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SubjectRe: [PATCH] mm: vmscan: scan anonymous pages on file refaults
On Mon 01-07-19 16:10:38, Kuo-Hsin Yang wrote:
> When file refaults are detected and there are many inactive file pages,
> the system never reclaim anonymous pages, the file pages are dropped
> aggressively when there are still a lot of cold anonymous pages and
> system thrashes. This issue impacts the performance of applications
> with large executable, e.g. chrome.
>
> With this patch, when file refault is detected, inactive_list_is_low()
> always returns true for file pages in get_scan_count() to enable
> scanning anonymous pages.
>
> The problem can be reproduced by the following test program.
>
> ---8<---
> void fallocate_file(const char *filename, off_t size)
> {
> struct stat st;
> int fd;
>
> if (!stat(filename, &st) && st.st_size >= size)
> return;
>
> fd = open(filename, O_WRONLY | O_CREAT, 0600);
> if (fd < 0) {
> perror("create file");
> exit(1);
> }
> if (posix_fallocate(fd, 0, size)) {
> perror("fallocate");
> exit(1);
> }
> close(fd);
> }
>
> long *alloc_anon(long size)
> {
> long *start = malloc(size);
> memset(start, 1, size);
> return start;
> }
>
> long access_file(const char *filename, long size, long rounds)
> {
> int fd, i;
> volatile char *start1, *end1, *start2;
> const int page_size = getpagesize();
> long sum = 0;
>
> fd = open(filename, O_RDONLY);
> if (fd == -1) {
> perror("open");
> exit(1);
> }
>
> /*
> * Some applications, e.g. chrome, use a lot of executable file
> * pages, map some of the pages with PROT_EXEC flag to simulate
> * the behavior.
> */
> start1 = mmap(NULL, size / 2, PROT_READ | PROT_EXEC, MAP_SHARED,
> fd, 0);
> if (start1 == MAP_FAILED) {
> perror("mmap");
> exit(1);
> }
> end1 = start1 + size / 2;
>
> start2 = mmap(NULL, size / 2, PROT_READ, MAP_SHARED, fd, size / 2);
> if (start2 == MAP_FAILED) {
> perror("mmap");
> exit(1);
> }
>
> for (i = 0; i < rounds; ++i) {
> struct timeval before, after;
> volatile char *ptr1 = start1, *ptr2 = start2;
> gettimeofday(&before, NULL);
> for (; ptr1 < end1; ptr1 += page_size, ptr2 += page_size)
> sum += *ptr1 + *ptr2;
> gettimeofday(&after, NULL);
> printf("File access time, round %d: %f (sec)\n", i,
> (after.tv_sec - before.tv_sec) +
> (after.tv_usec - before.tv_usec) / 1000000.0);
> }
> return sum;
> }
>
> int main(int argc, char *argv[])
> {
> const long MB = 1024 * 1024;
> long anon_mb, file_mb, file_rounds;
> const char filename[] = "large";
> long *ret1;
> long ret2;
>
> if (argc != 4) {
> printf("usage: thrash ANON_MB FILE_MB FILE_ROUNDS\n");
> exit(0);
> }
> anon_mb = atoi(argv[1]);
> file_mb = atoi(argv[2]);
> file_rounds = atoi(argv[3]);
>
> fallocate_file(filename, file_mb * MB);
> printf("Allocate %ld MB anonymous pages\n", anon_mb);
> ret1 = alloc_anon(anon_mb * MB);
> printf("Access %ld MB file pages\n", file_mb);
> ret2 = access_file(filename, file_mb * MB, file_rounds);
> printf("Print result to prevent optimization: %ld\n",
> *ret1 + ret2);
> return 0;
> }
> ---8<---
>
> Running the test program on 2GB RAM VM with kernel 5.2.0-rc5, the
> program fills ram with 2048 MB memory, access a 200 MB file for 10
> times. Without this patch, the file cache is dropped aggresively and
> every access to the file is from disk.
>
> $ ./thrash 2048 200 10
> Allocate 2048 MB anonymous pages
> Access 200 MB file pages
> File access time, round 0: 2.489316 (sec)
> File access time, round 1: 2.581277 (sec)
> File access time, round 2: 2.487624 (sec)
> File access time, round 3: 2.449100 (sec)
> File access time, round 4: 2.420423 (sec)
> File access time, round 5: 2.343411 (sec)
> File access time, round 6: 2.454833 (sec)
> File access time, round 7: 2.483398 (sec)
> File access time, round 8: 2.572701 (sec)
> File access time, round 9: 2.493014 (sec)
>
> With this patch, these file pages can be cached.
>
> $ ./thrash 2048 200 10
> Allocate 2048 MB anonymous pages
> Access 200 MB file pages
> File access time, round 0: 2.475189 (sec)
> File access time, round 1: 2.440777 (sec)
> File access time, round 2: 2.411671 (sec)
> File access time, round 3: 1.955267 (sec)
> File access time, round 4: 0.029924 (sec)
> File access time, round 5: 0.000808 (sec)
> File access time, round 6: 0.000771 (sec)
> File access time, round 7: 0.000746 (sec)
> File access time, round 8: 0.000738 (sec)
> File access time, round 9: 0.000747 (sec)

How does the reclaim behave with workloads with file backed data set
not fitting into the memory? Aren't we going to to swap a lot -
something that the heuristic is protecting from?

> Fixes: e9868505987a ("mm,vmscan: only evict file pages when we have plenty")
> Fixes: 7c5bd705d8f9 ("mm: memcg: only evict file pages when we have plenty")
> Signed-off-by: Kuo-Hsin Yang <vovoy@chromium.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Cc: <stable@vger.kernel.org> # 4.12+

--
Michal Hocko
SUSE Labs

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