[DRILL-2957] Netty Memory Manager doesn't move empty chunks between lists

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Not A Problem
• Fix Version/s: 1.0.0
• Component/s: Execution - Flow
• Assignee: Jacques Nadeau
• Reporter: Jacques Nadeau
• Created: 05/May/15 14:43
• Updated: 11/May/15 21:28
• Resolved: 11/May/15 20:44

Description

I'm seeing a pattern in the memory allocator and I need you to take a look at it. Here are the basic concepts:

1) We use an extension of PooledByteBufAllocator [1] called PooledByteBufAllocatorL.
2) We use many Direct Arenas (generally one per core)
3) Each arena has chunk lists for different occupancies (chunks that are empty, chunks 25% full, chunks 50% full, etc) [2]
4) Each of these chunk lists maintains a list of chunks. The chunks move from list to list as they get more or less full.
5) When no memory is being used, chunks move back to the empty list.
6) If there are excessive empty chunks, they are released back to the OS. (I don't remember the exact trigger here and I'm only seeing this sometimes right now.)

We're running on Netty 4.0.27.

What I'm seeing is that we don't seem to be moving the chunks back to the empty list as they are vacated. You can see an example output from my memory logging [3] that is enabled by [4]. I haven't replicated this at small scale but at large scale I see it consistently (30 node cluster, large group by query [5]).

I want to understand this behavior better, determine if it is a bug or not and determine whether or not this hurts memory for subsequent queries.

One other note, Netty will cache small amounts of memory that is allocated and released on the same thread for that thread. I don't believe this is a large amount of memory but be aware of it. It should be possible to control this using these settings [6].

[1] https://github.com/netty/netty/blob/master/buffer/src/main/java/io/netty/buffer/PooledByteBufAllocator.java
[2] https://github.com/netty/netty/blob/master/buffer/src/main/java/io/netty/buffer/PoolArena.java#L67
[3] Memory log output at idle after large query (one example arena out of 32 on perf cluster, see logs on those nodes for more info):
::snip::
Chunk(s) at 0~25%:
none
Chunk(s) at 0~50%:
Chunk(62194b16: 0%, 0/16777216)
Chunk(35983868: 1%, 8192/16777216)
Chunk(5bbfb16a: 1%, 163840/16777216)
Chunk(1c6d277e: 1%, 8192/16777216)
Chunk(2897b6bf: 2%, 204800/16777216)
Chunk(287d5c71: 0%, 0/16777216)
Chunk(s) at 25~75%:
Chunk(61bad0ee: 0%, 0/16777216)
Chunk(s) at 50~100%:
Chunk(2d79a032: 0%, 0/16777216)
Chunk(42415f4e: 0%, 0/16777216)
Chunk(33a3bade: 0%, 0/16777216)
Chunk(1ce7ca63: 0%, 0/16777216)
Chunk(531e1888: 0%, 0/16777216)
Chunk(54786a09: 0%, 0/16777216)
Chunk(5cdcb359: 0%, 0/16777216)
Chunk(3e40137b: 0%, 0/16777216)
Chunk(534f0fb3: 0%, 0/16777216)
Chunk(6301ee8a: 0%, 0/16777216)
Chunk(6a90c3aa: 0%, 0/16777216)
Chunk(s) at 75~100%:
none
Chunk(s) at 100%:
none
::snip::

[4] Enable the memory logger by enabling trace level debugging for the "drill.allocator" logger like this:
<logger name="drill.allocator" additivity="false">
<level value="trace" />
<appender-ref ref="FILE" />
<appender-ref ref="SOCKET" />
</logger>

[5] On perf cluster

1. sqllineTPCDS

ALTER SESSION SET `exec.errors.verbose` = true;
ALTER SESSION SET `planner.enable_multiphase_agg` = false;
ALTER SESSION SET `store.parquet.block-size` = 134217728;
ALTER SESSION SET `planner.enable_mux_exchange` = false;
ALTER SESSION SET `exec.min_hash_table_size` = 67108864;
ALTER SESSION SET `planner.enable_hashagg` = true;
ALTER SESSION SET `planner.memory.max_query_memory_per_node` = 29205777612;
ALTER SESSION SET `planner.width.max_per_node` = 23;

create table dfs.tmp.agg33 as
select ss_sold_date_sk , ss_sold_time_sk , ss_item_sk , ss_customer_sk , ss_cdemo_sk, count from `store_sales_dri30000`
group by ss_sold_date_sk , ss_sold_time_sk , ss_item_sk , ss_customer_sk , ss_cdemo_sk;

[6] https://github.com/netty/netty/blob/master/buffer/src/main/java/io/netty/buffer/PooledByteBufAllocator.java#L98