Re: uninteruptable sleep (D state => load_avrg++)

[Tim Wright]

On Wed, Apr 04, 2001 at 02:13:49PM +0200, christophe barbe wrote:
> The sleep should certainly be interruptible and I that's what I said to the GFS guy.
> But what the reason to increment the load average for each D process ?
> 

OK, the Unix history goes something like this. Synchronization was achieved
using two primitives, sleep() and wakeup(). These guys rendezvous'd on a
wait channel, which was simply an 'int', and by convention was actually the
address of a data structure (yes I know int and pointers aren't the same, this
is a long time ago, OK ? :-).
Anyway, when you called sleep, you also had an associated priority. Priority
values less than PZERO were "high" priority, and >= PZERO were "low" priority.
sleeping above PZERO was interruptible, and processes sleeping at this priority
did not count towards the load. The idea was to use this for events that
potentially might never happen. Sleeping at a priority < PZERO was intended
to be used for things that are absolutely 100% guaranteed to happen, preferably
sometime very soon. Disk I/O (real disks, not NFS) fell into this category,
and hence it counts towards the load since this could be deemed a "fast wait"
state, and the process is nominally runnable. All a bit hand-wavy I know, but
it worked well enough.

The really important part of all this is that you should never sleep
uninterruptibly for anything that you cannot absolutely guarantee will happen,
otherwise you wind up with a stuck process.

Regards,

Tim

-- 
Tim Wright - [e-mail] or [e-mail] or [e-mail]
IBM Linux Technology Center, Beaverton, Oregon
Interested in Linux scalability ? Look at http://lse.sourceforge.net/
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