Bismark Alignment very slow

[Nitin123-4]

Hi Team,

I am running Bismark with the below command. I can see it's really slow.

bismark --bowtie2 -N 1 --parallel 4 $RESOURCES2/HG38/ -1 $Read1 -2 $Read2 --output_dir $PWD --temp_dir $PWD/$SAMPLEID"_TEMP" --prefix $SAMPLEID

I have 218,289,382 total reads i.e. 32.96(Gb) data. I did pre processing using Trimmomatic. Filtered reads are used for this.
It took ~22 h to complete. Can you please help with this?

Bismark Version: v0.24.0

[FelixKrueger]

Increasing the the mismatches from the default (0) to -N 1 is probably slowing things down markedly. Lowering this and/or increasing the --parallel are your best options.

[Nitin123-4]

Thanks for the quick reply.

WARNING: Bismark Parallel (BP?) is resource hungry! Each value of --parallel specified
will effectively lead to a linear increase in compute and memory requirements, so --parallel 4 for
e.g. the GRCm38 mouse genome will probably use ~20 cores and eat ~40GB or RAM, but at the same time
reduce the alignment time to ~25-30%. You have been warned.

With --parallel 4 it is taking ~40GB RAM. If we increase it to --parallel 8 or 10 it will take a lot of memory.

I think this is difficult as it needs more RAM also.

Also Is it recommended to use mismatches (0) ?

[FelixKrueger]

This is from the Bowtie 2 manual:

-N <int>

Sets the number of mismatches to allowed in a seed alignment during multiseed alignment. Can be set to 0 or 1. Setting this higher makes alignment slower (often much slower) but increases sensitivity. Default: 0.

To be honest, I don't think I have ever changes this to 1 ever. It only really requires one multi-seed alignment in the read somewhere as anchor, which in my experience is pretty much always the case.

[Nitin123-4]

Okay. Thanks for your response.

Please suggest about : With --parallel 4 it is taking ~40GB RAM. If we increase it to --parallel 8 or 10 it will take a lot of memory.

Is there any solution for this?

[FelixKrueger]

I am afraid there isn't really a solution for this, if you ask for more memory it will also use more... You could potentially try to give each Bowtie2 thread some additional core (e.g. -p 4), but this will only get you so far to be honest. It should not use much more memory though. This could result in the following command line:

bismark -p 4 --parallel 4 $RESOURCES2/HG38/ -1 $Read1 -2 $Read2 --output_dir $PWD --temp_dir $PWD/$SAMPLEID"_TEMP" --prefix $SAMPLEID

[Nitin123-4]

Okay, so it means it will use 16 CPUs in total and ~40GB RAM?

[FelixKrueger]

If you open a second terminal and run top you should be able to monitor usage statistics