[rescue] Looking for I/O performance metrics

[Derrick D. Daugherty]

It's rumored that around Mon, Nov 26, 2001 at 04:25:20PM -0500
"Greg A. Woods" <woods at weird.com> wrote:
> [ On Monday, November 26, 2001 at 13:20:26 (-0600), Derrick D. Daugherty wrote: ]
> > Subject: Re: [rescue] Looking for I/O performance metrics
> >
> > does `sync` take a while to return?  Sounds like the interfaces aren't
> > happy
> 
> If you're talking about any modern "real" Unix (eg. SunOS-5, or even
> *BSD), the time it takes for "sync" to return is irrelevant.  If I'm not
> mistaken this statement has been in every sync(2) manual page since V6:
> 
>   BUGS
>      sync() may return before the buffers are completely flushed.
> 
> Most implementations of sync(8) normally should return immediately since
> they just call sync() and exit.

Well I just use this to kind of get a 'feel' for the sluggishness.  If
your system is heavily loaded `ls` and `ps` still return quickily.  Only
processes with a lot of expensive thread creation seem to slack. I've
observed, and kind of made it into superstition, that when I have a
heavily loaded system that's blocked for io that `sync` takes longer to
respond than on a quiesced box.  The only way to really tell what's
going on is the output from the suggested commands and then further
probing of the bottle necks.

Also, I've made an assumption that `sync` runs at the SYS (system class)
scheduling class for the kernel dispatcher, while the shell and others sit
in the IA/TS classes which have a lower priority.  It may still very
well be in the SYS class but it exits before it's executed...  The SYS
class does not have a time quantum calculated each tick, it just sits
in the run queue until they're blocked.

So for now assumptions and superstitions until the years grant me more
experience ;D

I think once upon a time I read of a "real sync" that would actually
wait for all registers to dump.  It was third party I think, but it may
have just been an idea and not reality.

^D

--
the dumber people think you are, the more surprised they'll be when
you kill them