C:知道何时读取或写入变量

Question

目前在 arm Xilinx 开发平台上工作。使用 linux 和 C。

我映射了一些页面。例如a = mmap(...) 。我想知道哪些页面最近最少使用，以便我可以将它们复制到其他地方。

Answer 1

Linux 内核本身会跟踪 LRU 页面，例如，找出哪些页面在内存争用时要交换或不交换。

如果你想在你的库中使用它，你可以从用户空间读取 LRU 标志:

• 在/proc/pid/pagemap 中遍历要跟踪的虚拟页面条目并记住页面框架编号
• 从/proc/kpageflags中读取这些页面对应的flags

现在你有几个标志，你可以将它们用于你的逻辑:

 3. UPTODATE  page has up-to-date data
              ie. for file backed page: (in-memory data revision >= on-disk one)
 4. DIRTY     page has been written to, hence contains new data
              ie. for file backed page: (in-memory data revision >  on-disk one)
 8. WRITEBACK page is being synced to disk

    [LRU related page flags]
 5. LRU         page is in one of the LRU lists
 6. ACTIVE      page is in the active LRU list
18. UNEVICTABLE page is in the unevictable (non-)LRU list
                It is somehow pinned and not a candidate for LRU page reclaims,
        eg. ramfs pages, shmctl(SHM_LOCK) and mlock() memory segments
 2. REFERENCED  page has been referenced since last LRU list enqueue/requeue
 9. RECLAIM     page will be reclaimed soon after its pageout IO completed
11. MMAP        a memory mapped page
12. ANON        a memory mapped page that is not part of a file
13. SWAPCACHE   page is mapped to swap space, ie. has an associated swap entry
14. SWAPBACKED  page is backed by swap/RAM

更多标志在 this document 中描述

还有一个名为 soft-dirty 的功能，它专门用于跟踪最近的写入(忽略读取)(请参阅 Soft Dirty 文档)。您可以通过/proc 清除软脏标志，然后通过/proc/pid/pagemap 再次读取它们。也许这对您的应用程序也很有用。