Changeset 229


Ignore:
Timestamp:
Jan 8, 2011, 3:48:55 PM (15 years ago)
Author:
dmik
Message:

README.OS2: Added information about memory requirements and about how Java calculates the default heap size.

File:
1 edited

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  • trunk/doc/README.OS2

    r225 r229  
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     57MEMORY REQUIREMENTS
     58
     59  Sometimes you may find out that starting a Java application fails with the
     60  following error message:
     61
     62     Error occured during initialization of VM
     63     Could not reserve enough space for object heap
     64     Could not create the Java virtual machine.
     65
     66  This means that the amount of memory Java wants to reserve for its heap is
     67  bigger than the maximum free block of memory available to the Java process.
     68  Note that the size of this free block does not directly depend on the amount
     69  of physical RAM installed in your computer (because the physical RAM may be
     70  extended using the swap file, for instance). It rather depends on the virtual
     71  address limit set by OS/2 for the process. In older OS/2 versions that don't
     72  support high memory (e.g. the ones based on pre-WSeB kernels) this limit
     73  is known to be 512M. In later versions it is controlled by the
     74  VIRTUALADDRESSLIMIT parameter in CONFIG.SYS (which is specified in megabytes
     75  and defaults to 1024M).
     76
     77  Furthermore, not all memory within the virtual address limit is available to
     78  the process. Some small fraction of it is used by the kernel and the rest is
     79  divided in two more or less equal parts: the private arena and the shared
     80  arena. As said, the size of these arenas does not depend on the amount of
     81  physical RAM and can be approximated using the following table. Note that the
     82  values in the table are not the initial arena sizes but rather the sizes of
     83  the maximum free block of memory available in the corresponding arena to a
     84  dummy process that does nothing but queries these system values (all numbers
     85  are in MB, the first column is for systems with no high memory support):
     86
     87  VIRTUALADDRESSLIMIT                *512 | 1024 | 1536 | 2048 | 3072
     88  -------------------------------------------------------------------
     89  Max free block in private arena     267 |  432 |  880 | 1328 | 2224
     90  Max free block in shared arena      228 |  404 |  852 | 1230 | 2196
     91
     92  Note that these values are gathered on a default eCS 2.0 GA system and may
     93  vary depending on what system DLLs get loaded into each process, they are
     94  given only as an example. You may get the real values on your system with
     95  a variety of tools gathering system information, such as THESEUS.
     96
     97  On the other hand, when calculating the default amount of memory to reserve
     98  for the the heap (which is called the maximum heap size in the documentation),
     99  Java uses the physical RAM size as a base, not the the size of the free block
     100  in the private arena (where Java actually allocates the heap). Below is a
     101  simplified version of the algorithm for these calculations:
     102
     103  1. Use MIN (MaxRAM, <physical_RAM>) as the base RAM value. MaxRAM is a Java[
     104     constant that defaults to 1G for the client (regular) Java virtual machine
     105     and 4G for the server JVM.
     106
     107  2. Divide this base RAM value by MaxRAMFraction (4 by default) and assign the
     108     result as the default value for the maximum heap size (MaxHeapSize).
     109
     110  3. Use the MaxHeapSize value increased by 20-30% (for the needs other than the
     111     Java heap) as the size of the private memory block to allocate.
     112
     113  So, if your machine has, say, 2G of RAM and you attempt to start it in server
     114  mode (using the -server command line option), Java will want 512M (2G/4) plus
     115  additional 20-30%. This would obviously not fit into 432M of free private
     116  memory available for the process when VIRTUALADDRESSLIMIT is set to 1024 and
     117  it was the case with earlier releases of OpenJDK 6 for OS/2 as well as with
     118  the releases of InnoTek Java 1.4.x for OS/2.
     119
     120  Starting with version 6 Beta 2, OpenJDK for OS/2 solves this problem by
     121  limiting the amount of memory Java wants for the heap to the actual size of
     122  the available memory block in the private arena. So, in the above case Java
     123  would actually get about 310M in server mode (instead of the requested 512M).
     124  You may change this limit by changing VIRTUALADDRESSLIMIT in CONFIG.SYS
     125  (according to the table above), but please note that values higher than 1024
     126  may cause problems with some drivers (for example, it is known that JFS and
     127  HPFS386 drivers cannot allocate a disk cache of the big size if the
     128  VIRTUALADDRESSLIMIT value is too high).
     129
     130  In either case, the above describes how Java calculates the defaulut maximum
     131  heap size. You may always override this default using the -Xmx<size> Java
     132  command line option if you are not satisfied with the default value for some
     133  reason or if your applcation gives you the "Could not reserve enough space for
     134  object heap" message at startup. However, keep in mind that if you a -Xmx
     135  value which is bigger than the maximum free block in the private arena, you
     136  will get the same error and will have to decrease the requested size until it
     137  succeeds.
     138
     139
     140
    57141CURRENT LIMITATIONS
    58142
    … …  
    69153     If you want to disable anti-aliasing, you may use the following Java
    70154     command line argument:
    71      
     155
    72156       -Dawt.useSystemAAFontSettings=off
    73157
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