Can I ask a reference letter from my Operating Systems professor while applying to Machine Learning programme? I am going to apply for a Computer Science master's programme in the field of Machine Learning or data science, but I asked a professor who taught me a different subject (Operating Systems) for a reference letter. Does it matter since these are two different subjects?
A good recommendation is one that highlights your potential as a researcher and your related technical and interpersonal skills. If the mentioned professor knows you and can vouch for your potential in these areas, the recommendation will be beneficial. Recommendations are not focused on what courses the professor teaches. A good recommendation can even come from a non-CS professor (but preferably in a related field like EE) whom you have worked with and knows you personally.