Re: [PATCH 2/2] mm/readahead: limit sync readahead while too many active refault

[Liu Shixin]

On 2024/2/2 1:31, Jan Kara wrote:
> On Thu 01-02-24 18:41:30, Liu Shixin wrote:
>> On 2024/2/1 17:37, Jan Kara wrote:
>>> On Thu 01-02-24 18:08:35, Liu Shixin wrote:
>>>> When the pagefault is not for write and the refault distance is close,
>>>> the page will be activated directly. If there are too many such pages in
>>>> a file, that means the pages may be reclaimed immediately.
>>>> In such situation, there is no positive effect to read-ahead since it will
>>>> only waste IO. So collect the number of such pages and when the number is
>>>> too large, stop bothering with read-ahead for a while until it decreased
>>>> automatically.
>>>>
>>>> Define 'too large' as 10000 experientially, which can solves the problem
>>>> and does not affect by the occasional active refault.
>>>>
>>>> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
>>> So I'm not convinced this new logic is needed. We already have
>>> ra->mmap_miss which gets incremented when a page fault has to read the page
>>> (and decremented when a page fault found the page already in cache). This
>>> should already work to detect trashing as well, shouldn't it? If it does
>>> not, why?
>>>
>>> 								Honza
>> ra->mmap_miss doesn't help, it increased only one in do_sync_mmap_readahead()
>> and then decreased one for every page in filemap_map_pages(). So in this scenario,
>> it can't exceed MMAP_LOTSAMISS.
> I see, OK. But that's a (longstanding) bug in how mmap_miss is handled. Can
> you please test whether attached patches fix the trashing for you? At least
> now I can see mmap_miss properly increments when we are hitting uncached
> pages...  Thanks!
>
> 								Honza
The patch doesn't seem to have much effect. I will try to analyze why it doesn't work.
The attached file is my testcase.

Thanks,
>

#!/bin/bash
  
while true; do
    flag=$(ps -ef | grep -v grep | grep alloc_page| wc -l)
    if [ "$flag" -eq 0 ]; then
        /alloc_page &
    fi

    sleep 30

    start_time=$(date +%s)
    yum install -y expect > /dev/null 2>&1

    end_time=$(date +%s)

    elapsed_time=$((end_time - start_time))

    echo "$elapsed_time seconds"
    yum remove -y expect > /dev/null 2>&1
done#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define SIZE 1*1024*1024 //1M

int main()
{
    void *ptr = NULL;
    int i;

    for (i = 0; i < 1024 * 6 - 50;i++) {
        ptr = (void *) malloc(SIZE);
        if (ptr == NULL) {
            printf("malloc err!");
            return -1;
        }

        memset(ptr, 0, SIZE);
    }

    sleep(99999);

    free(ptr);
    return 0;
}