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SubjectCFQ IOsched / CGROUPs: How can a process with realtime/0 IO priority starve for several seconds?
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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