corp: support batch coprocessor for tikv to reduce task and RPC numbers

[cfzjywxk]

Enhancement

Currently, in some web3 index lookup queries, the fanout issue may introduce latency and cost problems. Batching is a common way to reduce task and RPC numbers, and thus possibly reduce latency, cost, and parameter tuning work.

RFC

• The proto change cop: support batch coprocessor for tikv kvproto#1018
• The coprocessor client change store/copr: support batch coprocessor requests by store #39525
• The IndexLookupExecutor related change store/copr: support batch coprocessor requests by store #39525
• The tikv related change cop: support batch coprocessor processing in tikv tikv/tikv#13850

[XuHuaiyu]

It seems paging is disabled in batch cop, why?

[windtalker]

TiFlash already has BatchRequest and BatchCoprocessor in kvproto, why not just reuse/enhance that?

[cfzjywxk]

It seems paging is disabled in batch cop, why?
@XuHuaiyu
In the current stage, the aim is to resolve the fanout issue, more tests and investigations are needed to make it work with other features like adaptive concurrency and paging.

[cfzjywxk]

TiFlash already has BatchRequest and BatchCoprocessor in kvproto, why not just reuse/enhance that?

@windtalker The requirements are different and separating them may make development and maintenance easier.

[windtalker]

TiFlash already has BatchRequest and BatchCoprocessor in kvproto, why not just reuse/enhance that?

@windtalker The requirements are different and separating them may make development and maintenance easier.

OK, maybe you should use another name because now batch coprocessor means batch coprocessor to TiFlash, it may make reader/developer confusing if there are two batch coprocessor with completely different meanning.