Messages in this thread | | | Date | Mon, 27 Mar 2006 10:49:36 +0200 | From | Jens Axboe <> | Subject | Re: [cfq] sched_idle process stalled for 1 minute; strange ioprio too |
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On Fri, Mar 24 2006, Sanjoy Mahajan wrote: > I'm trying the cfq io scheduler (in vanilla 2.6.16-rc5, TP 600X, Debian > testing/unstable) and noticed that a massive 'apt-get upgrade' (300MB > downloaded, 266 packages to upgrade) stalled at the 'Setting up sed' > line below, for about 1 minute. The machine was otherwise idle (top > showed no processes chewing CPU), and no disk activity was going on -- > strange since an 'apt-get upgrade' usually makes 'dpkg' chew on the > disk. > > <snip> > (Reading database ... 184123 files and directories currently installed.) > Preparing to replace sed 4.1.2-8 (using .../archives/sed_4.1.4-5_i386.deb) ... > Unpacking replacement sed ... > Setting up sed (4.1.4-5) ... > <stalls here for about 1 minute> > <snip> > > I had ioniced the shell to SCHED_IDLE with > > ionice -p$$ -c3 > > Then I ran the 'apt-get upgrade' so that the setting would be inherited. > After the minute-long stall, it continued as if nothing had gone wrong. > Maybe nothing is wrong, and it's something strange about dpkg needing a > lock and having too low a priority to get one (but who would it fight > with)? > > While the dpkg was stalled, I did 'ps x' in another root window and > these were the interesting ones: > > 4823 pts/7 S+ 0:04 apt-get upgrade > 5228 ? D 0:00 [pdflush] > 5261 pts/7 S+ 0:01 /usr/bin/dpkg --status-fd 29 --unpack /var/cache/apt/ > 5267 ? S 0:00 [pdflush] > 5268 pts/7 D+ 0:00 /usr/bin/dpkg --status-fd 29 --unpack /var/cache/apt/
Are you sure no other disk activity was going on at that time? Just a single writeout or read from the disk will stall your idle prio task. If you expect things to finish within a bounded time, then you should not use idle :-)
> produced these entries for some of the processes above: > > pid=4733, 24583 > idle: prio 7 > > pid=4823, 24583 > idle: prio 7 > > pid=5228, 0 > none: prio 0 > > pid=5261, 24583 > idle: prio 7 > > pid=5268, 24583 > idle: prio 7 > > I don't understand the ", 24583" in the pid lines. In ionice.c it comes > from this code: > > > ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid); > printf("pid=%d, %d\n", pid, ioprio); > > So ioprio_get is returning a strange value -- or is 24583 correct?
It's a raw display of the priority, it would probably make more sense in hex. 24583 is 0x6007 - lower bits the priority, here 7. Shift it down by 13 (IOPRIO_CLASS_SHIFT) and you have the class - that would be 11b, or 3 decimal, which is IOPRIO_CLASS_IDLE. So 24583 is the idle io class, fixed priority of 7.
-- Jens Axboe
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