It's show on system indicator, I found solution in ubuntuforum to use command
sudo echo 3 | sudo tee /proc/sys/vm/drop_caches
It's work, but why my ubuntu not handle by itself?
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The cache has nothing to do with running programs. The cache is memory that is otherwise unused, so it is instead used to hold recently accessed data from the disk, in case it is needed again. This speeds up disk access by avoiding much of it since the data is already in ram. This is a good thing as it speeds up the system essentially for free, since that ram would otherwise be going to waste. Dropping the cache, therefore, is a bad idea except for doing cold cache performance benchmarking.
You can count memory used by the cache as free, which is why the
free program prints a line with the cache added to the free count and subtracted from the used count.