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EXCESS Concurrent Tree Library. Features the EXCESS’s locality-aware and energy- efficient concurrent search trees: DeltaTree and GreenBST. This library also contains various prominent and state-of-the-art concurrent search tree implementations.
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EXCESS Concurrent Search Tree Libraries

This repository contains the EXCESS’s locality-aware and energy- efficient concurrent search trees: DeltaTree and GreenBST, and EXCESS's experimental work-free and locality-aware concurrent search tree (BlueBST).

This repository also contains various prominent and state-of-the-art concurrent search tree implementations, such as: the concurrent B-tree; lock- and transaction-based dynamic cache-oblivious search trees; the non-blocking binary search trees; Read Copy Update (RCU)-based concurrent search tree; the portably scalable concurrent search tree; transaction-based red-black tree (RBtree), AVL tree (AVLtree), and speculation-friendly tree (SFtree) from Synchrobench (


  • BlueBST, DeltaTree, and GreenBST are developed by UiT and licensed under the Apache License, Version 2.0;

  • CBTree, VTMTree, and SVEB are developed by UiT based on the others’ work. They are licensed under the The GNU General Public License v3.0;

  • Other trees are developed by their respective authors and retain their original licenses

The available trees

1. DeltaTree and GreenBST

DeltaTree is a fine-grained locality-aware tree. GreenBST is a more compact (in tems of memory footprint) and more optimized variant of DeltaTree. DeltaTree and GreenBST are portable, namely it can maintain their locality-awareness on different computing platforms (platform-independent).

Also available in:

Related publications:

  • Ibrahim Umar, Otto J. Anshus, and Phuong H. Ha. GreenBST: An energy-efficient concurrent search tree. Proceedings of the 22nd International European Conference on Parallel and Distributed Computing (Euro-Par ’16), 2016, LNCS, pp. 502-517, Springer.
  • Ibrahim Umar, Otto J. Anshus, and Phuong H. Ha. Effect of portable fine-grained locality on energy efficiency and performance in concurrent search trees. Proceedings of the 21st ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming (PPoPP ’16), 2016, pp. 36:1-36:2, ACM.
  • Ibrahim Umar, Otto Johan Anshus, and Phuong Hoai Ha. DeltaTree: A Locality-aware Concurrent Search Tree. In Proceedings of the 2015 ACM SIGMETRICS International Conference on Measurement and Modeling of Computer Systems (SIGMETRICS '15), 2015, pp. 457-458, ACM.

2. Concurrent B-tree (CBTree)

CBTree is a prominent locality-aware concurrent B+tree [25]. CBTree is a representation of the classic coarse-grained locality-aware search concurrent trees that are usually platform-dependent. CBTree can only perform well if their node size is set correctly (e.g., to the system’s page size). This tree is also often referred as the B-link tree.

Related publication:

  • Philip L. Lehman and s. Bing Yao. 1981. Efficient locking for concurrent operations on B-trees. ACM Trans. Database Syst. 6, 4 (December 1981), 650-670.

3. Lock-based (SVEB) and transactional (VTMtree) dynamic cache-oblivious tree

SVEB and VTMtree are the concurrent implementation of the fine-grained locality- aware vEB binary search tree. SVEB uses a global mutex to serialize its concurrent tree operations, while VTMtree uses the transactional memory runtime of the GNU C Compiler.

Related publication:

  • Gerth Stølting Brodal, Rolf Fagerberg, and Riko Jacob. Cache oblivious search trees via binary trees of small height. In Proc. 13th ACM-SIAM Symp. Discrete algorithms, SODA ’02, pages 39–48, 2002.

4. Original non-blocking binary search tree (NBBST) and its improved variant (LFBST)

NBBST is the Ellen et al. non-blocking binary search tree implementation. LFBST is the improved variant of the original non-blocking binary search tree. These non-blocking search trees are locality-oblivious.

NBBST official repository:

LFBST official repository:

Related publications:

  • Aravind Natarajan and Neeraj Mittal. Fast concurrent lock-free binary search trees. In Proc. 19th ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming, PPoPP ’14, pages 317–328, 2014.

  • Faith Ellen, Panagiota Fatourou, Eric Ruppert, and Franck van Breugel. Non- blocking binary search trees. In Proc. 29th ACM SIGACT-SIGOPS Symp. Principles of distributed computing, PODC ’10, pages 131–140, 2010.

5. Transactional red-black tree (RBtree), AVL tree (AVLtree), and speculation-friendly tree (SFtree) from Synchrobench

Synchrobench is a micro-benchmark suite used to evaluate synchronization techniques on data structures and it contains several state-of-the-art concurrent trees such as red-black tree (developed by Oracle labs), AVLtree (developed by Stanford), and speculation-friendly tree implementation in C. These transactional trees are locality-oblivious.

Syncrobench official repository:

Related publications:

  • Vincent Gramoli. More than you ever wanted to know about synchronization: Synchrobench, measuring the impact of the synchronization on concurrent algorithms. In Proceedings of the 20th ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming, PPoPP 2015, pages 1–10, 2015.

  • Tyler Crain, Vincent Gramoli, and Michel Raynal. A speculation-friendly binary search tree. In Proc. 17th ACM SIGPLAN Symp. Principles and Practice of Parallel Programming, PPoPP ’12, pages 161–170, 2012.

  • Rudolf Bayer. Symmetric binary b-trees: Data structure and maintenance algorithms. Acta Informatica, 1(4):290–306, 1972.

  • G. M. Skii and Ye. M. Landis. An algorithm for the organization of information. Doklady Akad. Nauk SSSR, 1962.

6. RCU-based concurrent search tree (Citrus)

Citrus is a concurrent binary search tree that utilizes Read-Copy-Update (RCU) synchronization and fine-grained locking for synchronization among updaters. Citrus contain operation is wait-free. This concurrent search tree is locality-oblivious.

Citrus official repository:

Related publication:

  • Maya Arbel and Hagit Attiya. Concurrent updates with rcu: Search tree as an example. In Proc. 2014 ACM Symposium on Principles of Distributed Computing, PODC ’14, pages 196–205. ACM, 2014.

7. Portably scalable concurrent search tree (BSTTK)

BST-TK is the state-of- the-art lock-based concurrent search tree based on the asynchronous concurrency paradigm. BST-TK is portably scalable, namely it scales across different types of hardware platforms. BSTTK is a locality-oblivious tree.

BSTTK official repository:

Related publication:

  • Tudor David, Rachid Guerraoui, and Vasileios Trigonakis. Asynchronized concurrency: The secret to scaling concurrent search data structures. In Proc. 12th Intl. Conf. on Architectural Support for Programming Languages and Operating Systems, ASPLOS’15, pages 631–644, 2015

Using the trees

Library usage example

The concurrent search trees inside this repository can be used as a statically-linked library. However, only BlueBST, DeltaTree, GreenBST, and CBTree that currently provide complete interfaces. These trees provide a (.h) header file that contains public- callable functions of the commonly used tree functions such as: MAP INSERT(map, key), MAP REMOVE(map, key), and MAP CONTAINS(map, key). The code below shows a C program that uses the concurrent Btree (CBTree) as a library.

#define MAP_USE_CBTREE //Use the CBTree

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

#include "map_select.h"

int main ()
  long numData = 10;
  long i;
  MAP_T cbtreePtr = MAP_ALLOC(0, 0);

  for (i = 0; i < numData; i++){
    MAP_INSERT(cbtreePtr, i+1);

  for (i = 0; i < numData; i++){
    printf("%ld: %d\n", i+1, MAP_CONTAINS(cbtreePtr, i+1));

  for (i = 0; i < numData; i++){
    MAP_REMOVE(cbtreePtr, i+1);

  for (i = 0; i < numData; i++) {
    printf("%ld: %d\n", i+1, MAP_CONTAINS(cbtreePtr, i+1));


  return 0;

A sample code and its accompanying makefile is available in the sample/ directory.

Running the concurrent search trees benchmarks

Running all benchmarks

  1. Clone the repository
  2. Go to ./bench directory
  3. Generate the search tree binaries. Run ./
  4. Run qsub ./bench-short for short benchmark OR qsub ./bench-long for complete benchmark (>2 hours).
  5. Combined data in CSV format will be available in ./bench/combined directory
  6. Comparison chart in PDF format will be generated in ./bench/charts directory

Running individual benchmark

DeltaTree library also provides a collection of standalone benchmark programs for each tree. Below is an example of running a standalone benchmark program. These benchmark programs accept several runtime parameters (e.g., initial tree size, how many update/search operation ratios).

$ ./DeltaTree -h
DeltaTree v0.1
Use -h switch for help.

Accepted parameters
-r <NUM> : Range size
-u <0..100> : Update ratio. 0 = Only search; 100 = Only updates
-i <NUM> Initial tree size
-t <NUM> Triangle (DeltaNode/GNode) size
-n <NUM> Number of threads
-s <NUM> Random seed. 0=time()
-d <0..1> Density
-v <0 or 1> : Valgrind mode (less stats). 0 = False; 1 = True
-h : This help

Benchmark output format:
"0: range, insert ratio, delete ratio, #threads, attempted insert, attempted delete, attempted search, effective insert, effective delete, effective search, time (in msec)"

$ ./DeltaTree -r 5000000 -u 10 -i 1024000 -n 10 -s 0
DeltaTree v0.1

Use -h switch for help.

- Range size r: 5000000
- DeltaNode size t: 127
- Update rate u: 10%
- Number of threads n: 10
- Initial tree size i: 1024000
- Random seed s: 0
- Density d: 0.500000
- Valgrind mode v: 0

Finished building initial DeltaTree
The node size is: 25 bytes
Now pre-filling 1024000 random elements...

Finished init a DeltaTree using DeltaNode size 127, with initial 1024000 members

#TS: 1421050928, 511389
Starting benchmark...

0: 5000000, 5.00, 5.00, 10, 249410, 248857, 4501733, 195052, 53720, 1000568, 476

Active (alloc'd) triangle:258187(266398), Min Depth:12, Max Depth:30
Node Count:1165332, Node Count(MAX): 1217838, Rebalance (Insert) Done: 234, Rebalance (Delete) Done: 0, Merging Done: 1
Insert Count:195052, Delete Count:53720, Failed Insert:54358, Failed Delete:195137
Entering top: 0, Waiting at the top:0
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