shrink_mmap() change in ac-21

Zlatko Calusic
2000-06-19T20:14:52Z

Hi, Alan and others!

The shrink_mmap() change in your latest prepatch (ac12) doesn't look
very healthy. Removing the test for the wrong zone we effectively
discard lots of wrong pages before we get to the right one. That is
effectively flushing the page cache and we have unbalanced system.

For example, check the "vmstat 1" output below, done while I was
reading a big file from the disk. At some point in time, the page
cache shrunk to almost half of its size (75MB -> 42MB).

The reason is balancing of the DMA zone (which is much smaller on a
128MB machine than the NORMAL zone!). shrink_mmap() now happily evicts
wrong pages from the memory and continues doing so until it finally
frees enough pages from the DMA zone. That, of course, hurts caching
as the page cache gets shrunk a lot without a good reason.

procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 1 0 988 5556 172 78708 0 0 3811 0 342 859 0 6 93
0 1 0 1040 9712 176 74640 0 168 3043 42 309 801 0 6 93
0 1 0 1084 7272 184 76800 0 408 2659 102 317 762 0 7 93
0 1 0 1084 8704__ 212 75308 0 0 2730 0 285 782 0 7 93
3 0 0 1084 42400 \ 192 42780 0 0 2447 0 270 703 0 8 91
0 1 0 1084 30092 | 204 54684 0 0 2979 0 299 767 1 4 95
\
here! :)

The incriminating change is:

Index: 24001.28/mm/filemap.c
--- 24001.28/mm/filemap.c Wed, 14 Jun 2000 01:44:09 +0200 zcalusic (linux/F/b/16_filemap.c 1.6.1.3.2.4.1.1.2.2.2.1.1.21.1.1.3.2.3.1.4.1 644)
+++ 24001.31(w)/mm/filemap.c Sun, 18 Jun 2000 21:23:47 +0200 zcalusic (linux/F/b/16_filemap.c 1.6.1.3.2.4.1.1.2.2.2.1.1.21.1.1.3.2.3.1.4.2 644)
@@ -361,16 +361,6 @@
}
}

- /*
- * Page is from a zone we don't care about.
- * Don't drop page cache entries in vain.
- */
- if (page->zone->free_pages > page->zone->pages_high) {
- /* the page from the wrong zone doesn't count */
- count++;
- goto unlock_continue;
- }
-
/* Take the pagecache_lock spinlock held to avoid
other tasks to notice the page while we are looking at its
page count. If it's a pagecache-page we'll free it

Regards,
--
Zlatko