Re: paging behavior in Linux

[Andrew Morton]

David Mansfield wrote:
> 
> Manfred Spraul wrote:
> >
> > >
> > > When I run my program on a readhat linux machine, I dont get results as
> > > expected, work thread seems to be stuck when prefetch thread is waiting on
> > > a page fault
> > >
> > That's a known problem:
> >
> > The paging io for a process is controlled with a per-process semaphore.
> > The semaphore is held while waiting for the actual io. Thus the paging
> > in multi threaded applications is single threaded.
> > Probably your prefetch thread is waiting for disk io, and the worker
> > thread causes a minor pagefault --> worker thread sleeps until the disk
> > io is completed.
> 
> This behavior is actually pretty annoying.  There can be cases where a
> process wakes up from a page fault, does some work, goes back to sleep
> on a page fault, thereby keeping it's mmap_sem locked at all times (i.e.
> vmstat, top, ps unusable) on a UP system.  I posted this complaint a
> while ago, it was discussed by Linus and Andrew Morton about how it also
> boiled down to semaphore wakeup unfairness (and bugs?).  The current
> semaphore was determined to be too ugly to even look at.  So it was
> dropped.
> 
> Is there any way that the mmap_sem could be dropped during the blocking
> on I/O, and reclaimed after the handle_mm_fault?  Probably not, or it'd
> be done.
> 
> It can be a real DOS though, a 'well-written' clobbering program can
> make ps/vmstat useless.  (it's actually /proc/pid/stat that's the
> killer, IIRC).

Did the `goto inside' trick in the semaphore code actually
fix this unfairness issue?

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