fail to init variable during the init

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

[tidb]> set @@global.tidb_gogc_tuner_threshold=0.4;
Query OK, 0 rows affected (0.002 sec)
[tidb]> set @@global.tidb_server_memory_limit_gc_trigger=0.51;
Query OK, 0 rows affected (0.003 sec)
[tidb]> show variables like "%tidb_gogc_tuner_threshold%";
+---------------------------+-------+
| Variable_name             | Value |
+---------------------------+-------+
| tidb_gogc_tuner_threshold | 0.4   |
+---------------------------+-------+
1 row in set (0.001 sec)
[tidb]> show variables like "%tidb_server_memory_limit_gc_trigger%";
+-------------------------------------+-------+
| Variable_name                       | Value |
+-------------------------------------+-------+
| tidb_server_memory_limit_gc_trigger | 0.51  |
+-------------------------------------+-------+
1 row in set (0.001 sec)

restart tidb 

2. What did you expect to see? (Required)

succeed

3. What did you see instead (Required)

[tidb]> show variables like "%tidb_gogc_tuner_threshold%";
+---------------------------+-------+
| Variable_name             | Value |
+---------------------------+-------+
| tidb_gogc_tuner_threshold |       |
+---------------------------+-------+
1 row in set (0.002 sec)
[tidb]> show variables like "%tidb_server_memory_limit_gc_trigger%";
+-------------------------------------+-------+
| Variable_name                       | Value |
+-------------------------------------+-------+
| tidb_server_memory_limit_gc_trigger |       |
+-------------------------------------+-------+
1 row in set (0.001 sec)
[tidb]> set @@global.tidb_server_memory_limit_gc_trigger=0.51;
ERROR 1105 (HY000): tidb_server_memory_limit_gc_trigger should be greater than tidb_gogc_tuner_threshold + 0.05
[tidb]> set @@global.tidb_gogc_tuner_threshold=0.4;
ERROR 1105 (HY000): tidb_gogc_tuner_threshold should be less than tidb_server_memory_limit_gc_trigger - 0.05

4. What is your TiDB version? (Required)