Jalex Chang

2021.07.31

Memory Management in High-Performance Go Applications:

A Case Study of Pebble

Jalex Chang

Saff Software Engineer @ Umbo Computer Vision
Gopher
Love software engineering, database systems, and distributed systems

Contact:

jalex.cpc @ gmail.com
jalex.chang @ Facebook
JalexChang @ GitHub

Agenda

Introduction
Pebble & CockroachDB
Memory Management Approaches in Pebble
Discussions
Summary

References

[1] Introduction to Pebble, https://www.cockroachlabs.com/blog/pebble-rocksdb-kv-store

[2] Source code of Pebble (v21.1), https://github.com/cockroachdb/pebble/tree/crl-release-21.1
[3] Memo of memory management in Pebble, https://github.com/cockroachdb/pebble/blob/crl-release-21.1/docs/memory.md

Introduction

In this tech talk, we are going to introduce the memory management approaches in Pebble, a key-value data store written in GO.
The topics will be covered in the talk:
- What is Pebble?
- How does Pebble manage its memory usage efficiently?
- What are the pros and cons of the approach?
- What are the use cases of the approach?
The topics will not be covered in the talk:
- Concurrency control
- The details of data formats
  - WAL (write-ahead logging) and SSTable (String Sorted Table)

Go's memory allocation & managemet

Go's memory allocation and management mechanisms are complicated.
- Such as garbage collection (GC), TCMalloc, multi-layered memory allocator, escape analysis, and etc.
Some shared topics in recent years
- Garbage collection in Go (Meetup#37)
- Escape analysis in Go (GopherCon TW 2020)

Go memory management mechanisms are powerful enough in most cases, especially for Web-based applications.

However, does it also meet the needs of high-performance applications, such as database systems?

To find out the answer, I start to dig out the Go database system - CockroachDB and its underlying data engine Pebble.

CockroachDB & Pebble

CockroachDB

CockroachDB is a distributed SQL database
- Written in Go
- NewSQL: designed for high scalability and strong consistency
- Compatible with PostgreSQL wire protocol
- Built on a key-value storage engine with LSM trees (log-structured merge-tree)
- Developed by Cockroach Labs
- Under business source license (BSL)
  - Free to use
  - Forbid offering CockroachDB as a service without buying licenses

Original architecture layers (before 2020)

RocksDB is awesome, but......

RocksDB is written in C++.
Communication between CockroackDB and RocksDB leverages CGo.
Painpoints
- CGo generates around 70ns overhead per call.
- Copying memory from C to Go has performance penalties.

Pebble

Pebble is a RocksDB inspired key-value store
- Written in GO
- Inherit RocksDB's API and data format (use LSM tree as well)
- Focus on performance and internal usage by CockroachDB
- Under BSD license
History
- May 2020: introduced as an alternative storage engine to RocksDB in CockroachDB v20.1
- Nov 2020: made the default storage engine in CockroachDB v20.2

Source: https://www.cockroachlabs.com/blog/pebble-rocksdb-kv-store

Pebble vs RocksDB

Memory Management in Pebble

Significant memory usage sources

MemTable
- In-memory LSM tree
- Used for buffering data changes that
  - have been written to the WAL
  - have not been flushed to an SSTable
- mutable but append-only
Block Cache
- An in-memory cache for uncompressed SSTable blocks
- Adopt Clock-Pro for page replacement

Memory management problems in Pebble

MemTables and Block Cache generate a lot of heap garbage in seconds
- MemTables are discarded after flushing data to the disk.
- Cached blocks are evicted by others.
The garbage leads to a huge amount of pressure on GC
- The high pressure makes Pebble perform badly in extreme cases.
- Serving unpredictable performance is unacceptable to a database system.
Developers have tried improving memory usage and GC
- Such as reusing the memory in cached blokcs and tuning GOGC
- They turn out finding tuning Go GC may solve some problems but generate others......

Manual memory management in Pebble

Move memory used in MemTables and Block Cache out of the Go heap and use C memory allocator.
Go GC ignores the C allocated memory
- Extra lifetime tracking is needed, or memory leaks may happen.

//source code: pebble/internal/manual/manual.go
//go:linkname throw runtime.throw
func throw(s string)

func New(n int) []byte {
	if n == 0 {
		return make([]byte, 0)
	}
	ptr := C.calloc(C.size_t(n), 1)
	if ptr == nil {
		throw("out of memory")
	}
	// Interpret the C pointer as a pointer
	// to a Go array, then slice.
	return (*[MaxArrayLen]byte)(unsafe.Pointer(ptr))[:n:n]
}

func Free(b []byte) {
	if cap(b) != 0 {
		if len(b) == 0 {
			b = b[:cap(b)]
		}
		ptr := unsafe.Pointer(&b[0])
		C.free(ptr)
	}
}

The journey of C allocated memory

Write data

Flush memTables

Read data

Trivial cases

Free related blocks in Block Cache when flushing MemTables
- To avoid cache collision
Free related blocks in Block Cache when compacting SSTables

Observations

MemTable
- Each MemTable has a single but huge C memory space.
- The space is size-fixed and is allocated at the very beginning.
- C memory is encapsulated in the MemTable.
Block Cache
- Block Cache contains a huge amount of small C memory spaces.
- Each space is size-fixed but is allocated on-demanded.
- C memory can be manipulated out of the Block Cache.
  - Spaces may get lost if callers have not sent them back.

Memory leak detection

To avoid the memory leak in Block Cache, runtime.SetFinalizer is used in Pebble's testing and development.
- The finalizer is a function associated with an object.
- The finalizer is run when the object is no longer reachable => run when the object is going to be GC.

// source: pebble/internal/cache/value_invariants.go
func newValue(n int) *Value {
	b := manual.New(n)
	v := &Value{buf: b}
	// Note: this is a no-op if invariants and tracing are disabled or race is
	// enabled.
	invariants.SetFinalizer(v, func(obj interface{}) {
		v := obj.(*Value)
		if v.buf != nil {
			fmt.Fprintf(os.Stderr, "%p: cache value was not freed: refs=%d\n%s",
				v, v.refs(), v.ref.traces())
			os.Exit(1)
		}
	})
	return v
}

Disccussion

Q1: What are the pros and cons of the manual memory management approach?

Pros
- No need to tune GO GC anymore
- Have better and predictable performance on applications
Cons
- Should track the lifetime of C memory manually and carefully
- Memory leaks may happen

Disccussion

Q2: What are the use cases of the manual memory management approach?

Applications having a high GC pressure.
Applications having a heavy loading on disk I/O buffering.
The the adopted data is originally represented as bytes or pointers.

Takeaways

In this tech sharing, we have introduced Pebble and its memory management approach.

To avoid high pressure on Go GC, it uses the C memory allocator to its significant memory sources: MemTable and Block Cache.
To avoid memory leaks:
- Manual lifetime tracking is needed.
- A finalizer is used during the testing and development.

Through the case study of Pebble, we have learned:

Although Go's memory management mechanisms are powerful, they still cannot meet the requirements of really high-performance applications
Manual memory management is a possible alternative though risky.

Thanks for listening.

Memory Management in High-Performance Go Applications: A Case Study of Pebble

By Jalex Chang

Memory Management in High-Performance Go Applications: A Case Study of Pebble

A case study of memory management mechanism in Pebble, a high-performance key-value storage engine written in Go.

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Jalex Chang