Messages in this thread | | | Date | Mon, 13 Jun 2011 20:24:11 +0200 | From | marcel partap <> | Subject | CFQ IOsched / CGROUPs: How can a process with realtime/0 IO priority starve for several seconds? |
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Greetings Kernelings. Today i have a very curious issue to bring forward that has been bothering me for some time. But because i just got myself my first basic CGROUP setup, i considered hickups like this to be a thing of the past: Under heavy IO load, mpd starts skipping, sometimes even for a few seconds. That with mpd being in my 'swift' CGROUP which has, as can be seen below, a blkio.weight of 1000 - double the default of 500 and 100 times 'idle' class processes like munin-graph get. Nevertheless, it repeatedly happens that multiple processes accumulate in IOWAIT stage. What i would expect is that the drastically higher blkio.weight would prevent mpd to starve on disk input (it needs 30k/s or something), instead it stutters a lot making me want to #bash my head against a #shell or sumthing ^^ When i noticed the mpd process showed up as besteffort/4 in iotop, i re-ioniced it to realtime/0. That did nothing to improve the situation! That's in line with my earlier notion that a process in IOWAIT seems not to be unblocked by re-ionicing it to realtime!? That really confuses me. Doesn't realtime IO scheduling mean that... ???
So how comes? *How can a process even with high blkio.weight and realtime/0 IO priority starve for several seconds?*
Thx for any insights and please CC me.
#best regards/marcel.
P.S.: i did notice i did not have the CONFIG_CFQ_GROUP_IOSCHED option set, and i am going to try that now. But i guess that doesn't quite touch my basic question.
appendix --------
> CONFIG_CGROUPS=y > CONFIG_CGROUP_FREEZER=y > CONFIG_CGROUP_DEVICE=y > CONFIG_CGROUP_CPUACCT=y > CONFIG_CGROUP_MEM_RES_CTLR=y > CONFIG_CGROUP_MEM_RES_CTLR_SWAP=y > CONFIG_CGROUP_MEM_RES_CTLR_SWAP_ENABLED=y > CONFIG_CGROUP_PERF=y > CONFIG_CGROUP_SCHED=y > CONFIG_FAIR_GROUP_SCHED=y > CONFIG_RT_GROUP_SCHED=y > CONFIG_BLK_CGROUP=y > CONFIG_DEBUG_BLK_CGROUP=y > CONFIG_SCHED_AUTOGROUP=y > CONFIG_IOSCHED_NOOP=y > CONFIG_IOSCHED_DEADLINE=y > CONFIG_IOSCHED_CFQ=y > # CONFIG_CFQ_GROUP_IOSCHED is not set > CONFIG_DEFAULT_IOSCHED="cfq" > CONFIG_SCHED_OMIT_FRAME_POINTER=y > CONFIG_SCHED_MC=y > CONFIG_SCHED_HRTICK=y > CONFIG_NET_SCHED=y > CONFIG_SCHED_DEBUG=y > CONFIG_SCHEDSTATS=y > CONFIG_SCHED_TRACER=y
a screenshot of the situation: http://tinyurl.com/3-0-0-rc2-iosched-troubles-png (iotop in accumulative mode)
/etc/cgroup/cgconfig.conf > mount { > blkio = /sys/fs/cgroup; > cpu = /sys/fs/cgroup; > memory = /sys/fs/cgroup; > } > > group . { > blkio { > blkio.weight = 500; > } > } > > group idle { > blkio { > blkio.weight = 10; > } > cpu { > cpu.shares = 512; > } > memory { > memory.swappiness = 90; > } > } > > group swift { > blkio { > blkio.weight = 1000; > } > cpu { > cpu.shares = 2048; > } > memory { > memory.swappiness = 1; > } > }
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