used for tuples and indexes together (including allocated, but currently free slabs);
arena_usedis the efficient memory used for storing tuples and indexes together (omitting allocated, but currently free slabs).
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 ...
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.
mem_freeis the allocated, but currently unused memory;
mem_usedis the memory used for storing data items (tuples and indexes);
item_countis the number of stored items;
item_sizeis the size of each data item;
slab_countis the number of slabs allocated;
slab_sizeis the size of each allocated slab.
Here is a sample report for the first group:
tarantool> box.slab.stats() --- - 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_sizeis 16384 bytes, of which 16384 - 48 = 16232 bytes are 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 ...
mem_usedfor all groups in this report equals
arena_usedin box.slab.info() report.