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From: John Baldwin <jhb@freebsd.org>
To: freebsd-hackers@freebsd.org
Subject: Re: madvise() vs posix_fadvise()
Date: Thu, 27 Mar 2014 11:41:41 -0400
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Cc: hackers@freebsd.org, Dmitry Sivachenko <trtrmitya@gmail.com>,
 Trond =?utf-8?q?Endrest=C3=B8l?= <Trond.Endrestol@fagskolen.gjovik.no>
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On Monday, March 24, 2014 8:03:04 am Dmitry Sivachenko wrote:
>=20
> On 21 =D0=BC=D0=B0=D1=80=D1=82=D0=B0 2014 =D0=B3., at 20:27, Trond Endres=
t=C3=B8l=20
<Trond.Endrestol@fagskolen.gjovik.no> wrote:
>=20
> > On Fri, 21 Mar 2014 18:56+0400, Dmitry Sivachenko wrote:
> >=20
> >> Hello!
> >>=20
> >> I have a program which uses large data files (read-only, via mmap()).
> >>=20
> >> These machines have a bit more RAM that these files occupy, so it is=20
> >> possible to have all these data in memory.
> >>=20
> >> What techniques should I use to promote this data not to be purged=20
> >> from RAM:
> >>=20
> >> -- madvise(MADV_WILLNEED)
> >> -- posix_fadvise(POSIX_FADV_WILLNEED)
> >> -- both?
> >=20
> > Although a bit dangerous, mlock(2) might be your ticket. That system=20
> > call prevents your memory region from being swapped/paged away from=20
> > physical memory.
> >=20
>=20
>=20
> I know about mlock(2), it is a bit overkill.
> Can someone please explain the difference between madvise(MADV_WILLNEED) =
and=20
posix_fadvise(POSIX_FADV_WILLNEED)?

Right now FADV_WILLNEED is a nop.  (I have some patches to implement it for
UFS.)  I can't recall off the top of my head if MADV_WILLNEED is also a nop.
However, if both are fully implemented they should be similar in terms of
requesting async read-ahead.  MADV_WILLNEED might also conceivably
pre-create PTEs while FADV_WILLNEED can be used on a file that isn't
mapped but is accessed via read(2).

=2D-=20
John Baldwin