On Mon, 29 Jan 2001, Szabolcs Szakacsits wrote:
> On Mon, 29 Jan 2001, Chris Evans wrote:
>
> > Stephen Tweedie has a rather funky i/o stats enhancement patch which
> > should provide what you need. It comes with RedHat7.0 and gives decent
> > disk statistics in /proc/partitions.
>
> Monitoring via /proc [not just IO but close to anything] has the
> features:
> - slow, not atomic, not scalable
> - if kernel decides explicitely or due to a "bug" to refuse doing
> IO, you get something like this [even using a mlocked, RT monitor],
> procs memory swap io system cpu
> r b w swpd free buff cache si so bi bo in cs us sy id
> 0 1 1 27116 1048 736 152832 128 1972 2544 869 44 1812 2 43 55
> 5 0 2 27768 1048 744 153372 52 1308 2668 777 43 1772 2 61 37
> 0 2 1 28360 1048 752 153900 332 564 2311 955 49 2081 1 68 31
> <frozen>
> 1 7 2 28356 1048 752 153708 3936 0 2175 29091 494 27348 0 1 99
> 1 0 2 28356 1048 792 153656 172 0 7166 0 144 838 4 17 80
>
> In short, monitoring via /proc is unreliable.
Not really unreliable, but definitely with _serious_ latency issues :)
due to taking the mmap_sem. Acquiring the mmap_sem semaphore can take
a really long time under load.. and sys_brk downs this semaphore first
thing, as does task_mem() and proc_pid_stat()... If someone has the
mmap_sem you want, and is pushing disk I/O when that disk is saturated,
you are in for a long wait. This I think is what you see with your
mlocked RT monitor (pretty similar to my mlocked RT monitor I suspect)
In fact, that darn monitor can have a decidedly negative impact on system
performance because it can take an arbitrary task's mana connection and
then fault while throttling it... I think ;-)
-Mike
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