Messages in this thread | | | Date | Tue, 24 May 2005 18:11:45 +0200 | From | Ingo Molnar <> | Subject | Re: [patch] Voluntary Kernel Preemption, 2.6.12-rc4-mm2 |
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* Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> >remove it if it's not being used, but right now the only choice that > >distributions have is no preemption or full-blown CONFIG_PREEMPT. Ask > >the kernel maintainers at SuSE why they havent enabled CONFIG_PREEMPT in > >their kernels. > > > > I guess it is a number of reasons. Probably the main one had > traditionally been the chance of bugs. I guess the next big one is > return on overhead (ie. the scheduling latency soon runs into the > problem of long critical sections), although thanks to you and others, > I understand that is becoming less and less of an issue over time too. > > If a new SUSE kernel branch was started from 2.6.12 with VP turned on > rather than PREEMPT then I would probably argue against it a little > bit ;)
dont think of scheduling latencies as a binary thing a'ka "do we have good preemption latencies". It's a continuum, with almost a continuum number of techniques. One thing is sure: close to one end of the spectrum we have PREEMPT_NONE, and pretty close to the other end of the spectrum we have PREEMPT_RT.
both PREEMPT_VOLUNTARY and CONFIG_PREEMPT are at arbitrary points within that continuum, with different cost/benefit tradeoffs. Neither is perfect, and both are 'ugly' in the theoretical sense.
now, i dont intend to populate our .config with a continuum number of preemption models ;) But clearly the past 4 years have shown that no major distro was brave enough to go CONFIG_PREEMPT, so a solution inbetween is needed. -VP is precisely such a (very low-impact) solution. It has a ridiculously low impact:
include/linux/kernel.h | 18 +++++++++++----
we already talked an order of magnitude more about this feature than its size is (with help text included :). Lets go with it and let people know that the water is fine. If it's unused it can be zapped easily.
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