Performance Overview: Using top

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Using top

The top tool is an important tool for identifying potential problem areas in a process. This tool displays a periodically sampled set of statistics on system usage. Using top and understanding its output are an excellent way to identify potential performance problems.

The top tool displays periodically updated statistics on CPU usage, memory usage (in various categories), resource usage (such as threads and ports), and paging events. In the default mode, top displays CPU and memory utilization of all system processes. You can use this information to see how much memory your program is using and what percentage of the CPU time it is using. An idle program should not use any CPU time and an active one should consume a proportionate amount of CPU time based on the complexity of the task.

Note: If you want to track CPU usage and other statistics over time, use BigTop instead. BigTop graphs performance trends over time, providing a real-time display of memory usage, page faults, CPU usage, and other data.

Listing 4-1 shows a typical statistical output from top. For application developers, the statistics you should be most interested in are the CPU usage, resident private memory usage (RPRVT), and pagein/pageout rates. These values tell you some key things about your application’s resource usage. High CPU usage may mean that your application tasks are not tuned appropriately. Increased memory usage and page-in/page-out rates may indicate a need to reduce your application’s memory footprint.

Listing 4-1 Typical output of top

Processes:  36 total, 2 running, 34 sleeping... 81 threads

Load Avg:  0.24, 0.27, 0.23     CPU usage:  12.5% user, 87.5% sys, 0.0% idle

SharedLibs: num =   77, resident = 10.6M code, 1.11M data, 4.75M LinkEdit

MemRegions: num = 1207, resident = 16.4M + 4.94M private, 22.2M shared

PhysMem:  16.0M wired, 25.8M active, 48.9M inactive, 90.7M used, 37.2M free

VM:  476M + 39.8M   6494(6494) pageins, 0(0) pageouts

  PID COMMAND      %CPU   TIME      #TH #PRTS #MREGS RPRVT  RSHRD  RSIZE  VSIZE

  318 top           0.0%  0:00.36   1    23    13   172K   232K   380K  1.31M

  316 zsh           0.0%  0:00.08   1    18    12   168K   516K   628K  1.67M

  315 Terminal      0.0%  0:02.25   4   112    50  1.32M  3.55M  4.88M  31.7M

  314 CPU Monito    0.0%  0:02.08   1    63    35   896K  1.34M  2.14M  27.9M

  313 Clock         0.0%  0:01.51   1    57    38  1.02M  2.01M  2.69M  29.0M

  312 Dock          0.0%  0:03.72   2    77    78  2.18M  2.28M  3.64M  30.0M

  311 Finder        0.0%  0:07.68   4    86   171  7.96M  9.15M  15.1M  52.1M

  308 pbs           0.0%  0:01.37   4    76    40   928K   684K  1.77M  15.4M

  285 loginwindow   0.0%  0:07.19   2    70    58  1.64M  1.93M  3.45M  29.6M

  282 cron          0.0%  0:00.00   1    11    14    88K   228K   116K  1.50M

  245 sshd          0.0%  0:02.48   1    10    15   176K   312K   356K  1.41M

  222 SecuritySe    0.0%  0:00.14   2    21    24   476K   828K  1.29M  3.95M

  209 automount     0.0%  0:00.03   2    13    20   336K   748K   324K  4.36M

  200 nfsiod        0.0%  0:00.00   1    10    12     4K   224K    52K  1.22M

  199 nfsiod        0.0%  0:00.00   1    10    12     4K   224K    52K  1.2

[...]

In its header area, top displays statistics on the global state of the system. This information includes load averages; total process and thread counts; and total memory, broken down into various categories such as private, shared, wired, and free. It also includes global information concerning the system frameworks. At regular intervals, top updates these statistics to account for recent system activity.

Table 4-1 describes the columnar data that appears in the CPU and memory utilization mode using the -w parameter. For detailed information about how top reports information, see the top(1) man page.

**Table 4-1** Output from top using the -w option
Column	Description
`PID`	The BSD process ID.
`COMMAND`	The name of the executable or application package. (Note that Code Fragment Manager applications are named after the native process that launches them, `LaunchCFMApp`.)
`%CPU`	The percentage of CPU cycles consumed during the interval on behalf of this process (both kernel and user space).
`TIME`	The amount of CPU time (minute:seconds.hundreths) consumed by this process since it was launched.
`#TH`	The number of threads owned by this process.
`#PRTS (delta)`	The number of Mach port objects owned by this process. The delta value, which is enabled by the `-w` parameter, is relative to the value first displayed when `top` was launched.
`#MREG`	The number of memory regions.
`VPRVT`	The private address space currently allocated (with `-w` parameter only).
`RPRVT (delta)`	The resident private memory. The delta value, which is enabled by the `-w` parameter, is relative to the previous sample.
`RSHRD (delta)`	The resident shared memory. The delta value, which is enabled by the `-w` parameter, is relative to the previous sample
`RSIZE (delta)`	The total resident memory as real pages that this process currently has associated with it. Some may be shared by other processes. The delta value, which is enabled by the `-w` parameter, is relative to the previous sample.
`VSIZE (delta)`	The total address space currently allocated, including shared memory. The delta value, which is enabled by the `-w` parameter, is relative to the value first displayed when `top` was launched. This value is mostly irrelevant for Mac OS X processes. Every application has a large virtual size because of the shared region used to hold framework and library code.

The RPRVT data (for resident private pages) is a good measure of how much real memory an application is using. The RSHRD column (for resident shared pages) shows the resident pages of all the shared mapped files or memory objects that are shared with other tasks.

Note: The top tool does not provide a separate count of the number of pages in shared libraries that are mapped into the task.

The top tool reports memory usage of windows in the “shared memory” category because window buffers are shared with the window server.

Table 4-2 shows the columns displayed in the event-counting mode, which is enabled with either the -e, -d, or -a option on the command line.

**Table 4-2** Output from top using the -d option
Column	Description
`PID`	The BSD process ID.
`COMMAND`	The name of the executable or application package. (Note that Code Fragment Manager applications are named after the native process that launches them, `LaunchCFMApp`.)
`%CPU`	The percentage of CPU cycles consumed during the interval on behalf of this process (both kernel and user space).
`TIME`	The amount of CPU time consumed by this process (minute:seconds.hundreths) since it was launched.
`FAULTS`	The total number of page faults.
`PAGEINS`	The number of page-ins, requests for pages from a pager (each page-in represents a 4 kilobyte I/O operation).
`COW_FAULTS`	The number of faults that caused a page to be copied (generally caused by copy-on-write faults).
`MSGS_SENT`	The number of Mach messages sent by the process.
`MSGS_RCVD`	The number of Mach messages received by the process.
`BSDSYSCALL`	The number of BSD system calls made by the process.
`MACHSYSCALL`	The number of Mach system calls made by the process.
`CSWITCH`	The number of context switches to the process (the number of times the process has been given time to run by the kernel’s scheduler).

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Last updated: 2006-10-03

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