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Message-ID: <477EF8A4.7010507@tlinx.org>
Date: Fri, 04 Jan 2008 19:25:24 -0800
From: Linda Walsh <lkml@...nx.org>
To: LKML <linux-kernel@...r.kernel.org>
Subject: general config preemption Q: preempt-model and Big-Lock Preemption
A question that comes to mind every time I go through the settings
for "Preemption Model" and "Preempt The Big Kernel Lock".
Do each of the combinations "make sense", or are some "no-ops"?
For model, we have 1) no forced (server), 2) Voluntary (Desktop)
3) preemptible (low-latency Desktop), and for Big Lock Preemption (BLP),
we have "yes" or "no".
Questions:
1) What is the difference between "no forced" and "voluntary"? Doesn't
voluntary normally mean "not forced"?
2) If a process is not preemptible, then it seems this would be "doubly
so" in the kernel when the big-lock is held. So does the big-lock
preemption question have any effect (when preempt-model="no forced").
3) If a process is "fully preemptible" but "BLP=false", is that
much different than "voluntary preemption" & BLP=false? I.e. --
should 'preemptible kernel' also imply "BLP=true"...i.e.
_Should_ (would?) the following change be "somewhat" identical to
current config options:
O1) "Preempt Model"="no-forced preemption" (would set
preempt-big-kern-lock to FALSE and doesn't display that question).
O2) "Voluntary Preemption" (presuming there are voluntary preemption
points in the kernel in places where the big-K-lock is held), then
configuration would still ask whether to "Preempt The Big Kernel Lock".
O3) "Preemptible model"="allow-kernel-preemption" (would set
preempt-big-kern-lock to TRUE and doesn't display that question).
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