day4-jvm

Today's summary

In the morning, I went to the hospital to have a look at my throat , It's four o'clock for a nap in the afternoon , Chat with dad in the evening , I only watched a little all day jvm(76~83).

JVM

GC Differentiation and analysis

G1 When the old generation is insufficient , When the threshold is reached
Concurrent Tags + Mixed collection
If the recycling speed is lower than the speed of garbage generation , At this time, there is only a small pause
Conversely, it degenerates into serial
remark
In the concurrent tagging phase , Processed c after , Someone quoted c.
When the object reference changes , Will add a write barrier .
The writing barrier is c Join a queue and set to gray
After concurrency , Get into remark, Make further judgment .
In order to avoid being disposed of as garbage .

G1 Optimize

● String de duplication
String The bottom layer uses char Array
So add the newly allocated string to a queue , When recycling in the new era , Check whether there is string repetition , If the values are the same , Quote the same char[]
advantage ： Save memory
shortcoming ： Slightly occupied cpu Memory , The recycling time of the new generation is slightly increased .
Default on
● Concurrent tag class unloading

After concurrent marking , Know which classes are no longer used ,
Conditions ： When all classes of a class loader are no longer used , Then unload all the classes it loads .
● Giant object
○ def： Greater than region Half of
○ No copy
○ Priority should be given to recycling .
○ When no one quoted it in the old days , It can be disposed of when it is recycled in the new generation .
● Concurrent marking time adjustment
Dynamically adjust the threshold , Prevent degradation into full gc

reflection

I didn't expect it to be true to fish for three days and dry the net for two days , After three days of study, I began to slack off , I have to go out tomorrow … I hope to study before going out . A company notice passed today , Next, we need to prepare two sides .