Red Hat Bugzilla – Bug 319911
2.6.9 and interactive task starvation
Last modified: 2010-10-22 15:17:04 EDT
Description of problem: The scheduler in 2.6.9 is subject to starvation problems with interactive tasks. Insertion of newly activated tasks into the active runqueue array can prevent the scheduler draining the active array & switching to the inactive array, effectively starving those tasks in the inactive array. This is seen for e.g. with a large number of server processes being continually awoken to service requests. See: http://lwn.net/Articles/176635/ http://lwn.net/Articles/176638/ Version-Release number of selected component (if applicable): 2.6.9-55.EL How reproducible: 100% under the right conditions Steps to Reproduce: 1. Create a situation where a number of processes are being continuously awoken to service requests. E.g. configure apache httpd and generate a loadavg of ~10 using the ab tool. 2. Time the execution of some non-interactive tasks in a spare terminal, e.g.: $ time netstat -n | grep unix | wc -l Actual results: Execution of the non-interactive tasks is delayed by very long periods (minutes in some cases) due to starvation of the expired array. Expected results: With the attached __activate_task patch applied delays are reported to be drastically reduced (~10s of seconds). Additional info:
Created attachment 217181 [details] Mikes's patch to avoid expired starvation
Created attachment 217221 [details] patch re-diffed for 2.6.9-55.EL
Results from a dual core system running httpd at a 1m loadavg of ~6 (load generated with ab): RHEL4 baseline: [root@p380-1 ~]# uname -r 2.6.9-55.ELsmp [root@p380-1 ~]# time netstat|grep unix|wc -l 20 real 31m34.660s user 0m4.476s sys 0m48.278s RHEL4 with avoid-interactive-starvation-rhel4.patch: [root@p380-1 ~]# uname -r 2.6.9-55.EL.bz319911smp [root@p380-1 ~]# time netstat|grep unix|wc -l 20 real 3m13.364s user 0m4.583s sys 0m51.709s Raising severity as this has a huge impact on "victim" processes overall runtime.
Created attachment 217891 [details] re-diffed with correct directory prefix
This request was evaluated by Red Hat Product Management for inclusion in a Red Hat Enterprise Linux maintenance release. Product Management has requested further review of this request by Red Hat Engineering, for potential inclusion in a Red Hat Enterprise Linux Update release for currently deployed products. This request is not yet committed for inclusion in an Update release.