Improve for_select_default_busy

[sylzd]

Issue Number: #126

What problem does this PR solve?

Related to #126.
Reduce the cost of selectgo in writeInsert method, which can improve 30%+ perfomance of dumping data.

What is changed and how it works?

selectgo runs only in the case of buffer Reseting.

Check List

Tests

• Unit test
  Result:
  ? github.com/pingcap/dumpling/cmd/dumpling [no test files]
  ? github.com/pingcap/dumpling/v4/cli [no test files]
  ok github.com/pingcap/dumpling/v4/export 0.145s coverage: 51.6% of statements
  ? github.com/pingcap/dumpling/v4/log [no test files]

• Integration test
  Result:
  [2020/10/21 13:53:13.812 +08:00] [INFO] [main.go:253] ["dump data successfully, dumpling will exit now"]
  Cleaning up test output dir: /tmp/dumpling_test_result/sql_res.views
  Passed integration tests.

• Manual test (add detailed scripts or steps below)

  • Command:

  time ./dumpling -h 10.162.1.1 -u dump_test2 -p* -P 4000 --filetype sql -r 10000 --threads 16 -B sbtest -o dumpling_output

  • Before changing:

    • Costs:

                 real    0m21.262s
                user    3m7.963s
                sys     0m27.534s
      

  • Flamegraph:
    

  • After changing

    • Costs:

      real    0m15.175s
      user    1m48.148s
      sys     0m21.989s
      

    • Flamegraph:
      

Side effects

• Possible performance regression

  Maybe little delay to handle ctx.Done()

• Increased code complexity

  None

• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release note

[CLAassistant]

All committers have signed the CLA.

[lichunzhu]

Great! Please also update this part for WriteInsertInCsv.
https://github.com/pingcap/dumpling/blob/master/v4/export/writer_util.go#L299

[sylzd]

Great! Please also update this part for WriteInsertInCsv.
https://github.com/pingcap/dumpling/blob/master/v4/export/writer_util.go#L299

Done. unit test & integration test also passed.

[lichunzhu]

Done. unit test & integration test also passed.

OK. Please sign the Contributor License Agreement.

[sylzd]

Done. unit test & integration test also passed.

OK. Please sign the Contributor License Agreement.

signed.

[lichunzhu]

LGTM

[kennytm]

/reward 600

[ti-challenge-bot]

This PR already closed!

[Rustin170506]

/reward 600

Already added manually. Please reward before merging.