[global sort] tidb oom while add index with global sort

[aytrack]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. enable global config

set global tidb_cloud_storage_uri="s3://xxx"
set global tidb_enable_dist_task = 1;

2. alter table add index

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

add index success and tidb running stable

3. What did you see instead (Required)

tidb restart frequently with oom

4. What is your TiDB version? (Required)

+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v7.5.0-alpha                             |
| Edition: Community                                        |
| Git Commit Hash: 79db4ec58be3856995d31013e20f3faa57e3a3ac |
| Git Branch: heads/refs/tags/v7.5.0-alpha                  |
| UTC Build Time: 2023-10-16 11:42:39                       |
| GoVersion: go1.21.3                                       |
| Race Enabled: false                                       |
| Check Table Before Drop: false                            |
| Store: tikv                                               |
+-----------------------------------------------------------+