Java Heap and -Xms/-Xmx

Y Srinivas Ramakrishna Y.S.Ramakrishna at Sun.COM
Tue Jul 1 11:31:32 PDT 2008

Hi Keith --

> If I set -Xms1g -Xmx2g and I have 4gb RAM, then will the VM always 
> look to commit the 2GB Java heap to RAM? Or will it simply commit 2GB 
> to virtual memory? Opening with -Xms of 1GB to RAM, but perhaps at 
> runtime as the heap grows, the remaining 1GB java heap could be paged 
> out. I understand the Java heap should not be paged out by the OS on 
> account of performance overtones, but at start up when the heap is not 
> stuck, by setting -Xms < -Xmx, what happens vis-à-vis the addressable 
> memory that is the contiguous java heap? Spread out ENTIRELY on RAM, 
> if available, or if available, could it still be RAM and non-RAM?

When you set -Xms1G -Xmx2G, the committed heap (i.e. the physical heap
that we will actually use) starts at 1 G. That means there are pages
of memory that contain that state of the 1 G heap. Whether those
pages are physically on RAM or are in swap is something that the JVM
does not explicitly control. It's something that the OS controls, and
it depends on the "environment" (memory pressure) and the state of the host.
The 2G of address space is however reserved in the virtual address
space of the process. In other words, that space is not available for
use by other things in the process, such as C-heap, thread stacks or
mmapped libraries. However, at the start there may not be any physical
state (in an abstract sense) associated with that address space, because
there is no "memory" backing it, it's uninitialized unused memory that
has just been reserved. It may not even be available to be had when
we want to use it; see below. When the JVM believes it needs to expand the
Java heap to use that space, it will "commit" some of that reserved VA space,
as it expands the physical Java heap. When this happens, actual memory (i wanted
to say "physical", but it may not be RAM pages at that stage; it may just
be a swap reservation that is all) backs that virtual address space.
Typically, when that space is actually touched by a thread (say when copying
data into it for the first time) the OS will then allocate physical RAM pages
for it. [The swap backing that space is allocated at the time the process
"commits" that space, indicating an intention to use it -- that's how it
works in Solaris; some Linuxes will not actually reserve the swap space
until the physical pages are touched, by the way, so in those cases thewe may fail when actually touching the page for the first time because
we find that the swap is not to be had because we are maxed out on swap;
this can make Linuxes fragile wrt the JVM's current implementation.
I don't know if we catch this and translate it into a suitable OOM
on the vulnerable Linuxes, i suspect not. (That is my understanding
from a while ago; things may have changed since to deal with that

Once again however the question of whether the committed and used heap
is entirely in physical RAM or on RAM and swap depends on the OS
and the conditions of the host with the JVM not directly controlling

There was some talk of adding JVM options for locking the heap in
core (to use an archaic term), but I don't believe that has actually
been done in Java SE JDK. So my guess is that in order to lock the
heap in core, the user would need to do that separately via appropriate
OS interfaces.

Others please correct any mistakes I might have made in my description
above based on what might be somewhat obsolete information/understanding
of the JVM's status wrt these issues. (In particular there has been
talk on and off both with dealing with Linux's commit semantics
and with locking the Java heap in core via suitable JVM options.)

-- ramki

> Hope my question is not too confusing.
> thanks
> keith
> Keith R Holdaway
> Java Development Technologies
> SAS The Power to Know
> Carpe Diem
> -----Original Message-----
> From: Jon.Masamitsu at Sun.COM [mailto:Jon.Masamitsu at Sun.COM]
> Sent: Tuesday, July 01, 2008 1:56 PM
> To: Keith Holdaway
> Cc: hotspot-gc-dev at
> Subject: Re: Java Heap and -Xms/-Xmx
> Keith,
> Is this your question with numbers.
> If flag -Xmx1g -Xms500m are set on the command line
> and there is 2g of physical memory, will the VM
> increase the maximum heap size to 2g?
> No, the VM would not set the maximum heap size to
> 2g.  In general, the user gets what is requested
> on the command line.
> Was that the question?
> Jon
> Keith Holdaway wrote:
> > If -Xms is set to a smaller value than -Xmx, and -Xmx is set to a 
> value less than the total RAM, will the VM commit the -Xmx setting to 
> physical addressable space ONLY?
> >
> > thanks
> >
> > Keith R Holdaway
> > Java Development Technologies
> >
> > SAS The Power to Know
> >
> > Carpe Diem
> >
> >

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