Red Hat Bugzilla – Bug 185422
Zone balancing issue when low on highmem
Last modified: 2010-10-22 00:38:02 EDT
kernel-2.4.21-37.0.1.EL On an x86 machine with 1GB of memory (or with a 1GB of simulated memory, using the mem= kernel option), filling the pagecache and after that using a memory intensive application will evict running processes to the swap, instead of pruning the pagecache. 1. Create a 1.5GB file 2. run "cat bigfile > /dev/null" 3. run the attached test case 4. check the amount of swapped data: - with mem=890M (no HighMem zone), there's nothing in the swap - with mem=1G (129 megs in HighMem), there's about 2.6 megs of data in the swap Full test results also attached.
Created attachment 126111 [details] test.c
Created attachment 126112 [details] test-results.tgz
To jfautley, the latest RHEL3 is U7 (released yesterday), kernel version 2.4.21-40.EL. We are now doing development for U8, which is where any potential hot fix would be based. But there is no "previously-mentioned patch" in this bug report.
Sorry - my bad. I sent this to both bugzillas from issue-tracker. There is a patch for RHEL4 in Bug 185092. [See: https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=185092#c2]
The other bugzilla has nothing to do with this issue, so there wouldn't be much point in backporting it. Those issues shouldn't even be linked, and aren't anymore.
Created attachment 126919 [details] Patch that stops the swapping when the systeh should reclaim pagecache memory OK, the problem with the system is that the Highmem zone is only 1/8 of the system RAM where the Normal zone is the remaining 7/8. This combined with the fact that file system read and write operations deactivate pagecache pages after they are done with them results in the system swapping rather than reclaiming pagecache memory eventhough the total pagecache size is greater than /proc/sys/vm/pagecache.maxpercnet. After the creation of the large file the small Higmem zone is totally filled with pagecache pages although most of them are on the inactive_dirty list. When the test.c program is run the system quickly reclaims the pagecache pages from the Highmem zone and replaces them with anonymous memory pages from the memory region that test.c allocated. Once this occurs and the system runs out of memory it swaps out the Highmem because that zone is totally consumed with anonymous pages even though the Normal zon is not. To fix this, I added another test in launder_page() to reactivate anonymous pages if the total system pagecache size is less than pagecache.maxpercent. This fixes the problem described here but we still need to do significant performance testing before we can commit to addig this patch ro RHEL3-U8. Larry Woodman
This bug is filed against RHEL 3, which is in maintenance phase. During the maintenance phase, only security errata and select mission critical bug fixes will be released for enterprise products. Since this bug does not meet that criteria, it is now being closed. For more information of the RHEL errata support policy, please visit: http://www.redhat.com/security/updates/errata/ If you feel this bug is indeed mission critical, please contact your support representative. You may be asked to provide detailed information on how this bug is affecting you.