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Few tips to write low latency programs

People ask me can you write Low latency Java programs, and i always feel low latency is a relative terms and based on lots of things, not just my java code. But then i thought as a programmer i should think how my code could use my underlying resources in better way and make JVM performance better.
(My JVM or java code cant make harddisk run faster but i can write programs so that i can access hardisk less :) )


Few tips which i think we should keep in mind while writing code which should have low latency
  • Initialise/create Collection objects as actual size as near as possible.
If you can guess actual size of collection at run time, then its always better to create collections of that particular size.
e.g. if i know my list will have 100 items(approx) and those will be added at once, then its always good to create list like this
List<Object> list = new ArrayList<Object>(100);
instead of
List<Object> list = new ArrayList<Object>();

and for Hash based collection this is very important, as after resizing, rehashing is done for all elements which could affect your performance

  • make all private functions as static, though i expected that run time linking of private methods should be same as static methods as there must not be any lookup for private method actual implementation. No one can override private method
  • Maps usage , if you are using HashMap which will be read/write by many threads and you are planning to use synchronise to that, then better look into ConcurrentHashMap and initialise it properly with size and partition(default 16).
  • Which implementation of List to be used, Array List or Linked List, think hard before using one of them.
  • Which implementation of Set to be used.
  • Which Implementation of Map to be used.
  • Object reuse, more the object reuse less the task for Garbage Collector.
  • Make sure you give enough memory to JVM < the memory available from machine underlying, else you will have too much latency in memory swaps from disk.
  • Look into Garbage collector settings and see which one you should be using. Ofcourse concurrent GC would be better if you are running multiple processor :).
  • You can add Strings like this String a = b + c +d +e +f, earlier java version used to create lots of intermediate Strings like b+c, then b+c+d etc etc, but i saw in latest java compiled file that generated byte code, generate this code as if you have used StringBuilder, so don't worry about using StringBuilder or StringBuffer :). But should be used if you have dynamic Strings to be added.
  • If traversing a ArrayList then try to avoid new style looping for(Object oneObject:list), instead loop through using list.get(i) and for loop as
for(int i=0;i<list.size();i++){
list.get(i);//Do something with element
}

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