Re: [Re: paging behavior in Linux]

[Neelam Saboo]

Another observation. I have two independent programs. One program incurring
page faults and another program just doing some work.
When work program run undependently it takes ~19 seconds of CPU time, but when
it is run along with page faulting program on the same machine, it takes ~32
seconds of CPU time. Doesnt this indicate that page faults in a program slows
down all the program on the machine and not only threads in the same process
?

neelam

David Mansfield <[e-mail]> wrote:
> Manfred Spraul wrote:
> > 
> > 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).
> 
> David
> 
> -- 
> David Mansfield                                           [phone]
> [e-mail]
> Ultramaster Group, LLC                               www.ultramaster.com

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