unexpected cpu usage caused by tidb_server_memory_gc_trigger

[XuHuaiyu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. set global tidb_server_memory_limit = "760MB"
2. set global tidb_server_memory_limit_sess_min_size=107374182400
3. modify the tidb code to forbid pointget

diff --git a/planner/optimize.go b/planner/optimize.go
index b0c5b0070d..76da86e391 100644
--- a/planner/optimize.go
+++ b/planner/optimize.go
@@ -211,6 +211,7 @@ func Optimize(ctx context.Context, sctx sessionctx.Context, node ast.Node, is in
                        fp = fpv.Plan
                } else {
                        fp = core.TryFastPlan(sctx, node)
+                       fp = nil
                }
                if fp != nil {
                        return fp, fp.OutputNames(), nil

4. import tpcc warehouse 20 data
5. tiup bench tpcc run --db tpcc20wh --threads 16 --host --port --statusPort
6. check the grafana, we can see the cpu usage is high as the what I see instead part of following image show
7. modify the tidb code , and we can see the cpu usage is normal as the expect to see part of following image show

diff --git a/util/gctuner/memory_limit_tuner.go b/util/gctuner/memory_limit_tuner.go
index 204c569c0c..0e7d0d3ec4 100644
--- a/util/gctuner/memory_limit_tuner.go
+++ b/util/gctuner/memory_limit_tuner.go
@@ -128,6 +128,9 @@ func (t *memoryLimitTuner) GetPercentage() float64 {
 // UpdateMemoryLimit updates the memory limit.
 // This function should be called when `tidb_server_memory_limit` or `tidb_server_memory_limit_gc_trigger` is modified.
 func (t *memoryLimitTuner) UpdateMemoryLimit() {
+       if t.waitingReset.Load() {
+               return
+       }
        var memoryLimit = t.calcMemoryLimit(t.GetPercentage())
        if memoryLimit == math.MaxInt64 {
                t.isTuning.Store(false)

Analysis：

1. The tidb-server utilizes tidb_server_memory_limit_gc_trigger to set the memory threshold that actively triggers the golang garbage collection (GC). When the heapInUse memory reaches the threshold of tidb_server_memory_limit * tidb_server_memory_limit_gc_trigger, it actively triggers a golang GC. To avoid overly frequent GC, in the code implementation, after this threshold is touched once, it is adjusted from the default value of 0.7 to 1.1, and after 1 minute, it is reset back to 0.7.

2. Due to the periodic update of global system variables by tidb-server, the operation of increasing tidb_server_memory_limit_gc_trigger from 0.7 to 1.1 to reduce the GC frequency is almost ineffective. Therefore, when the heapInUse of tidb-server remains above the threshold of tidb_server_memory_limit * tidb_server_memory_limit_gc_trigger for an extended period, a high-frequency GC phenomenon occurs, leading to the CPU being in a near-maximum utilization state for an extended duration.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

v7.3.0