Subject: Re: mmap()/mlockall()/read()
From: Benno Senoner (sbenno_AT_gardena.net)
Date: ke loka 27 1999 - 14:03:00 EDT
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.
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