/proc/meminfo詳解

/proc/meminfo Explained "Free," "buffer," "swap," "dirty." What does it all mean? If you said, "something to do with the Summer of '68", you may need a primer on 'meminfo'. The entries in the /proc/meminfo can help explain what's going on with your memory usage, if you know how to read it. Example of "cat /proc/meminfo": root: total: used: free: shared: buffers: cached: Mem: 1055760384 1041887232 13873152 0 100417536 711233536 Swap: 1077501952 8540160 1068961792 MemTotal: 1031016 kB MemFree: 13548 kB MemShared: 0 kB Buffers: 98064 kB Cached: 692320 kB SwapCached: 2244 kB Active: 563112 kB Inact_dirty: 309584 kB Inact_clean: 79508 kB Inact_target: 190440 kB HighTotal: 130992 kB HighFree: 1876 kB LowTotal: 900024 kB LowFree: 11672 kB SwapTotal: 1052248 kB SwapFree: 1043908 kB Committed_AS: 332340 kB The information comes in the form of both high-level and low-level statistics. At the top you see a quick summary of the most common values people would like to look at. Below you find the individual values we will discuss. First we will discuss the high-level statistics. High-Level Statistics MemTotal: Total usable ram (i.e. physical ram minus a few reserved bits and the kernel binary code) MemFree: Is sum of LowFree+HighFree (overall stat) MemShared: 0; is here for compat reasons but always zero. Buffers: Memory in buffer cache. mostly useless as metric nowadays Cached: Memory in the pagecache (diskcache) minus SwapCache SwapCache: Memory that once was swapped out, is swapped back in but still also is in the swapfile (if memory is needed it doesn't need to be swapped out AGAIN because it is already in the swapfile. This saves I/O) Detailed Level Statistics VM Statistics VM splits the cache pages into "active" and "inactive" memory. The idea is that if you need memory and some cache needs to be sacrificed for that, you take it from inactive since that's expected to be not used. The vm checks what is used on a regular basis and moves stuff around. When you use memory, the CPU sets a bit in the pagetable and the VM checks that bit occasionally, and based on that, it can move pages back to active. And within active there's an order of "longest ago not used" (roughly, it's a little more complex in reality). The longest-ago used ones can get moved to inactive. Inactive is split into two in the above kernel (2.4.18-24.8.0). Some have it three. Active: Memory that has been used more recently and usually not reclaimed unless absolutely necessary. Inact_dirty: Dirty means "might need writing to disk or swap." Takes more work to free. Examples might be files that have not been written to yet. They aren't written to memory too soon in order to keep the I/O down. For instance, if you're writing logs, it might be better to wait until you have a complete log ready before sending it to disk. Inact_clean: Assumed to be easily freeable. The kernel will try to keep some clean stuff around always to have a bit of breathing room. Inact_target: Just a goal metric the kernel uses for making sure there are enough inactive pages around. When exceeded, the kernel will not do work to move pages from active to inactive. A page can also get inactive in a few other ways, e.g. if you do a long sequential I/O, the kernel assumes you're not going to use that memory and makes it inactive preventively. So you can get more inactive pages than the target because the kernel marks some cache as "more likely to be never used" and lets it cheat in the "last used" order. Memory Statistics HighTotal: is the total amount of memory in the high region. Highmem is all memory above (approx) 860MB of physical RAM. Kernel uses indirect tricks to access the high memory region. Data cache can go in this memory region. LowTotal: The total amount of non-highmem memory. LowFree: The amount of free memory of the low memory region. This is the memory the kernel can address directly. All kernel datastructures need to go into low memory. SwapTotal: Total amount of physical swap memory. SwapFree: Total amount of swap memory free. Committed_AS: An estimate of how much RAM you would need to make a 99.99% guarantee that there never is OOM (out of memory) for this workload. Normally the kernel will overcommit memory. That means, say you do a 1GB malloc, nothing happens, really. Only when you start USING that malloc memory you will get real memory on demand, and just as much as you use. So you sort of take a mortgage and hope the bank doesn't go bust. Other cases might include when you mmap a file that's shared only when you write to it and you get a private copy of that data. While it normally is shared between processes. The Committed_AS is a guesstimate of how much RAM/swap you would need worst-case. 在Linux下檢視記憶體我們一般用free指令： [[email protected] tmp]# free              total       used       free     shared    buffers     cached Mem:       3266180    3250004      16176          0     110652    2668236 -/+ buffers/cache:     471116    2795064 Swap:      2048276      80160    1968116 下面是對這些數值的解釋： total:總計實體記憶體的大小。 used:已使用多大。 free:可用有多少。 Shared:多個程序共享的記憶體總額。 Buffers/cached:磁盤緩存的大小。 第三行(-/+ buffers/cached): used:已使用多大。 free:可用有多少。 第四行就不多解釋了。 區 别：第二行(mem)的used/free與第三行(-/+ buffers/cache) used/free的差別。這兩個的差別在于使用的角度來看，第一行是從OS的角度來看，因為對于OS，buffers/cached 都是屬于被使用，是以他的可用記憶體是16176KB,已用記憶體是3250004KB,其中包括，核心（OS）使用+Application(X, oracle,etc)使用的+buffers+cached. 第三行所指的是從應用程式角度來看，對于應用程式來說，buffers/cached 是等于可用的，因為buffer/cached是為了提高檔案讀取的性能，當應用程式需在用到記憶體的時候，buffer/cached會很快地被回收。 是以從應用程式的角度來說，可用記憶體=系統free memory+buffers+cached。 如上例： 2795064=16176+110652+2668236 接下來解釋什麼時候記憶體會被交換，以及按什麼方交換。 當可用記憶體少于額定值的時候，就會開會進行交換。 如何看額定值： cat /proc/meminfo [[email protected] tmp]# cat /proc/meminfo MemTotal:      3266180 kB MemFree:         17456 kB Buffers:        111328 kB Cached:        2664024 kB SwapCached:          0 kB Active:         467236 kB Inactive:      2644928 kB HighTotal:           0 kB HighFree:            0 kB LowTotal:      3266180 kB LowFree:         17456 kB SwapTotal:     2048276 kB SwapFree:      1968116 kB Dirty:               8 kB Writeback:           0 kB Mapped:         345360 kB Slab:           112344 kB Committed_AS:   535292 kB PageTables:       2340 kB VmallocTotal: 536870911 kB VmallocUsed:    272696 kB VmallocChunk: 536598175 kB HugePages_Total:     0 HugePages_Free:      0 Hugepagesize:     2048 kB 用free -m檢視的結果： [[email protected] tmp]# free -m              total       used       free     shared    buffers     cached Mem:          3189       3173         16          0        107       2605 -/+ buffers/cache:        460       2729 Swap:         2000         78       1921 檢視/proc/kcore檔案的大小（記憶體鏡像）： [[email protected] tmp]# ll -h /proc/kcore -r-------- 1 root root 4.1G Jun 12 12:04 /proc/kcore 備注： 占用記憶體的測量 測量一個程序占用了多少記憶體，linux為我們提供了一個很友善的方法，/proc目錄為我們提供了所有的資訊，實際上top等工具也通過這裡來擷取相應的資訊。 /proc/meminfo 機器的記憶體使用資訊 /proc/pid/maps pid為程序号，顯示目前程序所占用的虛拟位址。 /proc/pid/statm 程序所占用的記憶體 [[email protected] ~]# cat /proc/self/statm 654 57 44 0 0 334 0 輸出解釋 CPU 以及CPU0。。。的每行的每個參數意思（以第一行為例）為： 參數 解釋 /proc//status Size (pages) 任務虛拟位址空間的大小 VmSize/4 Resident(pages) 應用程式正在使用的實體記憶體的大小 VmRSS/4 Shared(pages) 共享頁數 0 Trs(pages) 程式所擁有的可執行虛拟記憶體的大小 VmExe/4 Lrs(pages) 被映像到任務的虛拟記憶體空間的庫的大小 VmLib/4 Drs(pages) 程式資料段和使用者态的棧的大小 （VmData+ VmStk ）4 dt(pages) 04 檢視機器可用記憶體 /proc/28248/>free total used free shared buffers cached Mem: 1023788 926400 97388 0 134668 503688 -/+ buffers/cache: 288044 735744 Swap: 1959920 89608 1870312 我們通過free指令檢視機器空閑記憶體時，會發現free的值很小。這主要是因為，在linux中有這麼一種思想，記憶體不用白不用，是以它盡可能的cache和buffer一些資料，以友善下次使用。但實際上這些記憶體也是可以立刻拿來使用的。 是以 空閑記憶體=free+buffers+cached=total-used 轉自：http://blog.chinaunix.net/u2/78225/showart_1727609.html