Re: [2.4.17/18pre] VM and swap - it's really unusable

[Andrea Arcangeli]

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..).

Andrea