Messages in this thread | | | Date | Tue, 8 Jan 2002 17:38:09 +0100 | From | Andrea Arcangeli <> | Subject | Re: [2.4.17/18pre] VM and swap - it's really unusable |
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On Tue, Jan 08, 2002 at 04:54:58PM +0100, Daniel Phillips wrote: > On January 8, 2002 04:29 pm, Andrea Arcangeli wrote: > > > The preemptible approach is much less of a maintainance headache, since > > > people don't have to be constantly doing audits to see if something changed, > > > and going in to fiddle with scheduling points. > > > > this yes, it requires less maintainance, but still you should keep in > > mind the details about the spinlocks, things like the checks the VM does > > in shrink_cache are needed also with preemptive kernel. > > Yes of course, the spinlock regions still have to be analyzed and both > patches have to be maintained for that. Long duration spinlocks are bad > by any measure, and have to be dealt with anyway. > > > > Finally, with preemption, rescheduling can be forced with essentially zero > > > latency in response to an arbitrary interrupt such as IO completion, whereas > > > the non-preemptive kernel will have to 'coast to a stop'. In other words, > > > the non-preemptive kernel will have little lags between successive IOs, > > > whereas the preemptive kernel can submit the next IO immediately. So there > > > are bound to be loads where the preemptive kernel turns in better latency > > > *and throughput* than the scheduling point hack. > > > > The I/O pipeline is big enough that a few msec before or later in a > > submit_bh shouldn't make a difference, the batch logic in the > > ll_rw_block layer also try to reduce the reschedule, and last but not > > the least if the task is I/O bound preemptive kernel or not won't make > > any difference in the submit_bh latency because no task is eating cpu > > and latency will be the one of pure schedule call. > > That's not correct. For one thing, you don't know that no task is eating > CPU, or that nobody is hogging the kernel. Look at the above, and consider > the part about the little lags between IOs.
We agree. Actually "if the task is I/O bound", I meant "if nobody is hogging CPU", I exactly wanted to make the example of no task hogging CPU in general.
> > > Mind you, I'm not devaluing Andrew's work, it's good and valuable. However > > > it's good to be aware of why that approach can't equal the latency-busting > > > performance of the preemptive approach. > > > > I also don't want to devaluate the preemptive kernel approch (the mean > > latency it can reach is lower than the one of the lowlat kernel, however > > I personally care only about worst case latency and this is why I don't > > feel the need of -preempt), > > This is exactly the case that -preempt handles well. On the other hand, > trying to show that scheduling hacks satisfy any given latency bound is > equivalent to solving the halting problem. > > I thought you had done some real time work? > > > but I just wanted to make clear that the > > idea that is floating around that preemptive kernel is all goodness is > > very far from reality, you get very low mean latency but at a price. > > A price lots of people are willing to pay.
I'm not convinced that all those people knows exactly what they're buying then 8).
> > By the way, have you measured the cost of -preempt in practice?
dropping the lock in spin_unlock made a difference in the numbers, so the overhead must be definitely visible, by simply loading the system with threaded kernel computation (like webserving etc..etc..).
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