Re: [PATCH] make MADV_FREE lazily free memory

[Nick Piggin <nickpiggin@yahoo.com.au>]

Eric Dumazet wrote:
> Rik van Riel a écrit :
>
> > Eric Dumazet wrote:
> >
> > > Rik van Riel a écrit :
> > >
> > > > Make it possible for applications to have the kernel free memory
> > > > lazily.  This reduces a repeated free/malloc cycle from freeing
> > > > pages and allocating them, to just marking them freeable.  If the
> > > > application wants to reuse them before the kernel needs the memory,
> > > > not even a page fault will happen.
> >
> >
> > > I dont understand this last sentence. If not even a page fault 
> > > happens, how the kernel knows that the page was eventually reused by 
> > > the application, and should not be freed in case of memory pressure ?
> >
> >
> > Before maybe freeing the page, the kernel checks the referenced
> > and dirty bits of the page table entries mapping that page.
> >
> > > ptr = mmap(some space);
> > > madvise(ptr, length, MADV_FREE);
> > > /* kernel may free the pages */
> >
> >
> > All this call does is:
> > - clear the accessed and dirty bits
> > - move the page to the far end of the inactive list,
> >   where it will be the first to be reclaimed
> >
> > > sleep(10);
> > >
> > > /* what the application must do know before reusing space ? */
> > > memset(ptr, data, 10000);
> > > /* kernel should not free ptr[0..10000] now */
> >
> >
> > Two things can happen here.
> >
> > If this program used the pages before the kernel needed
> > them, the program will be reusing its old pages.
>
>
> ah ok, this is because accessed/dirty bits are set by hardware and not a 
> page fault.

No it isn't.

> Is it true for all architectures ?

No.

--
SUSE Labs, Novell Inc.
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