Con Kolivas has been working on his staircase scheduler patch for a while;
it was covered here in the beginning of
. That scheduler found its way into the 2.6.8-rc2-mm2 patch
, along with this comment
from Andrew Morton:
This will probably have to come out again because various people
are still fiddling with the CPU scheduler. But my feeling here is
that the current 1st-gen CPU scheduler has been tweaked as far as
it can go and is still not 100% right. It is time to start
thinking about a new design which addresses the requirements and
current problems by algorithmic means rather than by tweaking.
So it would seem that it is now open season for scheduler work.
Initial reports on the staircase scheduler are generally - but not
uniformly - good. Martin Bligh posted some
benchmark results showing some significant performance improvements for
the 2.6.8-rc2-mm2 kernel, especially for "low to mid loads." Ingo Molnar,
instead, has found a workload which
performs poorly with this scheduler; it involves running multiple processes
each of which wants most, but not all, of the CPU.
Con, meanwhile, has posted a couple of additional patches implementing
additional policies in the staircase scheduler. SCHED_BATCH is another attempt at an "idle
process" mode, where batch processes only run if nothing else wants the
processor. This patch attempts to avoid priority inversion problems by
scheduling SCHED_BATCH processes at normal priority when they are
running in kernel mode.
SCHED_ISO, instead, is an "isochronous" mode
intended for applications which need soft real-time response. Putting a
process into SCHED_ISO is an unprivileged operation, any user can
do it. Isochronous tasks start out with a relatively high priority, and
should get scheduled quickly. Their allocated time slices are half of what
they would otherwise be, however, and their priority drops especially quickly with CPU
usage. So this mode is suitable for I/O bound processes which need to
respond quickly (audio recording, CD burning, etc.), but it should not
allow a hostile user to take over the system.
Peter Williams has been working on a different set of scheduler patches.
His approach is to get rid of the "expired" array (where processes go to
languish when they have used up their time slices) and move everything to a
single array. The patch offers two modes, being the traditional
priority-based mode and a new "entitlement" mode which tries to figure how
much processor time each task is entitled to, then works to ensure that
each is given at least that much time. His patches are available in a dizzying number of varieties; they seem to
have seen less testing so far, but Andrew has said that one of them might
get a turn in -mm for a while.
Nick Piggin's -np trees
also contain a new scheduler. Nick's work tries to simplify many of the
scheduler calculations while retaining logic which tries to evaluate the
"interactivity" of each process. Unlike some implementations, this
scheduler gives longer time slices to higher-priority processes. All slices
are scaled depending on the job mix, however; low-priority processes will
get longer slices if there are no high-priority processes around.
Ingo Molnar has continued his work on voluntary preemption; his voluntary-preempt-2.6.8-rc2-O2 patch features a
new implementation of the interrupt threads feature. The available reports
indicate that, with this patch, latency problems in the 2.6 kernel are
becoming few and far between.
There is no way to tell, at this point, which of these scheduler approaches
- if any - will find its way into the mainline kernel. Evaluating
schedulers takes a long time, and, for any given scheduler, there always
seems to be some strange workload out there which makes it fall apart. The
approaches described above (with the exception of voluntary preemption)
share one nice feature, however, which is likely to argue in favor of
including one of them: they all remove a significant amount of code and
make the scheduler simpler and easier to understand. That, in and of
itself, may be a worthwhile step toward the implementation of a top-quality
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