I agree no all topics,
using mlockall(MCL_CURRENT|MCL_FUTURE) on a low mem box,
and trying to mmap a large file would simply not work.
I would definitively use MAP_UNLOCKED on Linux,
becaue it could give us the best possible performance.
(and maybe provide a fallback solution for other OSes)
I think the best fallback solution is the following (but not 100% reliable
in the case of syscalls):
mlock(MCL_CURRENT);
mlock() malloec()ed aread
( or is there a better way to do this ?)
but strange,
MAP_UNLOCKED seems to not exist on my 2.2.12 kernel !!
I grepped through the entire kernel source and include files but
nothing !!!!!
in /usr/include/bits/mman.h
there are some flags:
----
/* These are Linux-specific. */
#ifdef __USE_MISC
# define MAP_GROWSDOWN 0x0100 /* Stack-like segment. */
# define MAP_DENYWRITE 0x0800 /* ETXTBSY */
# define MAP_EXECUTABLE 0x1000 /* Mark it as an executable. */
# define MAP_LOCKED 0x2000 /* Lock the mapping. */
# define MAP_NORESERVE 0x4000 /* Don't check for reservations. */
#endif
----
no MAP_UNLOCKED present ...
is this a 2.3.x feature ?
> > What I'm trying to say: read() doesn't work better than mmap() in terms of
> > behaviour during swapping, because the kernel could easily swap out
> > your buffer where you read() in the data.
>
> Actually, the last time I tested this extensively (with a 2.0.x kernel
> I think), mmap() was much better performing than read()..for a while.
> When sequential accessing of a file got to a point around my RAM
> limits, the disk activity became insane. Maybe things are better now.
>
> > I will add a feature to the pagefaulter thread which uses mlock()/munlock if
> > root privileges are available, so that even
> > Eric is satisfied.
> > :-)
>
> Eric is hard to satisfy. :)
You are not the only !
:-)
>
> > PS: mlockall(MCL_CURRENT|MCL_FUTURE) is a bad idea because when you do the
> > mmap() of the large file the process tries to load all into mem,and my scheme
> > would not work. Better to use mlockall(MCL_CURRENT) and mlock()/munlock()
> > areas on demand.
>
> This is a useful solution in some circumstances. However, it means I
> can't use malloc()/new, may have trouble with shared libraries or
> dynamically loaded plugins and have to worry about reserving stack
> space. It's not the way I want to work.
agreed for shared libs, but if you don't make any strange syscalls ,
I think there are not very much problems.
As for malloc : just mlock() the area after the allocation.
regards,
Benno.
