vmtouch－系统pagecache查看和操纵器
这个工具很好使，对查找定位问题很有帮助，特别是对大量使用mmap的系统，比如搜索引擎中的索引。
vmtouch的源文件只有一个c文件，很简单；主要使用了两个系统函数
1. int mincore(void *start, size_t length, unsigned char *vec): get information on whether pages are in core
判断文件数据页是否已经存在于内存中，使用这个函数可以知道一个文件有多少数据已经在page-cache中了。
2. int posix_fadvise(int fd, off_t offset, off_t len, int advice): predeclare an access pattern for file data
声明文件的预访问模式，其中有一个模式是POSIX_FADV_DONTNEED，使用这个模式，系统会尽量把该文件的数据从page-cache中踢出去。
有个朋友原来也写了一个类似工具，原理完全一样，只差没开源呐。
转载自：http://blog.yufeng.info/archives/1903
今天看到dbanote这篇博文介绍的技术清单，里面列到了很多有意思的技术。其中提到了一个小工具vmtouch，主页见这里。
vmtouch – the Virtual Memory Toucher
Portable file system cache diagnostics and control
vmtouch is a tool for learning about and controlling the file system cache of unix and unix-like systems. It is BSD licensed so you can basically do whatever you want with it.
我之前写过不少之类的博文，但是基本都是用systemtap,工具也零碎，现在vmtouch比较系统的把功能整合在一起，源码写的虽然简单，但是用到了还是挺管用的。
顺手贴了下vmtouch的使用:
Example 1
How much of the /bin/ directory is currently in cache?
$ vmtouch /bin/
Files: 92
Directories: 1
Resident Pages: 348/1307 1M/5M 26.6%
Elapsed: 0.003426 seconds
Example 2
How much of big-dataset.txt is currently in memory?
$ vmtouch -v big-dataset.txt
big-dataset.txt
[ ] 0/42116
Files: 1
Directories: 0
Resident Pages: 0/42116 0/164M 0%
Elapsed: 0.005182 seconds
None of it. Now let’s bring part of it into memory with tail:
$ tail -n 10000 big-dataset.txt > /dev/null
Now how much?
$ vmtouch -v big-dataset.txt
big-dataset.txt
[ oOOOOOOO] 4950/42116
Files: 1
Directories: 0
Resident Pages: 4950/42116 19M/164M 11.8%
Elapsed: 0.006706 seconds
vmtouch tells us that 4950 pages at the end of the file are now resident in memory.
Example 3
Let’s bring the rest of big-dataset.txt into memory (pressing enter a few times to illustrate the animated progress bar you will see on your terminal):
$ vmtouch -vt big-dataset.txt
big-dataset.txt
[OOo oOOOOOOO] 6887/42116
[OOOOOOOOo oOOOOOOO] 10631/42116
[OOOOOOOOOOOOOOo oOOOOOOO] 15351/42116
[OOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 19719/42116
[OOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 24183/42116
[OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 28615/42116
[OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 31415/42116
[OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 36775/42116
[OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOo oOOOOOOO] 39431/42116
[OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO] 42116/42116
Files: 1
Directories: 0
Touched Pages: 42116 (164M)
Elapsed: 12.107 seconds
Example 4
We have 3 big datasets, a.txt, b.txt, and c.txt but only 2 of them will fit in memory at once. If we have a.txt and b.txt in memory but would now like to work with b.txt and c.txt, we could just start loading up c.txt but then our system would evict pages from both a.txt (which we want) and b.txt (which we don’t want).
So let’s give the system a hint and evict a.txt from memory, making room for c.txt:
$ vmtouch -ve a.txt
Evicting a.txt
Files: 1
Directories: 0
Evicted Pages: 42116 (164M)
Elapsed: 0.076824 seconds
Example 5
Daemonise and lock all files in a directory into physical memory:
vmtouch -dl /var/www/htdocs/critical/
祝玩得开心！
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