[concurrent programming 045] what is pseudo shared memory sequence conflict? How to avoid?

What is pseudo shared memory order conflict ? How to avoid ?

Due to storage in CPU The cache line is a block of memory, not a single variable , Therefore, it is possible to store multiple variables in Same cache line in , When multiple threads modify When there are multiple variables in this cache line , Since only one thread can operate on the cache line at the same time , At this time, two threads modify two different variables under the same cache line at the same time , This is pseudo sharing , Also known as memory order conflict .

When pseudo sharing occurs , CPU You have to clear the assembly line , Can cause CPU It's a big expense .

How to avoid ：

JDK1.8 Previously, this problem was generally avoided by byte filling , That is, when creating a variable, make

Fill the cache line where the variable is located with the fill field , This avoids storing multiple variables in the same cache line , For example, the following code ：

If the cache behavior 64 byte , So we're in FilledLong Class padding 了 6 individual long A change of type

One long Type variables occupy 8 byte , Add your own value Variables occupy 8 Bytes , in total 56 byte . in addition , here FilledLong Is a class object , The object header of the bytecode of the class object occupies 8 byte , So a FilledLong The object actually takes up 64 Bytes of memory , This happens to fit into the same cache line .

JDK Provides sun.misc Contended annotation , To solve the pseudo sharing problem . Modify the above code as follows .

Particular attention

By default , ＠Contended Annotations are only used for Java Core class , such as rt Class under package .

If the class under the user's class path needs to use this annotation , Need to add JVM Parameters ： － XX:-RestrictContended . The width of the fill is... By default 128, To customize the fill width, you can use the parameter -XX:ContendedPaddingWidth Parameters Set it up .