Messages in this thread | | | Subject | Re: [PATCH 2/2] mm/readahead: limit sync readahead while too many active refault | From | Liu Shixin <> | Date | Fri, 2 Feb 2024 17:02:00 +0800 |
| |
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@huawei.com> >>> 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; }
| |