Speed tradeoff of Java s -Xms and -Xmx options

Question

Given these two commands  A     java -Xms10G -Xmx10G myjavacode input txt   B     java -Xms5G -Xmx5G myjavacode input txt   I have two questions    Since command A reserves more memory with its parameters  will A run faster than B   How do -Xmx and -Xms affect the running process and the output of my program

User · Accepted Answer

It depends on the GC your java is using  Parallel GCs might work better on larger memory settings - I m no expert on that though   In general  if you have larger memory the less frequent it needs to be GC-ed - there is lots of room for garbage  However  when it comes to a GC  the GC has to work on more memory - which in turn might be slower

User · Answer

It is difficult to say how the memory allocation will affect your speed   It depends on the garbage collection algorithm the JVM is using   For example if your garbage collector needs to pause to do a full collection  then if you have 10 more memory than you really need then the collector will have 10 more garbage to clean up     If you are using java 6 you can use the jconsole  in the bin directory of the jdk  to attach to your process and watch how the collector is behaving   In general the collectors are very smart and you won t need to do any tuning  but if you have a need there are numerous options you have use to further tune the collection process

User · Answer

The -Xmx argument defines the max memory size that the heap can reach for the JVM  You must know your program well and see how it performs under load and set this parameter accordingly  A low value can cause OutOfMemoryExceptions or a very poor performance if your program s heap memory is reaching the maximum heap size  If your program is running in dedicated server you can set this parameter higher because it wont affect other programs    The -Xms argument sets the initial heap memory size for the JVM  This means that when you start your program the JVM will allocate this amount of memory instantly  This is useful if your program will consume a large amount of heap memory right from the start  This avoids the JVM to be constantly increasing the heap and can gain some performance there  If you don t know if this parameter is going to help you  don t use it   In summary  this is a compromise that you have to decide based only in the memory behavior of your program

User · Answer

I have found that in some cases too much memory can slow the program down     For example I had a hibernate based transform engine that started running slowly as the load increased   It turned out that each time we got an object from the db  hibernate was checking memory for objects that would never be used again   The solution was to evict the old objects from the session   Stuart

User · Answer

Allocation always depends on your OS  If you allocate too much memory  you could end up having loaded portions into swap  which indeed is slow  Whether your program runs slower or faster depends on the references the VM has to handle and to clean  The GC doesn t have to sweep through the allocated memory to find abandoned objects  It knows it s objects and the amount of memory they allocate by reference mapping  So sweeping just depends on the size of your objects  If your program behaves the same in both cases  the only performance impact should be on VM startup  when the VM tries to allocate memory provided by your OS and if you use the swap  which again leads to 1

User · Answer

This was always the question I had when I was working on one of my application which created massive number of threads per request   So this is a really good question and there are two aspects of this   1  Whether my Xms and Xmx value should be same  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp - Most websites and even oracle docs suggest it to be the same  However  I suggest to have some 10-20  of buffer between those values to give heap resizing an option to your application in case sudden high traffic spikes OR a incidental memory leak  2  Whether I should start my Application with lower heap size  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp - So here s the thing - no matter what GC Algo you use  even G1   large heap always has some trade off  The goal is to identify the behavior of your application to what heap size you can allow your GC pauses in terms of latency and throughput       nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp -  For example  if your application has lot of threads  each thread has 1 MB stack in native memory and not in heap  but does not occupy heavy object space  then I suggest have a lower value of Xms       nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp  nbsp - If your application creates lot of objects with increasing number of threads  then identify to what value of Xms you can set to tolerate those STW pauses  This means identify the max response time of your incoming requests you can tolerate and according tune the minimum heap size

User · Answer

gt  C  java -X  -Xmixed           mixed mode execution  default  -Xint             interpreted mode execution only -Xbootclasspath  lt directories and zip jar files separated by   gt                    set search path for bootstrap classes and resources -Xbootclasspath a  lt directories and zip jar files separated by   gt                    append to end of bootstrap class path -Xbootclasspath p  lt directories and zip jar files separated by   gt                    prepend in front of bootstrap class path -Xnoclassgc       disable class garbage collection -Xincgc           enable incremental garbage collection -Xloggc  lt file gt     log GC status to a file with time stamps -Xbatch           disable background compilation -Xms lt size gt         set initial Java heap size -Xmx lt size gt         set maximum Java heap size -Xss lt size gt         set java thread stack size -Xprof            output cpu profiling data -Xfuture          enable strictest checks  anticipating future default -Xrs              reduce use of OS signals by Java VM  see documentation  -Xcheck jni       perform additional checks for JNI functions -Xshare off       do not attempt to use shared class data -Xshare auto      use shared class data if possible  default  -Xshare on        require using shared class data  otherwise fail    The -X options are non-standard and subject to change without notice    copy-paste

User · Answer

The speed tradeoffs between various settings of -Xms and -Xmx depend on the application and system that you run your Java application on  It also depends on your JVM and other garbage collection parameters you use  This question is 11 years old  and since then the effects of the JVM parameters on performance have become even harder to predict in advance  So you can try different values and see the effects on performance  or use a free tool like Optimizer Studio that will find the optimal JVM parameter values automatically

[java] Speed tradeoff of Java's -Xms and -Xmx options

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