[olug] understanding Linux memory usage

Thu Feb 21 05:38:48 UTC 2008

Another thing you may want to do with your Java applications is deploy them
using the Sun JDK 1.5+. This will allow you to take advantage of the JMX
reporting tools, and using jconsole (or even rolling your own application to
report) you can monitor each application from the JVM directly. You'll see
thread count, memory usages, etc...
(used exhaustively at my work -- unfortunately, I'm not a Java developer,
merely a sys-admin)

HTH,
Travis

On Wed, Feb 20, 2008 at 8:07 PM, Christopher Cashell <topher-olug at zyp.org>
wrote:

> I'm going to simplify some aspects of Linux memory management below to
> make for shorter explanations.  If anyone wishes to expand on any of
> the oversimplified places, feel free.  Memory management, and trying
> to sort out exact memory usage, can be complicated and convoluted.
>
> On Wed, Feb 20, 2008 at 2:58 PM, Ryan Stille <ryan at cfwebtools.com> wrote:
> > I have a process (a java virtual machine) that according to ps and top,
> >  is taking up 3GB of "virtual memory" size.  I would think "virtual
> >  memory" would be swap (not real memory, but virtual, like swap, right?)
> >  but the man page says it is swap + resident (physical) combined.
>
> The Linux Virtual Memory subsystem takes all available memory,
> physical and swap, and manages it internally, while presenting a
> single memory "view" to an application.  For this reason, many
> references to virtual memory, such as the one you point out, are
> meaning memory allocated by the virtual memory system, as opposed to
> disk-based "virtual" memory (swap).
>
> >  Here is the ps output:
> >  USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
> >  nobody    7147 21.1  6.6 3013876 271796 pts/0 Sl  14:44   1:04
> >  /opt/coldfusion8/runtime/bin/....
> >
> >  Its corresponding line from top agrees:
> >  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
> >  7147 nobody    25   0 2943m 265m  12m S    0  6.7   1:06.01 coldfusion8
>
> The first thing to note, is that this information can be a bit
> misleading, as the information is kind of an idealized report of how
> much memory the given application would use if it were the only
> application running on the system.  The 265M resident memory
> represents memory being used by this program alone, along with memory
> being used by shared libraries that are shared among multiple
> programs.
>
> For an example of how misleading ps/top can seem to be, I have a web
> server that is running apache.  It shows that each instance of apache
> is using 22M of resident (RAM) memory.  And there are 50 apache
> processes running on that web server.  Which should mean that apache
> is using 1100MB of RAM, right?  Except this box only has 512MB of RAM,
> and of that, almost 200MB is currently free.  The reason is because
> each of those apache processes is sharing most of the memory with all
> other apache processes, and only about 1M each is probably unique.
>
> >  So why does the summary info at the top of the top output say that
> there
> >  is only 687400k being used on the whole box, and that 0k swap is being
> used?
> >
> >  Here are those lines from top:
> >  Mem:   4090516k total,   687400k used,  3403116k free,    42012k
> buffers
> >  Swap:   787176k total,        0k used,   787176k free,   206808k cached
> >
> >  I know that the 265m of resident memory is contributing to the 687400k
> >  total used... but its still not all making sense.  I don't understand
> >  how top and "free -m" can say there is over 3GB of free memory, when
> the
> >  java process is supposed to be taking up 3GB or so (there is only 4GB
> on
> >  the box).
>
> This has to do with quirks of Java and Linux memory.  An application
> can do a number of things to reserve memory, even though they aren't
> using them.  Common examples are linked libraries, shared memory,
> mmap()ed files, special memory mapped to a process (i.e. video card's
> RAM mapped to X), COW (Copy On Write) semantics, and Linux memory
> overcommit.
>
> When the JVM starts up, it is reserving enough memory for all of the
> libraries it thinks it will need.  It also reserves memory for it's
> heap, as specified.  However, when it does that, the kernel is giving
> it a pointer to this memory address space, but internally, it will
> allocate that memory as needed.  Because the memory isn't actually in
> use, just reserved, it doesn't show up as used memory.
>
> Once the application actually makes use of the memory, it will start
> to show up in top and ps as being taken by the application.
>
> >  The end goal here is to allow this java process to allocate as much
> >  memory as possible for itself.  If free -m is showing 3GB free,
> couldn't
> >  other processes end up using that memory?  My java memory switches are
> >  -Xmx2500m -Xms2500m, BTW.
>
> Setting the max memory isn't a bad idea, to prevent the application
> from going crazy.  Setting the minimum is rarely a good idea, in my
> experience.  It limits the amount of memory anything else on the box
> can potentially allocate, even though Java may not be using it.  Also,
> if you do end up with other programs on the box requesting memory,
> it's possible for Linux to start killing processes as OOM, even though
> it isn't actually out of memory, it's just that all of the memory is
> reserved (even though it may not be used).  Your java program could
> even end up getting killed that way.
>
> Also don't forget that Linux will use all available (unused) memory
> for disk cache, and that can have a significant impact on the
> performance of a machine, especially if it's dealing with any
> significant amount of disk IO.  I always make it a point to keep a
> certain amount of memory free on my Linux boxes specifically for disk
> cache use.
>
> Hopefully this made a little bit of sense.  If you want to know a
> little more about your specific application, check out the 'pmap'
> command.  It will give you a very detailed breakdown of how memory is
> mapped for it.  If you really want to get down and dirty with Linux
> memory, google for "linux memory management".  There's a number of
> really good references that'll come up from that.
>
> >  -Ryan
>
> --
> Christopher
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-- 
Travis Owens

Let every nation know, whether it wishes us well or ill, that we shall pay
any price, bear any burden, meet any hardship, support any friend, oppose
any foe, to assure the survival and the success of liberty. This much we
pledge and more.   -- John F. Kennedy, inaugural speech, January 20, 1961