I think this mostly depends on what "cat /proc/sys/vm/zone_reclaim_mode" is set to by default on Ubuntu 12.04 ... If you have it at 1, then you should be able to reproduce it ... at least in theory ... because possible changes to the kernel might have already changed the behavior too. Here we have added it to our standard installation procedure, to always and explicitly set cat "zone_reclaim_mode=0".
-- You received this bug notification because you are a member of Kernel Packages, which is subscribed to linux in Ubuntu. https://bugs.launchpad.net/bugs/605773 Title: Wrong kernel setting zone_reclaim_mode leads to performance problems Status in “linux” package in Ubuntu: Incomplete Bug description: Binary package hint: linux-image-server -------------------------------------------------- Description: Ubuntu 10.04 LTS Release: 10.04 -------------------------------------------------- linux-image-server version: Installed: 2.6.32.22.23 -------------------------------------------------- WORKAROUND: setting: zone_reclaim_mode = 0 The background of this problem is - or how I discovered it - a migration of PostgreSQL database server from old hardware+old OS to a new hardware+new OS. Transition was no problem, but after we started using the server in production, we discovered a strange problem during nightly backups. The runtime of the backups went up from 2 1/2 hours to 6 1/2 hours (despite the fact, that the new hardware was designed to have much more power ... which positively showed up in most other tasks!). A longer research of the issue using the knowledge of many helpful guys on the PostgreSQL mailing list finally helped to find the reason for this slow down. It turned out to be a problem around the VM part of the kernel! Under some situations, where a lot of memory - for caching purposes - was consumed (which easily happens while backing up 100 GByte DBs), a congestion happened in the VM which slowed down the process dramatically. In depth analysis of many parts (vie /proc file system, ps, strace etc.) and comparing with settings on the old machines, I finally found an essential kernel setting, vm.zone_reclaim_mode, that was solely responsible for the issue. Luckily I could construct a simple test scenario (COPY-to-STDOU - exporting the data from a database table via stdout ... and writing this via pipe to the file system) where I could reproduce the issue. Our server had the value zone_reclaim_mode = 1 set, whereas our old servers used zone_reclaim_mode = 0. By switching (via sysctl) this values back and forth, I could easily bring down the experimental export process to crouching speed, or let it run again. The complete path of the analysis can be viewed at the PostgreSQL mailing list here: (there ia also a description, how the problem can be reproduced, and what the many symptoms are) http://archives.postgresql.org/pgsql-general/2010-07/msg00267.php Now, the conclusion to use "zone_reclaim_mode = 0" on our type of hardware was further strengthened by a very interesting thread at LKML, where the kernel developer discussed potential issues with this setting. You can read it here: http://lkml.org/lkml/2009/5/12/586 That discussion boils down to the fact, that for some reasons (described there in detail), the Linux kernel thinks on modern CPU architectures (out new Servers use Core i7 generation CPUs which are explicitly mentioned!) that it has a NUMA architecture. And for NUMA architectures it automatically enables "zone_reclaim_mode = 1" ... even though it is wrong, and not even recommended under many circumstances. Interestingly, even most posters at the LKML thread think, that it would be better to always(!) default this value to "zone_reclaim_mode = 0" instead of some automatic decision. Some more detail on what zone_reclaim_mode does can also be found here: http://www.linuxinsight.com/proc_sys_vm_zone_reclaim_mode.html Now, I don't know why this "defaulting to 0" is still not in the mainline kernels. That discussion from May 2009 at LKML died down, and obviously no one feeled responsible to commit the patches (even though, obvioulsy one of the guys had already prepared some!). BUT, I would ask the Ubuntu team, to maybe act on their own and provide a way in the Ubuntu 10.04 LTS to fix this issue (because, some reports on the net suggest, that "zone_reclaim_mode = 1" can do harm to performance in many ways)! And I believe, that I will not be the only PostgreSQL admin being affected by this issue! To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/605773/+subscriptions -- Mailing list: https://launchpad.net/~kernel-packages Post to : kernel-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~kernel-packages More help : https://help.launchpad.net/ListHelp
