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Performance Q & A by Adrian Cockcroft

Sizing up memory in Solaris

How can you tell how much memory you need? Introducing Sun's memory guru, who has written a white paper and some very powerful tools to help solve this problem

March  1998
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How much memory do I need? This is a question we keep hearing over and over, and memory sizing has been the subject of several Performance Q&A columns. This month, we take another stab at explaining memory in Solaris by reviewing the different types of memory and introducing a white paper and a set of tools, the RMCmem package, by Sun Systems Engineer Richard McDougall. (2,200 words)

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Q: How can I tell how much memory I need?

A: I've tried to answer this question a few times, but the real problem is that Solaris doesn't give you enough information on what memory is being used for. A few years ago a Sun systems engineer based in Adelaide, Australia, started looking for solutions to this problem. He came to work with me for a month in November 1995 and developed some new tools. In April 1997 we hired him to work full-time in our group, and now Richard McDougall is ready to share his knowledge with the world via a new white paper The Solaris Memory System -- Sizing Tools and Architecture (see Resources section), and a new tool set, the RMCmem package.


The RMCmem tools
The white paper contains details on how to obtain the tools. The package includes a kernel module that provides extra instrumentation, and is not a supported Sun product. As with any unsupported package, you should not depend upon it in a production environment. Bugs, patches, or interactions with other kernel modules could crash your system, and there is some additional data-collection overhead. Part of the package was integrated into Solaris 2.6, and we're using the tools as a proof-of-concept prototype to guide future enhancements to Solaris. An earlier collection of these tools was known as the RMCbunyip package -- the Bunyip is a legendary monster in Australia.

The four main uses of memory
Solaris is a virtual memory system. The total amount of memory that you can use is increased by adding swap space to the system. If you ever see "out of memory" messages, adding swap space is the usual fix. Performance of the system is very dependent on how much physical memory (RAM) you have. If you don't have enough RAM to run your workload, performance degrades rapidly. In this discussion I'm mainly concerned with RAM usage, and knowing how much is enough.

Physical memory usage can be classified into four groups:

RMCmem includes a simple command to summarize this:

% prtmem

Total memory:             123 Megabytes
Kernel Memory:             12 Megabytes
Application memory:        16 Megabytes
Buffercache memory:        34 Megabytes
Free memory:               59 Megabytes

Total physical memory
The total physical memory can be seen using prtconf. Memory is allocated in units called pages, and you can use the pagesize command to see whether you have 4096 or 8192 bytes per page:

% /usr/sbin/prtconf | grep Memory
Memory size: 128 Megabytes
% /usr/bin/pagesize

From an application you can call sysconf(_SC_PHYS_PAGES) to find out how many pages of memory you have, and getpagesize() to see the size. In some cases the result you get is less than the total -- as during boot, the first few megabytes are grabbed by the kernel, and what remains is made available to the system as pages (counted by the kernel as physmem).

Kernel memory
Kernel memory is allocated to hold the initial kernel code at boot time, then grows dynamically as new device drivers and kernel modules are used. Kernel tables also grow dynamically, unlike some older versions of Unix. As you add hardware and processes to a system, the kernel will grow. In particular, to keep track of all the memory in a system, the kernel allocates a page table structure. If you have several gigabytes of RAM this table gets quite large. The dynamic kernel memory allocator grabs memory in large "slabs," then allocates smaller blocks more efficiently. This means that the kernel tends to grab a bit more memory than it's really using. If there is a severe memory shortage, the kernel unloads unused kernel modules and devices and frees unused slabs. The simplest summary of kernel memory usage comes from sar. The old memory allocator architecture is still reflected in sar's output format, which lists a pool of small allocations (under 512 bytes each), large allocations (512 bytes to a page), and oversize allocations (a page or more). The new allocator is more sophisticated and is best seen by running the crash command as root and using the kmastat option to list the individual memory pools:

% sar -k 1
12:44:23 sml_mem   alloc  fail  lg_mem   alloc  fail  ovsz_alloc  fail
12:44:24 2842624 2003764     0 6561792 5503480     0     2678784     0

Totaling up the small, large, and oversize allocations, the kernel has grabbed 12,083,200 bytes of memory and is actually using 10,186,028 at present.

Application process memory
Application processes consist of an address space divided into segments, where each segment maps either to a file, anonymous memory (the swap space), System V shared memory, or a memory mapped device. The mapped files include the code and initialized data for the command and all its shared libraries. Files that the process has accessed via the mmap call may also be included, but not files that have been read and written in the normal manner. The segments can be viewed using /usr/proc/bin/pmap on any system running Solaris 2.5 or later.

% /usr/proc/bin/pmap 3687
3687:   cat misc/vmstat.run64
00010000    8K read/exec          dev:  32,0   ino: 108245
00020000    8K read/write/exec    dev:  32,0   ino: 108245
00022000    8K read/write/exec
EF680000  512K read/exec          /usr/lib/
EF70E000   32K read/write/exec    /usr/lib/
EF716000    8K read/write/exec
EF730000   48K read/shared        dev:  32,8   ino: 226
EF740000   16K read/exec          /usr/platform/SUNW,Ultra-2/lib/
EF752000    8K read/write/exec    /usr/platform/SUNW,Ultra-2/lib/
EF770000   32K read/exec          /usr/lib/
EF786000    8K read/write/exec    /usr/lib/
EF790000   16K read/exec          /usr/lib/
EF7A2000    8K read/write/exec    /usr/lib/
EF7B0000    8K read/exec/shared   /usr/lib/
EF7C0000    8K read/write/exec
EF7D0000  104K read/exec          /usr/lib/
EF7F8000   16K read/write/exec    /usr/lib/
EFFFC000   16K read/write/exec
EFFFC000   16K     [ stack ]

What is shown here is the base address and size of each segment, together with its mapping. Inode 108245 is the cat command itself. The cat command uses mmap for its input file, and inode 226 matches the file being read.

What we really want to know, however, is the amount of RAM used by each segment. This is shown by the pmem command in the RMCmem package in Solaris 2.5.1. The new kernel measurement used by this command was added to Solaris 2.6, where it can be seen using /usr/proc/bin/pmap-x.

% pmem 3687
3687:   cat misc/vmstat.dread.384run64
Addr       Size    Res Shared   Priv Prot               Segment-Name
-------- ------ ------  ----- ------ -----------------  -----------------------------
00010000     8K     8k     8k     0k read/exec          dev:  32,0   ino: 108245
00020000     8K     8k     8k     0k read/write/exec    dev:  32,0   ino: 108245
00022000     8K     8k     0k     8k read/write/exec    
EF680000   512K   504k   480k    24k read/exec          /usr/lib/
EF70E000    32K    32k     8k    24k read/write/exec    /usr/lib/
EF716000     8K     8k     0k     8k read/write/exec    
EF730000    48K    40k     8k    32k read/shared        dev:  32,8   ino: 226
EF740000    16K    16k    16k     0k read/exec          /usr/platform/SUNW,Ultra-2/lib/
EF752000     8K     8k     8k     0k read/write/exec    /usr/platform/SUNW,Ultra-2/lib/
EF770000    32K    32k    32k     0k read/exec          /usr/lib/
EF786000     8K     8k     8k     0k read/write/exec    /usr/lib/
EF790000    16K    16k    16k     0k read/exec          /usr/lib/
EF7A2000     8K     8k     8k     0k read/write/exec    /usr/lib/
EF7B0000     8K     8k     8k     0k read/exec/shared   /usr/lib/
EF7C0000     8K     8k     0k     8k read/write/exec    
EF7D0000   104K   104k   104k     0k read/exec          /usr/lib/
EF7F8000    16K    16k     8k     8k read/write/exec    /usr/lib/
EFFFC000    16K    16k     0k    16k read/write/exec    
EFFFC000    16K                                          [ stack ]
-------- ------ ------  ----- ------ 
           864K   848k   720k   128k

Now we can see that the process address space size is 864 kilobytes; 846 kilobytes of that are currently resident in main memory, wherein 720 kilobytes are shared with other processes while 128 kilobytes are private. When this command started only the 128 kilobytes of private memory were taken from the free list.

If we now go through all the processes on the system, add up how much private memory they use, and also add in the shared memory for each mapped file, we'll know how much application memory is in use. This summary is shown by prtmem as we saw in the beginning, and the detail is listed by the memps command in RMCmem. Note that the X server maps over 100 megabytes of Creator3D framebuffer hardware address space in this example, and processes that run only in the kernel address space do not have any address space of their own.

# memps

SunOS crun 5.5.1 Generic_103640-14 sun4u    02/21/98

   PID      Size Resident   Shared  Private  Process                              
   3349   11712k   10240k    1408k    8832k  /usr/dist/pkgs/framemaker,v5.1/bin/
   1479  126080k    9160k    1088k    8072k  /usr/openwin/bin/X :0 -dev /dev/fbs
    355    2992k    2552k    1632k     920k  rpc.ttdbserverd
   1513    5144k    3664k    2800k     864k  cm -Wp 769 40 -Ws 485 440 -WP 205 3
   1511    3864k    3152k    2296k     856k  calctool -Wp 60 60 -Ws 438 264 -WP 
   1498    3552k    2728k    1984k     744k  ttsession -s
...and so on...
   3732     880k     872k     760k     112k  more
    180     832k     728k     648k      80k  /usr/lib/power/powerd
   1443    1128k     928k     864k      64k  -csh
   1525    1104k     928k     864k      64k  /bin/csh
      3       0k       0k       0k       0k  fsflush
      2       0k       0k       0k       0k  pageout
      0       0k       0k       0k       0k  sched

Filesystem cache memory
This is the part of memory that is most confusing, as it is invisible. You can only tell it's there if you access the same file twice and it's quicker the second time. The RMCmem package adds kernel instrumentation that counts up all the pages for each cached file. The memps -m command lists the files that are cached in order of the amount of memory they're consuming. One problem is that within the kernel the file is only known by its inode number and filesystem mount point. The directory pathname for the file may not be known. The RMCmem package tries to solve this problem by catching file names as files are opened (by interposing on the vnode open code) and making an inode-to-name lookup cache in the kernel. This cache size is limited (to 8192 entries by default), and the file may have been opened before the kernel module was loaded, so it can't always find the name. You see a mount point (in this case /800000 or /800008) and the inode number for that mount point:

# memps -m | more

SunOS crun 5.5.1 Generic_103640-14 sun4u    02/21/98

   Size  Filename                               
  5336k  /export/home5/framemaker,v5.1/bin/sunxm.s5.sparc/maker5X.exe
  1256k  /usr/openwin/server/lib/
   960k  /usr/openwin/lib/
   608k  /usr/openwin/lib/
   528k  /usr/lib/
   448k  /usr/openwin/bin/Xsun
   344k  /usr/openwin/server/modules/
   336k  /usr/openwin/lib/
   312k  /usr/lib/
   232k  /usr/lib/sendmail
   216k  /800000:  586782
   200k  /export/home5/framemaker,v5.1/bin/sunxm.s5.sparc/fa.htmllite
   192k  /800000:  586694
   184k  /800000:  586702
   168k  /800000:  586758
   160k  /usr/bin/csh
   160k  /usr/openwin/server/lib/
   144k  /usr/openwin/server/lib/
   136k  /800008:      27
   136k  /usr/dt/bin/rpc.cmsd
   128k  /export/home5/framemaker,v5.1/bin/sunxm.s5.sparc/fa.tooltalk
   128k  /opt/RMCmem/bin/memps551
   112k  /usr/openwin/server/lib/
   112k  /usr/lib/
   112k  /usr/openwin/platform/sun4u/server/lib/
 ... and so on down to lots of files with only 8KB each ...

To add up all the memory taken up by shared library files another command is provided by RMCmem:

# prtlibs

Library (.so) Memory:     6496 K-Bytes

Free memory
Free memory is maintained so that when a new process starts up or an existing process needs to grow, the additional memory is immediately available. Just after boot, or if large processes have just exited, free memory can be quite large. As the filesystem cache grows, free memory settles down to a level set by the kernel variable lotsfree. When free memory falls below lotsfree, all the other memory in the system is scanned to find pages that have not been referenced recently. These pages are moved to the free list.

Graphical user interface tools
There is now a lot of new data available. To view it more easily and interactively, Richard wrote a Motif-based GUI tool. The memtool program comes up with a list of the files that are in memory and also shows the pageins and pageouts that have occurred for each file in the last measurement interval.

Example display of filesystem cache using memtool

There are three displays available. The second one shows process memory usage, and clicking on a process gives the detailed segment list as shown here for the X server.

Example display of process memory usage using memtool

The third display is a matrix that combines the two views. It shows the processes across the screen and the files down the side, with each cell holding the amount of memory mapped from that file by that process.

Example memtool process and file matrix display

Wrap up
So, finally, we can tell where all the memory is going. Richard's white paper contains a lot more information, including typical memory sizes for many common commands and daemons, and a full explanation of how the memory system works, and how to size memory configurations for desktop and server systems.

The white paper is at

The tools can be obtained by sending e-mail to:

New book update
On another recurring subject, my new book is now in the final stages of the production process. It should become available sometime in April. It ended up being more than 600 pages long (the old book was about 250 pages). The title is Sun Performance and Tuning -- Java and the Internet, by Adrian Cockcroft and Richard Pettit, Sun Press/PTR Prentice Hall, ISBN 0-13-095249-4.

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Other Cockcroft columns at

About the author
Adrian Cockcroft joined Sun Microsystems in 1988, and currently works as a performance specialist for the Server Division of SMCC. He wrote Sun Performance and Tuning: SPARC and Solaris, published by SunSoft Press PTR Prentice Hall. Reach Adrian at

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