Re: mmap() better than read() fro streaming, Was: Re: Streaming disk I/O kills file buffering and ma

Benno Senoner (sbenno@gardena.net)
Wed, 25 Aug 1999 19:05:25 +0200

On Wed, 25 Aug 1999, Alan Cox wrote:
> > > ptr=mmap(blah)
> > > mlock(ptr, ...)
> > > munmap(ptr, len)
> >
> > will mlock() automatically cause all pages to be read from the file ?
> > (I don't think)
>
> > But since I already do an mlockall() at the beginnig, there is no need of
> > additional mlock()s
>
> >From man mlock
>
> All pages which contain a
> part of the specified memory range are guaranteed be resi-
> dent in RAM when the mlock system call returns success-
> fully and they are guaranteed to stay in RAM until the
> pages are unlocked by munlock or munlockall

nice, so I can avoid referencing the mmap()ed region in steps of 4096 bytes.

in your suggested solution:
ptr=mmap(blah)
mlock(ptr, ...)
munmap(ptr, len)

can I avoid an munlock(ptr,...) call after mmap()ed buffer is no longer needed,
will mmap() re-utilize that region on the next call, or could this avoiding of
munlock() cause the kernel to get short of free pages ?
( Just a paranoid idea :-) )

But as stated in man munmap :

---
       The munmap system call deletes the mappings for the speci-
       fied address  range,  and  causes  further  references  to
       addresses within the range to generate invalid memory ref-
       erences.

---
I guess since the page does not longer exist , there is no reason to
keep the page locked into mem.


regards,
Benno.




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