理解Oracle在AIX平台上的内存使用

1.理解Oracle进程 首先我们要做的是理解Oracle的3种进程类型：后台进程( background process)和服务进程(也叫前台进程)还有用户进程。当我们尝试启动Oracle实例，首先受到召唤的是后台进程，一组后台进程和内存组建构成了 Oracle 实例，这些后台进程包括 日志记录进程lgwr,数据库写出进程 dbwr, 系统监控进程smon, 进程监控进程pmon, 分布式恢复进程reco, 检查点进程ckpt, 11g后变得更多了，多到我记不住。 这些进程在UNIX上的具体args总是形如ora_functionname_sid, 这里的functionname即后台进程的功能名而sid 即 $ORACLE_SID所指出的值。 第二类是用户进程，它可能是一个sqlplus命令行，可能是imp/exp工具，也可能是用户开发的一个java程序，当用户进程在本地启动时它们不直接操作SGA或者PGA,但毫无疑问它们也是需要消耗一定数量的虚拟内存的。 第三类进程就是我们说的服务进程，启动一个sqlplus 连接(这个连接可能是连到本地的 也可能的是远程的，这在我们讨论内存使用时无关紧要)的同时我们需要用到一个服务进程，它直接向我们的sqlplus终端负责。我们有时候也称服务进程为影子进程。影子进程总是和每一个用户进程一一对应的映射除非我们用到了MTS(多线程服务器)时。影子进程一般形如oracleSID,这里的sid和前文所指一般。 2.理解Oracle的内存使用 Oracle对内存的使用可以划分为2个大类型，即私有的和共享的。私有内存仅供单个进程使用。相反的，共享内存可以供多个进程使用且在具体使用上要复杂得多。在合计共享内存时，我们只需将所有进程所共享的内存段累加一次即可(Oracle 的SGA具体反映到OS层可能是多个shared memory segment,我们只需要将这一个或多个段的大小加到一起就可以了)。 我们可能使用到的最大的共享内存段毫无疑问会是SGA(SYSTEM GLOBAL AREA),我们看到的SGA被映射成虚拟地址且被每一个后台进程和前台进程attach到自己身上，以便随时能够利用到SGA; 我们有很多性能工具可以反映这部分的内存使用， 好比'top','ps -lf', 但他们都无法分辨前后台进程内存使用中私有内存和共享内存分别得使用状况(我们往往只能得到一个Oracle使用内存很多的结论，却不知道是PGA还是 SGA消耗的更多的内存)。如果我们把从这些途径中获得每个进程的内存使用量相加，我们会发现这样计算的总内存使用量是SGA+PGA的几十倍，这是违反常识的，实际也分配不到那么多内存。 要真正了解Oracle内存使用，你使用的内存窥测命令需要能够分离Oracle使用的私有内存和共享内存。在Aix平台上有这样一个svmon(在其他 UNIX平台上有一个我认为更好的工具是pmap,与之对应AIX上有一个procmap命令，但这个命令并不能窥测Oracle 私有或共享内存的使用，所以我们只能退而求其次了)。您可能在AIX的安装光盘上通过安装文件(filesets) "perfagent.tools"来获取该工具。使用"smit install_lastest"命令可以配备这个命令。对于svmon，作为一个非AIX操作系统专家而言，我推荐您读一下我引用的这篇文档：

The svmon Command The svmon command provides a more in-depth analysis of memory usage. It is more informative, but also more intrusive, than the vmstat and ps commands. The svmon command captures a snapshot of the current state of memory. However, it is not a true snapshot because it runs at the user level with interrupts enabled. To determine whether svmon is installed and available, run the following command: # lslpp -lI perfagent.tools The svmon command can only be executed by the root user. If an interval is used (-i option), statistics will be displayed until the command is killed or until the number of intervals, which can be specified right after the interval, is reached. You can use four different reports to analyze the displayed information: Global (-G) Displays statistics describing the real memory and paging space in use for the whole system. Process (-P) Displays memory usage statistics for active processes. Segment (-S) Displays memory usage for a specified number of segments or the top ten highest memory-usage processes in descending order. Detailed Segment (-D) Displays detailed information on specified segments. Additional reports are available in AIX 4.3.3 and later, as follows: User (-U) Displays memory usage statistics for the specified login names. If no list of login names is supplied, memory usage statistics display all defined login names. Command (-C) Displays memory usage statistics for the processes specified by command name. Workload Management Class (-W) Displays memory usage statistics for the specified workload management classes. If no classes are supplied, memory usage statistics display all defined classes. To support 64-bit applications, the output format of the svmon command was modified in AIX 4.3.3 and later. Additional reports are available in operating system versions later than 4.3.3, as follows: Frame (-F) Displays information about frames. When no frame number is specified, the percentage of used memory is reported. When a frame number is specified, information about that frame is reported. Tier (-T) Displays information about tiers, such as the tier number, the superclass name when the -a flag is used, and the total number of pages in real memory from segments belonging to the tier. How Much Memory is in Use To print out global statistics, use the -G flag. In this example, we will repeat it five times at two-second intervals. # svmon -G -i 2 5 m e m o r y i n u s e p i n p g s p a c e size inuse free pin work pers clnt work pers clnt size inuse 16384 16250 134 2006 10675 2939 2636 2006 0 0 40960 12674 16384 16254 130 2006 10679 2939 2636 2006 0 0 40960 12676 16384 16254 130 2006 10679 2939 2636 2006 0 0 40960 12676 16384 16254 130 2006 10679 2939 2636 2006 0 0 40960 12676 16384 16254 130 2006 10679 2939 2636 2006 0 0 40960 12676 The columns on the resulting svmon report are described as follows: memory Statistics describing the use of real memory, shown in 4 K pages. size Total size of memory in 4 K pages. inuse Number of pages in RAM that are in use by a process plus the number of persistent pages that belonged to a terminated process and are still resident in RAM. This value is the total size of memory minus the number of pages on the free list. free Number of pages on the free list. pin Number of pages pinned in RAM (a pinned page is a page that is always resident in RAM and cannot be paged out). in use Detailed statistics on the subset of real memory in use, shown in 4 K frames. work Number of working pages in RAM. pers Number of persistent pages in RAM. clnt Number of client pages in RAM (client page is a remote file page). pin Detailed statistics on the subset of real memory containing pinned pages, shown in 4 K frames. work Number of working pages pinned in RAM. pers Number of persistent pages pinned in RAM. clnt Number of client pages pinned in RAM. pg space Statistics describing the use of paging space, shown in 4 K pages. This data is reported only if the -r flag is not used. The value reported starting with AIX 4.3.2 is the actual number of paging-space pages used (which indicates that these pages were paged out to the paging space). This differs from the vmstat command in that vmstat's avm column which shows the virtual memory accessed but not necessarily paged out. size Total size of paging space in 4 K pages. inuse Total number of allocated pages. In our example, there are 16384 pages of total size of memory. Multiply this number by 4096 to see the total real memory size (64 MB). While 16250 pages are in use, there are 134 pages on the free list and 2006 pages are pinned in RAM. Of the total pages in use, there are 10675 working pages in RAM, 2939 persistent pages in RAM, and 2636 client pages in RAM. The sum of these three parts is equal to the inuse column of the memory part. The pin part divides the pinned memory size into working, persistent and client categories. The sum of them is equal to the pin column of the memory part. There are 40960 pages (160 MB) of total paging space, and 12676 pages are in use. The inuse column of memory is usually greater than the inuse column of pg spage because memory for file pages is not freed when a program completes, while paging-space allocation is. In AIX 4.3.3 and later, systems the output of the same command looks similar to the following: # svmon -G -i 2 5 size inuse free pin virtual memory 65527 64087 1440 5909 81136 pg space 131072 55824 work pers clnt pin 5918 0 0 in use 47554 13838 2695 size inuse free pin virtual memory 65527 64091 1436 5909 81137 pg space 131072 55824 work pers clnt pin 5918 0 0 in use 47558 13838 2695 size inuse free pin virtual memory 65527 64091 1436 5909 81137 pg space 131072 55824 work pers clnt pin 5918 0 0 in use 47558 13838 2695 size inuse free pin virtual memory 65527 64090 1437 5909 81137 pg space 131072 55824 work pers clnt pin 5918 0 0 in use 47558 13837 2695 size inuse free pin virtual memory 65527 64168 1359 5912 81206 pg space 131072 55824 work pers clnt pin 5921 0 0 in use 47636 13837 2695 The additional output field is the virtual field, which shows the number of pages allocated in the system virtual space. Who is Using Memory? The following command displays the memory usage statistics for the top ten processes. If you do not specify a number, it will display all the processes currently running in this system. # svmon -Pau 10 Pid Command Inuse Pin Pgspace 15012 maker4X.exe 4783 1174 4781 2750 X 4353 1178 5544 15706 dtwm 3257 1174 4003 17172 dtsession 2986 1174 3827 21150 dtterm 2941 1174 3697 17764 aixterm 2862 1174 3644 2910 dtterm 2813 1174 3705 19334 dtterm 2813 1174 3704 13664 dtterm 2804 1174 3706 17520 aixterm 2801 1174 3619 Pid: 15012 Command: maker4X.exe Segid Type Description Inuse Pin Pgspace Address Range 1572 pers /dev/hd3:62 0 0 0 0..-1 142 pers /dev/hd3:51 0 0 0 0..-1 1bde pers /dev/hd3:50 0 0 0 0..-1 2c1 pers /dev/hd3:49 1 0 0 0..7 9ab pers /dev/hd2:53289 1 0 0 0..0 404 work kernel extension 27 27 0 0..24580 1d9b work lib data 39 0 23 0..607 909 work shared library text 864 0 7 0..65535 5a3 work sreg[4] 9 0 12 0..32768 1096 work sreg[3] 32 0 32 0..32783 1b9d work private 1057 1 1219 0..1306 : 65307..65535 1af8 clnt 961 0 0 0..1716 0 work kernel 1792 1146 3488 0..32767 : 32768..65535 ... The output is divided into summary and detail sections. The summary section lists the top ten highest memory-usage processes in descending order. Pid 15012 is the process ID that has the highest memory usage. The Command indicates the command name, in this case maker4X.exe. The Inuse column (total number of pages in real memory from segments that are used by the process) shows 4783 pages (each page is 4 KB). The Pin column (total number of pages pinned from segments that are used by the process) shows 1174 pages. The Pgspace column (total number of paging-space pages that are used by the process) shows 4781 pages. The detailed section displays information about each segment for each process that is shown in the summary section. This includes the segment ID, the type of the segment, description (a textual description of the segment, including the volume name and i-node of the file for persistent segments), number of pages in RAM, number of pinned pages in RAM, number of pages in paging space, and address range. The Address Range specifies one range for a persistent or client segment and two ranges for a working segment. The range for a persistent or a client segment takes the form '0..x,' where x is the maximum number of virtual pages that have been used. The range field for a working segment can be '0..x : y..65535', where 0..x contains global data and grows upward, and y..65535 contains stack area and grows downward. For the address range, in a working segment, space is allocated starting from both ends and working towards the middle. If the working segment is non-private (kernel or shared library), space is allocated differently. In this example, the segment ID 1b9d is a private working segment; its address range is 0..1306 : 65307..65535. The segment ID 909 is a shared library text working segment; its address range is 0..65535. A segment can be used by multiple processes. Each page in real memory from such a segment is accounted for in the Inuse field for each process using that segment. Thus, the total for Inuse may exceed the total number of pages in real memory. The same is true for the Pgspace and Pin fields. The sum of Inuse, Pin, and Pgspace of all segments of a process is equal to the numbers in the summary section. You can use one of the following commands to display the file name associated with the i-node: * ncheck -i i-node_number volume_name * find file_system_associated_with_lv_name -xdev -inum inode_number -print To get a similar output in AIX 4.3.3 and later, use the following command: # svmon -Put 10 ------------------------------------------------------------------------------ Pid Command Inuse Pin Pgsp Virtual 64-bit Mthrd 2164 X 15535 1461 34577 37869 N N Vsid Esid Type Description Inuse Pin Pgsp Virtual Addr Range 1966 2 work process private 9984 4 31892 32234 0..32272 : 65309..65535 4411 d work shared library text 3165 0 1264 1315 0..65535 0 0 work kernel seg 2044 1455 1370 4170 0..32767 : 65475..65535 396e 1 pers code,/dev/hd2:18950 200 0 - - 0..706 2ca3 - work 32 0 0 32 0..32783 43d5 - work 31 0 6 32 0..32783 2661 - work 29 0 0 29 0..32783 681f - work 29 0 25 29 0..32783 356d f work shared library data 18 0 18 24 0..310 34e8 3 work shmat/mmap 2 2 2 4 0..32767 5c97 - pers /dev/hd4:2 1 0 - - 0..0 5575 - pers /dev/hd2:19315 0 0 - - 0..0 4972 - pers /dev/hd2:19316 0 0 - - 0..5 4170 - pers /dev/hd3:28 0 0 - - 0..0 755d - pers /dev/hd9var:94 0 0 - - 0..0 6158 - pers /dev/hd9var:90 0 0 - - 0..0 ------------------------------------------------------------------------------ Pid Command Inuse Pin Pgsp Virtual 64-bit Mthrd 25336 austin.ibm. 12466 1456 2797 11638 N N Vsid Esid Type Description Inuse Pin Pgsp Virtual Addr Range 14c3 2 work process private 5644 1 161 5993 0..6550 : 65293..65535 4411 d work shared library text 3165 0 1264 1315 0..65535 0 0 work kernel seg 2044 1455 1370 4170 0..32767 : 65475..65535 13c5 1 clnt code 735 0 - - 0..4424 d21 - pers /dev/andy:563 603 0 - - 0..618 9e6 f work shared library data 190 0 2 128 0..3303 942 - pers /dev/cache:16 43 0 - - 0..42 2ca3 - work 32 0 0 32 0..32783 49f0 - clnt 10 0 - - 0..471 1b07 - pers /dev/andy:8568 0 0 - - 0..0 623 - pers /dev/hd2:22539 0 0 - - 0..1 2de9 - clnt 0 0 - - 0..0 1541 5 mmap mapped to sid 761b 0 0 - - 5d15 - pers /dev/andy:487 0 0 - - 0..3 4513 - pers /dev/andy:486 0 0 - - 0..45 cc4 4 mmap mapped to sid 803 0 0 - - 242a - pers /dev/andy:485 0 0 - - 0..0 ... The Vsid column is the virtual segment ID, and the Esid column is the effective segment ID. The effective segment ID reflects the segment register that is used to access the corresponding pages. Detailed Information on a Specific Segment ID The -D option displays detailed memory-usage statistics for segments. # svmon -D 404 Segid: 404 Type: working Description: kernel extension Address Range: 0..24580 Size of page space allocation: 0 pages ( 0.0 Mb) Inuse: 28 frames ( 0.1 Mb) Page Frame Pin Ref Mod 12294 3320 pin ref mod 24580 1052 pin ref mod 12293 52774 pin ref mod 24579 20109 pin ref mod 12292 19494 pin ref mod 12291 52108 pin ref mod 24578 50685 pin ref mod 12290 51024 pin ref mod 24577 1598 pin ref mod 12289 35007 pin ref mod 24576 204 pin ref mod 12288 206 pin ref mod 4112 53007 pin mod 4111 53006 pin mod 4110 53005 pin mod 4109 53004 pin mod 4108 53003 pin mod 4107 53002 pin mod 4106 53001 pin mod 4105 53000 pin mod 4104 52999 pin mod 4103 52998 pin mod 4102 52997 pin mod 4101 52996 pin mod 4100 52995 pin mod 4099 52994 pin mod 4098 52993 pin mod 4097 52992 pin ref mod The detail columns are explained as follows: Page Specifies the index of the page within the segment. Frame Specifies the index of the real memory frame that the page resides in. Pin Specifies a flag indicating whether the page is pinned. Ref Specifies a flag indicating whether the page's reference bit is on. Mod Specifies a flag indicating whether the page is modified. The size of page space allocation is 0 because all the pages are pinned in real memory. An example output from AIX 4.3.3 and later, is very similar to the following: # svmon -D 629 -b Segid: 629 Type: working Address Range: 0..77 Size of page space allocation: 7 pages ( 0.0 Mb) Virtual: 11 frames ( 0.0 Mb) Inuse: 7 frames ( 0.0 Mb) Page Frame Pin Ref Mod 0 32304 N Y Y 3 32167 N Y Y 7 32321 N Y Y 8 32320 N Y Y 5 32941 N Y Y 1 48357 N N Y 77 47897 N N Y The -b flag shows the status of the reference and modified bits of all the displayed frames. After it is shown, the reference bit of the frame is reset. When used with the -i flag, it detects which frames are accessed between each interval. Note: Use this flag with caution because of its performance impacts. List of Top Memory Usage of Segments The -S option is used to sort segments by memory usage and to display the memory-usage statistics for the top memory-usage segments. If count is not specified, then a count of 10 is implicit. The following command sorts system and non-system segments by the number of pages in real memory and prints out the top 10 segments of the resulting list. # svmon -Sau Segid Type Description Inuse Pin Pgspace Address Range 0 work kernel 1990 1408 3722 0..32767 : 32768..65535 1 work private, pid=4042 1553 1 1497 0..1907 : 65307..65535 1435 work private, pid=3006 1391 3 1800 0..4565 : 65309..65535 11f5 work private, pid=14248 1049 1 1081 0..1104 : 65307..65535 11f3 clnt 991 0 0 0..1716 681 clnt 960 0 0 0..1880 909 work shared library text 900 0 8 0..65535 101 work vmm data 497 496 1 0..27115 : 43464..65535 a0a work shared library data 247 0 718 0..65535 1bf9 work private, pid=21094 221 1 320 0..290 : 65277..65535 All output fields are described in the previous examples. An example output from AIX 4.3.3 and later is similar to the following: # svmon -Sut 10 Vsid Esid Type Description Inuse Pin Pgsp Virtual Addr Range 1966 - work 9985 4 31892 32234 0..32272 : 65309..65535 14c3 - work 5644 1 161 5993 0..6550 : 65293..65535 5453 - work 3437 1 2971 4187 0..4141 : 65303..65535 4411 - work 3165 0 1264 1315 0..65535 5a1e - work 2986 1 13 2994 0..3036 : 65295..65535 340d - work misc kernel tables 2643 0 993 2645 0..15038 : 63488..65535 380e - work kernel pinned heap 2183 1055 1416 2936 0..65535 0 - work kernel seg 2044 1455 1370 4170 0..32767 : 65475..65535 6afb - pers /dev/notes:92 1522 0 - - 0..10295 2faa - clnt 1189 0 - - 0..2324 Correlating svmon and vmstat Outputs There are some relationships between the svmon and vmstat outputs. The svmon report of AIX 4.3.2 follows (the example is the same with AIX 4.3.3 and later, although the output format is different): # svmon -G m e m o r y i n u s e p i n p g s p a c e size inuse free pin work pers clnt work pers clnt size inuse 16384 16254 130 2016 11198 2537 2519 2016 0 0 40960 13392 The vmstat command was run in a separate window while the svmon command was running. The vmstat report follows: # vmstat 5 kthr memory page faults cpu ----- ----------- ------------------------ ------------ ----------- r b avm fre re pi po fr sr cy in sy cs us sy id wa 0 0 13392 130 0 0 0 0 2 0 125 140 36 2 1 97 0 0 0 13336 199 0 0 0 0 0 0 145 14028 38 11 22 67 0 0 0 13336 199 0 0 0 0 0 0 141 49 31 1 1 98 0 0 0 13336 199 0 0 0 0 0 0 142 49 32 1 1 98 0 0 0 13336 199 0 0 0 0 0 0 145 49 32 1 1 99 0 0 0 13336 199 0 0 0 0 0 0 163 49 33 1 1 92 6 0 0 13336 199 0 0 0 0 0 0 142 49 32 0 1 98 0 The global svmon report shows related numbers. The vmstatfre column relates to the svmon memory free column. The number that vmstat reports as Active Virtual Memory (avm) is reported by the svmon command as pg space inuse (13392). The vmstat avm column provides the same figures as the pg space inuse column of the svmon command except starting with AIX 4.3.2 where Deferred Page Space Allocation is used. In that case, the svmon command shows the number of pages actually paged out to paging space whereas the vmstat command shows the number of virtual pages accessed but not necessarily paged out (see Looking at Paging Space and Virtual Memory). Correlating svmon and ps Outputs There are some relationships between the svmon and ps outputs. The svmon report of AIX 4.3.2 follows (the example is the same with AIX 4.3.3 and later, although the output format is different): # svmon -P 7226 Pid Command Inuse Pin Pgspace 7226 telnetd 936 1 69 Pid: 7226 Command: telnetd Segid Type Description Inuse Pin Pgspace Address Range 828 pers /dev/hd2:15333 0 0 0 0..0 1d3e work lib data 0 0 28 0..559 909 work shared library text 930 0 8 0..65535 1cbb work sreg[3] 0 0 1 0..0 1694 work private 6 1 32 0..24 : 65310..65535 12f6 pers code,/dev/hd2:69914 0 0 0 0..11 Compare with the ps report, which follows: # ps v 7226 PID TTY STAT TIME PGIN SIZE RSS LIM TSIZ TRS %CPU %MEM COMMAND 7226 - A 0:00 51 240 24 32768 33 0 0.0 0.0 telnetd SIZE refers to the virtual size in KB of the data section of the process (in paging space). This number is equal to the number of working segment pages of the process that have been touched (that is, the number of paging-space pages that have been allocated) times 4. It must be multiplied by 4 because pages are in 4 K units and SIZE is in 1 K units. If some working segment pages are currently paged out, this number is larger than the amount of real memory being used. The SIZE value (240) correlates with the Pgspace number from the svmon command for private (32) plus lib data (28) in 1 K units. RSS refers to the real memory (resident set) size in KB of the process. This number is equal to the sum of the number of working segment and code segment pages in memory times 4. Remember that code segment pages are shared among all of the currently running instances of the program. If 26 ksh processes are running, only one copy of any given page of the ksh executable program would be in memory, but the ps command would report that code segment size as part of the RSS of each instance of the ksh program. The RSS value (24) correlates with the Inuse numbers from the svmon command for private (6) working-storage segments, for code (0) segments, and for lib data (0) of the process in 1-K units. TRS refers to the size of the resident set (real memory) of text. This is the number of code segment pages times four. As was noted earlier, this number exaggerates memory use for programs of which multiple instances are running. This does not include the shared text of the process. The TRS value (0) correlates with the number of the svmon pages in the code segment (0) of the Inuse column in 1 K units. The TRS value can be higher than the TSIZ value because other pages, such as the XCOFF header and the loader section, may be included in the code segment. The following calculations can be made for the values mentioned: SIZE = 4 * Pgspace of (work lib data + work private) RSS = 4 * Inuse of (work lib data + work private + pers code) TRS = 4 * Inuse of (pers code) Calculating the Minimum Memory Requirement of a Program To calculate the minimum memory requirement of a program, the formula would be: Total memory pages (4 KB units) = T + ( N * ( PD + LD ) ) + F where: T = Number of pages for text (shared by all users) N = Number of copies of this program running simultaneously PD = Number of working segment pages in process private segment LD = Number of shared library data pages used by the process F = Number of file pages (shared by all users) Multiply the result by 4 to obtain the number of kilobytes required. You may want to add in the kernel, kernel extension, and shared library text segment values to this as well even though they are shared by all processes on the system. For example, some applications like CATIA and databases use very large shared library modules. Note that because we have only used statistics from a single snapshot of the process, there is no guarantee that the value we get from the formula will be the correct value for the minimum working set size of a process. To get working set size, one would need to run a tool such as the rmss command or take many snapshots during the life of the process and determine the average values from these snapshots (see Assessing Memory Requirements Through the rmss Command). If we estimate the minimum memory requirement for the program pacman, shown in Finding Memory-Leaking Programs, the formula would be: T = 2 (Inuse of code,/dev/lv01:12302 of pers) PD = 1632 (Inuse of private of work) LD = 12 (Inuse of lib data of work) F = 1 (Inuse of /dev/hd2:53289 of pers That is: 2 + (N * (1632+ 12)) + 1, equal to 1644 * N + 3 in 4 KB units.

需要注意一点是，svmon会将UNIX上的文件系统缓存对应到曾经申请过这些文件页的进程身上。可笑的是，这些文件系统缓存是不受Oracle本身控制的，他既不是PGA亦不是SGA,这些缓存是受AIX操作系统分配并被排他式地控制着(controlled exclusively).以缓存文件为目的的这部分内存不在我们考虑的Oracle内存使用问题的范畴内，因为这部分内存实际是被AIX所支配着，与我们讨论的PGA/SGA没有联系，如果我们的环境中全部是裸设备(raw device)的话(当然这不太可能),就不存在大量文件系统缓存的问题了。当然这也不意味着这部分在我们考虑总的内存使用时被忽略或漠视，因为这部分文件系统缓存同样会消耗大量物理内存并可能引起不必要的换页操作。我们可以通过"svmon -Pau 10"来了解这部分内存的使用状况；在AIX上著名的性能调优工具virtual memory optimizer,原先的vmtume，现在的vmo工具，可以帮助我们调节文件系统内存的具体阀值如 maxperm,minperm,strict_maxperm(这里不做展开)。有兴趣的话可以参考下面引用的这篇文档:

Tuning VMM Page Replacement with the vmtune Command The memory management algorithm, discussed in Real-Memory Management, tries to keep the size of the free list and the percentage of real memory occupied by persistent segment pages within specified bounds. These bounds can be altered with the vmtune command, which can only be run by the root user. Changes made by this tool remain in effect until the next reboot of the system. To determine whether the vmtune command is installed and available, run the following command: # lslpp -lI bos.adt.samples Note: The vmtune command is in the samples directory because it is very VMM-implementation dependent. The vmtune code that accompanies each release of the operating system is tailored specifically to the VMM in that release. Running the vmtune command from one release on a different release might result in an operating-system failure. It is also possible that the functions of vmtune may change from release to release. Do not propagate shell scripts or /etc/inittab entries that include the vmtune command to a new release without checking the vmtune documentation for the new release to make sure that the scripts will still have the desired effect. Executing the vmtune command on AIX 4.3.3 with no options results in the following output: # /usr/samples/kernel/vmtune vmtune: