Problem of history cache

I think that there is a problem at history cache when the ZABBIX server observes the log.

The log of 700Line/Sec enters History_text_cache, and sync is done correctly when it is normal.
As for pfree of History_text_cache at that time, 80-90% changes, and is normal.

However, movement becomes illegal when becoming the following afterwards.

1.The log in profusion (1200Line/sec) is output and History_text_cache becomes Full.
2.The output of the log decreases. (300Line/sec)
3.Pfree of History_text_cache doesn't increase from 0.001 to 2%.

Originally, I think that it is necessary to be updated from Cache to DB at the speed of 700Line/sec that the watch is possible.
However, it is updated at the speed of from 20 to 200Line/sec.

Under such a condition, there is a problem of not returning to the origin when one degree History_text_cache becomes Full.