Hi,

One rule in my `snakemake` workflow (in which I run `kraken2`) has a large RAM requirement of ~50 GB. When running the workflow on many samples in parallel (e.g., `snakemake --cores 12 -s workflow.smk`), I get a bunch of errors because the system's RAM is exhausted.

As a result, I've tried limiting the amount of RAM used by `snakemake` by setting a maximum memory usage within the `kraken2` rule: 

```
    resources:
        mem_mb=55000
```

Intuitively, I thought that this would mean that `snakemake` would run each job at a time since running >1 job would exceed 55000 MB (=55 GB) of RAM usage. However, the workflow still fails by running out of RAM despite my system having 64 GB of RAM.

Next, I tried also setting a maximum amount of RAM on the command line using the `--resources` flag:

`snakemake -j12 -s workflow.smk --resources mem_mb=55000`

However, I get a traceback error:

```
Traceback (most recent call last):
  File "/home/cfos/.local/bin/snakemake", line 8, in <module>
    sys.exit(main())
  File "/home/cfos/.local/lib/python3.8/site-packages/snakemake/__init__.py", line 2400, in main
    resources = parse_resources(args.resources)
  File "/home/cfos/.local/lib/python3.8/site-packages/snakemake/resources.py", line 85, in parse_resources
    for res, val in resources_args.items():
AttributeError: 'list' object has no attribute 'items'
```

What am I doing wrong here? Any advice would be appreciated.



UPDATE: This has been fixed in version [6.5.3](https://github.com/snakemake/snakemake/releases/tag/v6.5.3)