[DRILL-5593] Modernize Drill's memory allocator to reflect current usage

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.10.0
• Assignee: Unassigned
• Reporter: Paul Rogers
• Created: 18/Jun/17 19:11
• Updated: 18/Jun/17 19:12

Description

Drill's memory allocator is quite sophisticated. But, as Drill moves toward improved resource management, the design of the current allocator no longer aligns well with the overall resource management design.

The current allocator:

• Provides a separate allocator and accountant for each operator.
• Enforces a hard memory limit for each operator, causing an OOM error when the operator exceeds the per-operator limit.
• Provides a complex transfer mechanism that moves memory ownership from one operator to another as batches move downstream.
• Allows a buffer to be shared by multiple allocators, with one allocator being the "owning" allocator.
• Allows a memory block to be shared by multiple buffers (as occurs when deserializing a record batch from the wire.)
• Provides a tree of allocators in which child allocators can ask parents for more memory and parents provide that memory out of their own allocation.

The current design appears to have been an attempt to allow operators to negotiate among themselves for memory usage. The idea seems to be that any given operator uses its assigned memory. If it needs more, it asks the parent allocator for more. If the parent can't provide more, the child operator sends a OUT_OF_MEMORY signal downstream and some downstream operator must give up some of its memory (perhaps by spilling) so that the upstream operator can proceed.

The challenge is that only the framework was implemented, not the intended negotiation mechanisms. As a result, the current allocator presents challenges:

• Drill is moving toward a planned memory allocation system: the planner assigns memory limits to each fragment (for the in-flight batch overhead) and to each buffering operator.
• Memory is then managed at the fragment level, and per-opeartor, but only for buffering operators.
• Memory for other operators (scan, select, project, etc.) is completely determined by batch size, th operators have no way to deal with OOM conditions.
• The OUT_OF_MEMORY iterator status never worked. (It is hard to imagine how, say, a scan operator would run out of memory on column d within (a, b, c, d, e, f), remember its state, hold onto the d value, send the signal downstream, then resume where it left off. The code would become even more complex than it already is.
• Code now must rediscover the memory used by each batch just to ensure that it never exceeds the per-operator memory limits. The sort, in particular is infamous for OOM on SV2 allocation because a batch is so large that it fills up the allocator, causing the next allocation (the SV2) to fail – but only for accounting reasons.

One very important part of the current allocator to be retained is the "fresh" (one buffer per vector) and deserialized (shared buffer for all vectors) modes. Also, the ability for a single deserialized buffer to be shared by multiple fragments.

As a result, this is a complex design task, not a simple bug fix.

Issue Links

• relates to: DRILL-5522 OOM during the merge and spill process of the managed external sort