[fix](memory) Modify `mem_limit` default value

[xinyiZzz]

Proposed changes

Issue Number: close #xxx

Problem summary

Modify the default value of mem_limit to auto. auto means process mem limit is equal to max(physical mem * 0.9, 6.4G). 6.4G is the maximum memory reserved for the system.

Checklist(Required)

• [ ] Does it affect the original behavior
• [ ] Has unit tests been added
• [ ] Has document been added or modified
• [ ] Does it need to update dependencies
• [ ] Is this PR support rollback (If NO, please explain WHY)

Further comments

If this is a relatively large or complex change, kick off the discussion at [email protected] by explaining why you chose the solution you did and what alternatives you considered, etc...

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[xinyiZzz]

run buildall

[morningman]

Please update the document

[xinyiZzz]

Please update the document

done, cc~ @morningman

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[xinyiZzz]

run buildall

[xinyiZzz]

run buildall

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"