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

[Andrew Morton]

Daniel Phillips wrote:
> 
> On January 8, 2002 11:21 pm, Andrew Morton wrote:
> > Daniel Phillips wrote:
> > > The preemptible kernel can reschedule, on average, sooner than the
> > > scheduling-point kernel, which has to wait for a scheduling point to roll
> > > around.
> >
> > Yes.  It can also fix problematic areas which my testing
> > didn't cover.
> 
> I bet, with a minor hack, it can help you *find* those problem areas too.
> You compile the two patches together and automatically log any event along
> with the execution address, where your explicit schedule points failed to
> reschedule in time.  Sort of like a profile but suited exactly to your
> problem.

Well, one of the instrumentation patches which I use detects a
scheduling overrun at interrupt time and emits an all-CPU backtrace.
You just feed the trace into ksymoops or gdb then go stare at
the offending code.  

That's the easy part - the hard part is getting sufficient coverage.
There are surprising places.  close_files(), exit_notify(), ...

> This just detects the problem areas in normal kernel execution, not
> spinlocks, but that is probably where most of the maintainance will be anyway.
> 
> By the way, did you check for latency in directory operations?

Yes.  They can be very bad for really large directories.  Scheduling
on the found-in-cache case in bread() kills that one easily for most
local filesystems.  There may still be a problem in ext2.

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