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Ligra: A Lightweight Graph Processing Framework for Shared Memory
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Ligra (and Ligra+): A Lightweight Graph Processing Framework for Shared Memory


The code for the Ligra (and Ligra+) framework is located in the ligra/ directory. The code for the applications is in the apps/ directory, which is where compilation should be performed. Example inputs are provided in the inputs/ directory. Graph utilities are provided in the utils/ directory.


Compilation is done from within the apps/ directory. The compiled code will work on both uncompressed and compressed graphs.


  • g++ >= 5.3.0 with support for Cilk Plus
  • g++ >= 5.3.0 with OpenMP
  • Intel icpc compiler

To compile with g++ using Cilk Plus, define the environment variable CILK. To compile with icpc, define the environment variable MKLROOT and make sure CILK is not defined. To compile with OpenMP, define the environment variable OPENMP and make sure CILK and MKLROOT are not defined. Using Cilk Plus seems to give the best parallel performance in our experience. To compile with g++ with no parallel support, make sure CILK, MKLROOT and OPENMP are not defined.

Note: OpenMP support in Ligra has not been thoroughly tested. If you experience any errors, please send an email to Julian Shun.

For processing compressed graph files, there are three compression schemes currently implemented that can be used---byte codes, byte codes with run-length encoding and nibble codes. By default, the code is compiled for byte codes with run-length encoding. To use byte codes instead, define the environment variable BYTE, and to use nibble codes instead, define the environment variable NIBBLE. Parallel decoding within a vertex can be enabled by defining the environment variable PD (by default, a vertex's edge list is decoded sequentially).

After the appropriate environment variables are set, to compile, simply run

$ make -j  #compiles with all threads

The following commands cleans the directory:

$ make clean #removes all executables
$ make cleansrc #removes all executables and linked files from the ligra/ directory

Running code in Ligra

The applications take the input graph as input as well as an optional flag "-s" to indicate a symmetric graph. Symmetric graphs should be called with the "-s" flag for better performance. For example:

$ ./BFS -s ../inputs/rMatGraph_J_5_100
$ ./BellmanFord -s ../inputs/rMatGraph_WJ_5_100

For BFS, BC and BellmanFord, one can also pass the "-r" flag followed by an integer to indicate the source vertex. rMat graphs along with other graphs can be generated with the graph generators in the utils/ directory. By default, the applications are run four times, with times reported for the last three runs. This can be changed by passing the flag "-rounds" followed by an integer indicating the number of timed runs.

On NUMA machines, adding the command "numactl -i all " when running the program may improve performance for large graphs. For example:

$ numactl -i all ./BFS -s <input file>

Running code on compressed graphs (Ligra+)

When using Ligra+, graphs must first be compressed using the encoder program provided. The encoder program takes as input a file in the format described in the next section, as well as an output file name. For symmetric graphs, the flag "-s" should be passed before the filenames, and for weighted graphs, the flag "-w" should be passed before the filenames. For example:

$ ./encoder -s ../inputs/rMatGraph_J_5_100 ../inputs/rMatGraph_J_5_100.compressed
$ ./encoder -s -w ../inputs/rMatGraph_WJ_5_100 ../inputs/rMatGraph_WJ_5_100.compressed

After compressing the graphs, the applications can be run in the same manner as on uncompressed graphs, but with an additional "-c" flag. For example:

$ ./BFS -s -c ../inputs/rMatGraph_J_5_100.compressed
$ ./BellmanFord -s -c ../inputs/rMatGraph_WJ_5_100.compressed

Make sure that the compression method used for compilation of the applications is consistent with the method used to compress the graph with the encoder program.

Input Format for Ligra applications and the Ligra+ encoder

The input format of unweighted graphs should be in one of two formats (the Ligra+ encoder currently only supports the first format).

  1. The adjacency graph format from the Problem Based Benchmark Suite ( The adjacency graph format starts with a sequence of offsets one for each vertex, followed by a sequence of directed edges ordered by their source vertex. The offset for a vertex i refers to the location of the start of a contiguous block of out edges for vertex i in the sequence of edges. The block continues until the offset of the next vertex, or the end if i is the last vertex. All vertices and offsets are 0 based and represented in decimal. The specific format is as follows:


This file is represented as plain text.

  1. In binary format. This requires three files NAME.config, NAME.adj, and NAME.idx, where NAME is chosen by the user. The .config file stores the number of vertices in the graph in text format. The .idx file stores in binary the offsets for the vertices in the CSR format (the <o>'s above). The .adj file stores in binary the edge targets in the CSR format (the <e>'s above).

Weighted graphs: For format (1), the weights are listed at the end of the file (after <e(m-1)>), and the first line of the file should store the string "WeightedAdjacencyGraph". For format (2), the weights are stored after all of the edge targets in the .adj file.

By default, format (1) is used. To run an input with format (2), pass the "-b" flag as a command line argument.

By default the offsets are stored as 32-bit integers, and to represent them as 64-bit integers, compile with the variable LONG defined. By default the vertex IDs (edge values) are stored as 32-bit integers, and to represent them as 64-bit integers, compile with the variable EDGELONG defined.

Graph Utilities

Several graph utilities are provided in the utils/ directory and can be compiled using "make".

Graph Generators

Three graph generators from the PBBS project are provided. rMatGraph generators an rMat graph (described by Chakrabarti, Zhan and Faloutsos in SDM '04). The required parameters are the number of vertices and the output file. By default the number of directed edges is set to 10 times the number of vertices, and can be changed by specifying the "-m" flag followed by the number of edges. The default parameters are (a=.5, b=.1, c=.1 and d=.3), and can be changed by specifying the "-a", "-b", and "-c" flags each followed by the desired probability (d=1-a-b-c). The "-s" flag followed by an integer specifies the random seed (default value of 1). randLocalGraph generators a random graph, and the required parameters are the number of vertices (10 times the number of edges are generated by default, and can be changed with the "-m" flag) and the output file. gridGraph takes the same parameters generates a 2 or 3 dimensional graph, specified by the "-d" flag (default value is 2). The grid graphs are symmetrized, and the rMat and random graphs are not symmetrized but can be symmetrized by passing the "-s" flag. For graphs that are symmetrized, the total number of edges can be up to twice the number specified.


$ ./rMatGraph 10000000 rMat_10000000
$ ./rMatGraph -a .57 -b .19 -c .19 10000000 rMat_10000000 #modify rMat parameters
$ ./randLocalGraph 10000000 rand_10000000
$ ./randLocalGraph -m 50000000 10000000 rand_10000000 #modify edge count
$ ./gridGraph 10000000 2Dgrid_10000000
$ ./gridGraph -d 3 10000000 3Dgrid_10000000 

Graph Converters

SNAPtoAdj converts a graph in SNAP format and converts it to Ligra's adjacency graph format. The first required parameter is the input (SNAP) file name and second required parameter is the output (Ligra) file name. The "-s" flag may be used to symmetrize the input file. This converter works for any format that lists the two endpoints of each edge separated by white space per line, with lines starting with '#' ignored.

adjGraphAddWeights adds random integer weights in the range [1,...,log2(number of vertices)] to an unweighted Ligra graph in adjacency graph format, and takes as input the input file name followed by the output file name.

adjToBinary converts a Ligra graph in adjacency graph format to binary format. The arguments are the adjacency graph file name followed by the 3 binary file names (.idx, .adj and .config). For a weighted graph, pass the "-w" flag before the file names.


$ ./SNAPtoAdj SNAPfile LigraFile
$ ./adjGraphAddWeights unweightedLigraFile weightedLigraFile
$ ./adjToBinary rMatGraph_J_5_100 rMatGraph_J_5_100.idx rMatGraph_J_5_100.adj rMatGraph_J_5_100.config 
$ ./adjToBinary -w rMatGraph_WJ_5_100 rMatGraph_WJ_5_100.idx rMatGraph_WJ_5_100.adj rMatGraph_WJ_5_100.config 

Ligra Data Structure and Functions

Data Structure

vertexSubset: represents a subset of vertices in the graph. Various constructors are given in ligra.h


edgeMap: takes as input 3 required arguments and 3 optional arguments: a graph G, vertexSubset V, struct F, threshold argument (optional, default threshold is m/20), an option in {DENSE, DENSE_FORWARD} (optional, default value is DENSE), and a boolean indicating whether to remove duplicates (optional, default does not remove duplicates). It returns as output a vertexSubset Out (see section 4 of paper for how Out is computed).

The F struct contains three boolean functions: update, updateAtomic and cond. update and updateAtomic should take two integer arguments (corresponding to source and destination vertex). In addition, updateAtomic should be atomic with respect to the destination vertex. cond takes one argument corresponding to a vertex. For the cond function which always returns true, cond_true can be called.

struct F {
  inline bool update (intT s, intT d) {
  //fill in
  inline bool updateAtomic (intT s, intT d){ 
  //fill in
  inline bool cond (intT d) {
  //fill in 

The threshold argument determines when edgeMap switches between edgemapSparse and edgemapDense---for a threshold value T, edgeMap calls edgemapSparse if the vertex subset size plus its number of outgoing edges is less than T, and otherwise calls edgemapDense.

DENSE and is a read-based version where all vertices not satisfying Cond loop over their incoming edges and DENSE_FORWARD is a write-based version where each frontier vertex loops over its outgoing edges. This optimization is described in Section 4 of the paper.

Note that duplicate removal can only be avoided if updateAtomic returns true at most once for each vertex in a call to edgeMap.

vertexMap: takes as input 2 arguments: a vertexSubset V and a function F which is applied to all vertices in V. It does not have a return value.

vertexFilter: takes as input a vertexSubset V and a boolean function F which is applied to all vertices in V. It returns a vertexSubset containing all vertices v in V such that F(v) returns true.

struct F {
  inline bool operator () (intT i) {
  //fill in

To write your own Ligra code, it would be helpful to look at the code for the provided applications as reference.

Currently the results of the computation are not used, but the code can be easily modified to output the results to a file.

To develop a new implementation, simply include "ligra.h" in the implementation files. When finished, one may add it to the ALL variable in Makefile. The function that is passed to the Ligra/Ligra+ driver is the following Compute function, which is filled in by the user. The first argument is the graph, and second argument is a structure containing the command line arguments, which can be extracted using routines in parseCommandLine.h, or manually from P.argv and P.argc.

template<class vertex>
void Compute(graph<vertex>& GA, commandLine P){ 


For weighted graph applications, add "#define WEIGHTED 1" before including ligra.h.

To write a parallel for loop in your code, simply use the parallel_for construct in place of "for".

Graph Applications

Implementation files are provided in the apps/ directory: BFS.C (breadth-first search), BFS-Bitvector.C (breadth-first search with a bitvector to mark visited vertices), BC.C (betweenness centrality), Radii.C (graph eccentricity estimation), Components.C (connected components), BellmanFord.C (Bellman-Ford shortest paths), PageRank.C, PageRankDelta.C, BFSCC.C (connected components based on BFS), MIS.C (maximal independent set), KCore.C (K-core decomposition), Triangle.C (triangle counting), and CF.C (collaborative filtering).

Eccentricity Estimation

Code for eccentricity estimation is available in the apps/eccentricity/ directory: kBFS-Ecc.C (2 passes of multiple BFS's), kBFS-1Phase-Ecc.C (1 pass of multiple BFS's), FM-Ecc.C (estimation using Flajolet-Martin counters; an implementation of a variant of HADI from TKDD '11), LogLog-Ecc.C (estimation using LogLog counters; an implementation of a variant of HyperANF from WWW '11), RV.C (a parallel implementation of the algorithm by Roditty and Vassilevska Williams from STOC '13), CLRSTV.C (a parallel implementation of a variant of the algorithm by Chechik, Larkin, Roditty, Schoenebeck, Tarjan, and Vassilevska Williams from SODA '14), kBFS-Exact.C (exact algorithm using multiple BFS's), TK.C (a parallel implementation of the exact algorithm by Takes and Kosters from Algorithms '13), Simple-Approx-Ecc.C (simple 2-approximation algorithm). Follow the same instructions as above for compilation, but from the apps/eccentricity/ directory.

For kBFS-Ecc.C, kBFS-1Phase-Ecc.C, FM-Ecc.C, LogLog-Ecc.C, and kBFS-Exact.C, the "-r" flag followed by an integer indicates the maximum number of words to associate with each vertex. For all implementations, the "-s" flag should be used as the current implementations are designed for undirected graphs. To output the eccentricity estimates to a file, use the "-out" flag followed by the name of the output file. The file format is one integer per line, with the eccentricity estimate for vertex i on line i.


Julian Shun and Guy E. Blelloch. Ligra: A Lightweight Graph Processing Framework for Shared Memory. Proceedings of the ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming (PPoPP), pp. 135-146, 2013.

Julian Shun, Laxman Dhulipala, and Guy E. Blelloch. Smaller and Faster: Parallel Processing of Compressed Graphs with Ligra+. Proceedings of the IEEE Data Compression Conference (DCC), pp. 403-412, 2015.

Julian Shun. An Evaluation of Parallel Eccentricity Estimation Algorithms on Undirected Real-World Graphs. Proceedings of the ACM SIGKDD Conference on Knowledge Discovery and Data Mining (KDD), pp. 1095-1104, 2015.

Julian Shun, Farbod Roosta-Khorasani, Kimon Fountoulakis, and Michael W. Mahoney. Parallel Local Graph Clustering. Proceedings of the International Conference on Very Large Data Bases (VLDB), 9(12), pp. 1041-1052, 2016.

Laxman Dhulipala, Guy E. Blelloch, and Julian Shun. Julienne: A Framework for Parallel Graph Algorithms using Work-efficient Bucketing. Proceedings of the ACM Symposium on Parallelism in Algorithms and Architectures (SPAA), pp. 293-304, 2017.

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