Cmapv2: A high performance, concurrent map
15 comments
·June 16, 2025kiitos
sapiogram
Thank you for debunking it so I didn't have to. I don't think I've ever seen someone post a low-level/concurrent data structure in Go that wasn't wildly unsound, so I assumed this was too.
diagraphic
There are many good implementations.
kiitos
Be more specific? OK
---
CopyNode broken
`CopyNode` duplicates only the parent; every child pointer is still shared
nodeCopy.setChildren(make([]*node, len(n.children)))
copy(nodeCopy.children, n.children) // pointers reused
Any later mutation (for example `setValue`) writes through those shared pointers, so readers and historical snapshots are modified concurrently -- invalid, data race, memory model violation
---
Bitmap corruption
`SetBit` uses XOR rather than “set”:
func SetBit(bitmap uint32, position int) uint32 { return bitmap ^ (1 << position) }
Calling it twice on the same index flips the bit back to 0. During branch-creation on insert and during delete, this function is invoked multiple times on the same index, clearing a bit that should remain set and leaving orphaned children.
---
Invalid assumptions re: interior pointers
Only the root pointer is read with `atomic.LoadPointer`. All deeper fields like `children[pos]`, `bitmap`, and the byte-slice keys/values, are accessed directly after a successful CAS. Readers therefore race with writers that mutate these fields in place -- race condition, memory model violation, etc.
pos := cMap.getPosition(node.Bitmap(), hash, level)
if node.Child(pos).IsLeaf() && bytes.Equal(key, node.Child(pos).Key()) {
return node.Child(pos).Value()
}
---
All xxxRecursive functions rely on those invalid interior pointer assumptions
Sequence in `putRecursive` / `deleteRecursive` is
1. `curr := atomic.LoadPointer(ptr)`
2. Build `nodeCopy`
3. Recurse; grandchildren are mutated in place
4. `atomic.CompareAndSwap(ptr, curr, nodeCopy)`
If another goroutine has already swapped in a different copy of `curr` (and mutated it) the CAS still succeeds because the pointer value is unchanged, merging incompatible sub-tries and corrupting the data
---
Use-after-free in sync.Pool
On CAS failure the freshly built `nodeCopy` is immediately returned to a `sync.Pool` -- undefined behavior
cMap.pool.PutNode(nodeCopy) // may race with outstanding readers
Other goroutines still holding references to that node can now access a reclaimed object, oops.
---
K/V Aliasing
Keys and values (both []byte slices, which are not safe for concurrent r/w access) are stored by reference, a mistake:
n.setKey(key)
n.setValue(value)
---
Reader panics, etc.
- `getRecursive` accesses `children[pos]` without bounds or nil checks, concurrent shrink can make `pos` invalid
- `GetIndex` allows a negative `shiftSize` once `level >= 7` with `chunkSize = 5`, producing nonsense indices and potential slice-out-of-boundsjohnisgood
> both []byte slices, which are not safe for concurrent r/w access
You must clone the slice on both write and read, right?
I get that cloning incurs a memory allocation and a copy operation, but this is the price for safety when concurrent access is possible or your data may be bodified outside your structure.
You could probably intern immutable keys, or avoid storing if keys already exist and are immutable, or use an object pool (like sync.Pool) to reduce allocations if this happens at scale. Anything else I am missing?
sirgallo
Performance comparisons are made against go sync.Map, with cmapv2 on par or sometimes exceeding is on different workloads. It is both lock-free and thread safe, using atomic operations. It also supports sharding out of the box. cmapv2 with 10m k/v pairs where keys and values are 64bytes was able to achieve 1.5m w/s and 3m r/s.
latchkey
I've been using this one for years, can you do comparisons against it?
jzelinskie
Another good comparison would be against https://pkg.go.dev/github.com/puzpuzpuz/xsync/v3#Map
sirgallo
definitely, I can expand my comparisons and benchmarks
sirgallo
yes I can take a look, thanks for passing that along
gttalbot
Is it anywhere close to swisstables?
vlovich123
No because swisstables generally don't do concurrency (i.e. concurrency ==> atomics which are inherently more expensive due to HW reasons).
This repo is completely unsound, code like [1] is pervasive and demonstrates a total misunderstanding of what guarantees are provided -- or, really, not provided -- by "atomic" reads of unsafe.Pointer values. Data races everywhere!
Not safe, do not pass go, do not collect $200, absolutely do not use.
[1] https://github.com/sirgallo/cmapv2/blob/280e3017ae4ba212f6f8...