Biowulf High Performance Computing at the NIH
gmap-gsnap on Biowulf


GMAP is a tools for rapidly and accurately mapping and aligning cDNA sequences to genomic sequences.

GSNAP is designed to align short reads from NGS data and allow detection of short and long range splicing de novo or with a database of know juctions. In addtion, SNP-tolerant alignements and alignments of reads obtained from bisulfite treated DNA are implemented.

There may be multiple versions of gmap-gsnap available. An easy way of selecting the version is to use modules. To see the modules available, type

module avail gmap-gsnap 

To select a module use

module load gmap-gsnap/[version]

where [version] is the version of choice.

gmap-gsnap is a multithreaded application. Make sure to match the number of cpus requested with the number of threads.

Environment variables set



Index files

gmap/gsnap indices are stored under


in a structure analogous to the igenomes package. In addition, commonly used genomes are also available at /fdb/gmap_post20210308/common_genomes. This path is also the default path gmap and gsnap use to look for genome files, so a particular genome is available there it is sufficient to provide the genome name with -d genome_name to any of the tools without also specifying the genome directory with -D.

Performance considerations

Gsnap throughput was measured using different numbers of threads with an ENCODE data set (100nt single ended RNA-Seq, mouse Purkinje cells; accession ENCFF220BOE; biosample ENCSR758TAD). Discovery of novel splice sites was disabled but known splice sites were provided as input. The input file was compressed and stored on local scratch space. The output was saved unmodified on local scratch space. Each process was allocated 30GB of memory. The exact command used was

gsnap -d mm10 --nthreads=${SLURM_CPUS_PER_TASK} \
    --use-shared-memory=0 -B 5 \
    --novelsplicing=0 --gunzip \
    --use-splicing=/lscratch/${SLURM_JOBID}/mm10.iit \
    --format=sam -o /lscratch/${SLURM_JOBID}/${n}.sam \
gsnap thread scaling

Reads aligned per second per thread for different versions of gsnap with different numbers of threads.

The lower throughput of version 2015-05-01 with default settings is largely the result of a change in the default for one setting ( --end-detail) from medium to high which has the effect of improving alignments in the small number of cases where the distal end of a splice or indel contains a mismatch. Changing this version back to medium recovers most the loss in throughput. It is therefore largely a tradeoff between throughput and sensitivity. As of version 2016-06-30, the default reverted to medium.

Most benchmarks were run on the phase1 norm nodes (x2650; 10g). Benchmarks ending in x2695 were run on the newer phase2 norm nodes (x2695; ibfdr).

Gsnap scales well with the number of threads all the way to 32, though the most efficient use is probably around 24 threads with this input data set. Note that gsnap does not start any extra processes or threads, so --nthreads=$SLURM_CPUS_PER_TASK is appropriate unless the output is piped into another process such as samtools. In that case the number of threads used by the second process has to be subtracted from the number of threads given to gsnap.

Performance was comparable when input and output were kept on the shared storage space (/data).

On interactive session

Gmap and gsnap are multi-threaded applications. The thread count is determined with -t / --nthreads and defaults to 1.

Map and align a single cDNA to the mouse genome. Note that the mouse genome is in common genomes, so no path has to be provided.

[user@biowulf]$ sinteractive

[user@cn3144]$ module load gmap-gsnap
[user@cn3144]$ cd /data/$USER/test_data
[user@cn3144]$ gmap -d mm10 -A /usr/local/apps/gmap-gsnap/TEST_DATA/myc.fa

Map and align single end RNA-Seq data from encode to the mouse genome with database of known splice junctions. Supress search for novel juctions:

[user@cn3144]$ module load samtools/1.2
[user@cn3144]$ gtf_splicesites < /fdb/igenomes/Mus_musculus/UCSC/mm10/Annotation/Genes/genes.gtf \
  | iit_store -o mm10.iit

[user@cn3144]$ gsnap -d mm10 --nthreads=2 --novelsplicing=0 --use-splicing=mm10.iit --gunzip \
            --format=sam \
            /usr/local/apps/gmap-gsnap/TEST_DATA/ENCFF220BOE.fastq.gz \
            2> test.log \
          | samtools view -S -b -F4 -q 20 - \
          > bam/gsnap.bam

A number of other small tools are available as part of the package.

Batch job on Biowulf

Batch scripts should use the $SLURM_CPUS_PER_TASK environment variable to determine the number of threads to use. This variable is set by slurm according to the number of requested cores. An example script for gsnap might look like the following:

#! /bin/bash
set -o pipefail
set -e

function fail {
    echo "$@" >&2
    exit 1

module load samtools/1.2 || fail "could not load samtools module"
module load gmap-gsnap || fail "could not load gmap-gsnap module"
cd /data/$USER/test_data || fail "no such directory"

gtf_splicesites < /fdb/igenomes/Mus_musculus/UCSC/mm10/Annotation/Genes/genes.gtf \
  | iit_store -o mm10.iit

gsnap -d mm10 --nthreads=$(( SLURM_CPUS_PER_TASK - 2 ))\
        --novelsplicing=0 --use-splicing=mm10.iit --gunzip \
        --format=sam \
        /usr/local/apps/gmap-gsnap/TEST_DATA/ENCFF220BOE.fastq.gz \
        2> test.log \
      | samtools view -S -b -F4 -q 20 -@ 2 - \
      > bam/gsnap.bam

The script is submitted to the batch system requesting, for example, 8 cores and 25GB of memory:

biowulf$ sbatch -c 20 --mem=30g
Swarm of jobs on Biowulf

Set up a command file for two jobs, each aligning single ended reads. Note that swarm allows line continuations.

# gsnap.swarm
cd /data/$USER/test_data \
  && gsnap -d mm10 --nthreads=$(( SLURM_CPUS_PER_TASK - 1 ))\
        --novelsplicing=0 --use-splicing=mm10.iit --gunzip \
        --format=sam file1.fastq.gz \
        2> test1.log \
  | samtools view -S -b -F4 -q 20 - \
  > bam/gsnap.bam
cd /data/$USER/test_data \
  && gsnap -d mm10 --nthreads=$(( SLURM_CPUS_PER_TASK - 1 ))\
        --novelsplicing=0 --use-splicing=mm10.iit --gunzip \
        --format=sam file2.fastq.gz \
        2> test2.log \
  | samtools view -S -b -F4 -q 20 - \
  > bam/gsnap2.bam

Create the known splice site database as shown above and start the jobs with swarm

biowulf$ swarm -f swarm_command_file -t 20 -g 30 \
                --module samtools/1.2 gmap-gsnap

Interactive job on Biowulf

Interactive work requiring significant resources should be carried out on interactive compute nodes, not on the head node or [user@cn3144]. Interactive nodes are allocated with sinteractive. For example, to request a 8 core interactive job with 25GB memory:

biowulf$ sinteractive -c 8 --mem=30g

On the interactive node, gmap and gsnap are then used essentially as decribed above.