CFQ IOsched / CGROUPs: How can a process with realtime/0 IO prioritystarve for several seconds?
From: marcel partap
Date: Mon Jun 13 2011 - 14:24:22 EST
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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