Helge Hafting wrote:
On Wed, Dec 21, 2005 at 05:32:52PM +1100, Peter Williams wrote:
Trond Myklebust wrote:
[...]
Sorry. That theory is just plain wrong. ALL of those case _ARE_
interactive sleeps.
It's not a theory. It's a result of observing a -j 16 build with the
sources on an NFS mounted file system with top with and without the
patches and comparing that with the same builds with the sources on a
local file system. Without the patches the tasks in the kernel build
all get the same dynamic priority as the X server and other interactive
programs when the sources are on an NFS mounted file system. With the
patches they generally have dynamic priorities between 6 to 10 higher
than the X server and other interactive programs.
A process waiting for NFS data looses cpu time, which is spent on running
something else. Therefore, it gains some priority so it won't be
forever behind when it wakes up. Same as for any other io waiting.
That's more or less independent of this issue as the distribution of CPU
to tasks is largely determined by the time slice mechanism and the
dynamic priority is primarily about latency. (This distinction is a
little distorted by the fact that, under some circumstances,
"interactive" tasks don't get moved to the expired list at the end of
their time slice but this usually won't matter as genuine interactive
tasks aren't generally CPU hogs.) In other words, the issue that you
raised is largely solved by the time tasks spend on the active queue
before moving to the expired queue rather than the order in which they
run when on the active queue.
This problem is all about those tasks getting an inappropriate boost to
improve their latency because they are mistakenly believed to be
interactive. Having had a closer think about the way the scheduler
works I'm now of the opinion that completely ignoring sleeps labelled as
TASK_NONINTERACTIVE may be a mistake and that it might be more
appropriate to treat them the same as TASK_UNITERRUPTIBLE but I'll bow
to Ingo on this as he would have a better understanding of the issues
involved.
Perhaps expecting a 16-way parallel make to have "no impact" is
a bit optimistic. How about nicing the make, explicitly telling
linux that it isn't important?
Yes, but that shouldn't be necessary. If I do the same build on a local
file system everything works OK and the tasks in the build have dynamic
priorities 8 to 10 slots higher than the X server and other interactive
programs.
Or how about giving important
tasks extra priority?
Only root can do that. But some operating systems do just that e.g.
Solaris has an IA scheduling class (which all X based programs are run
in) that takes precedence over programs in the TS class (which is the
equivalent of Linus's SCHED_NORMAL). I'm not sure how they handle the
privileges issues related to stopping inappropriate programs misusing
the IA class. IA is really just TS with a boost which is effectively
just the reverse implementation of what the new SCHED_BATCH achieves.
Arguably, SCHED_BATCH is the superior way of doing this as it doesn't
cause any privilege issues as shifting to SCHED_BATCH can be done by the
owner of the task.
The main drawback to the SCHED_BATCH approach is that it (currently)
requires the user to explicitly set it on the relevant tasks. It's long
term success would be greatly enhanced if programmers could be convinced
to have their programs switch themselves to SCHED_BATCH unless they are
genuine interactive processes.
Peter
--
Peter Williams [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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