restore: split & scatter regions concurrently

[YuJuncen]

Port of pingcap/br#1363

What problem does this PR solve?

Before, when restoring many small tables, the batcher would probably send small batch due to the so called AutoCommit feature of the batcher. By this, we can make the split & scatter & restore worker more active.

But frequently send small batches isn't free. The split step is costly and I/O bounded for even small batches. For example, it costs about 3s to splitting 60 ranges, but restore those ranges typically costs only 1s. Then the restore worker get idle at most time. The restore hence has slowed down.

What is changed and how it works?

Instead of using a single split worker, this PR allow multi restore batches be split concurrently.
We added two hidden flags, --batch-flush-interval and --pd-concurrency, the former for better tuning the behavior of batcher, the latter for tweaking the concurrent split.
Also, more logs were added so the create table speed, download, ingest time cost can be observed via log.

Check List

Tests

• Integration test
• Manual test (add detailed scripts or steps below)
  A internal test shows, in a 190GB, 6000 tables workload, this PR can speed up the restoration: the original version takes over 2 hours for restoring, and this version takes about 30mins for restoring. The latter is nearly equal to the time cost of creating tables(see figure below).

Release note

Restore many small tables would be faster now.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Little-Wallace
• kennytm

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[YuJuncen]

/sig migrate

[YuJuncen]

/cc Little-Wallace
/cc 3pointer

[3pointer]

/run-integration-tests

[3pointer]

/run-integration-tests

[kennytm]

/component br

[YuJuncen]

/run-integration-test

[YuJuncen]

/run-integration-tests

[YuJuncen]

/run-check_dev_2

So many unit tests >= 5s...

[Little-Wallace]

LGTM

[kennytm]

rest LGTM

[YuJuncen]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: afe9bd3

[ti-chi-bot]

@YuJuncen: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.