How can I write a program to trigger memory direct reclaim ?

 Directly triggering memory direct reclaim in a program is generally not recommended, as the Linux kernel's memory management should handle memory reclamation automatically. However, if you have a specific use case that requires manual memory reclamation, you can use the `madvise` system call or `mincore` to influence memory management behavior. Here's a basic example using `madvise` to advise the kernel to reclaim memory:

```c

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <sys/mman.h>

int main() {

    // Allocate memory

    size_t size = 100 * 1024 * 1024; // 100 MB

    void* mem = malloc(size);

    if (mem == NULL) {

        perror("malloc");

        exit(1);

    }

    // Use the memory (touch it)

    for (size_t i = 0; i < size; i += getpagesize()) {

        ((char*)mem)[i] = 0;

    }

    // Advise the kernel to reclaim memory

    madvise(mem, size, MADV_DONTNEED);

    // You can also use `madvise` with other flags for different behaviors

    // MADV_WILLNEED advises that the data will be accessed soon

    // MADV_REMOVE advises the kernel to remove the data from the page cache

    free(mem); // Free the memory

    return 0;

}

```

In this example, we allocate a large block of memory, touch it (to make sure it's in physical RAM), and then use `madvise` with the `MADV_DONTNEED` flag to advise the kernel to reclaim the memory. Please note that this is a simplified example, and in real-world applications, you would need to be cautious about when and how you use `madvise` to avoid performance issues and potential data loss.

Manual memory reclamation is generally discouraged because the kernel's memory management system is optimized to handle memory efficiently. You should only consider this approach if you have a specific use case or requirement that necessitates manual control over memory reclamation.