Set maximum global limit on TiDB memory usage

[zhuboshuai]

Feature Request

Is your feature request related to a problem? Please describe:

i always found that some tidb instances restart because of out of memory ,the result is slow query peak in the Duration panel of grafana. it's a severe problem. because it affect the query speed of the business, and consequent result is a fault report. it's irksome.
At some point it will happen, but sooner or later. so it's a problem.

Describe the feature you'd like:

a parameter that can limit a tidb instance's memory usage.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[morgo]

TiDB currently permits limiting memory on a per session basis, as well as the max number of connections to an individual server. This is a good feature request, but there are some challenges:

• If the limit is reached, what is the expected behavior?
• Is the session that last tried to allocate memory blocked, or the one that is the most greedy (similar to OOM killer)?
• What happens if it was not a user session that triggered the OOM?
• Should there be a soft limit, so that when the limit is reached it is permitted to go over briefly, but the connection is marked as killed? (This would permit a more graceful cleanup.)

[zz-jason]

@SunRunAway PTAL

[yahonda]

Does server-memory-quota feature satisfy this feature request?