memory usage

[gerry.feldman at compaq.com (Jerry Feldman)]

Just adding a bit to what David said.
Unix systems buffer all I/O. The buffers are allocated by the kernel for 
all system-wide I/O. So, when you write a file (to just about any device), 
you are writing to a buffer. Note that these kernel buffers have nothing to 
do with the C language stdio package. These buffers are used by device 
drivers. The Unix sync(8) command will cause all buffers to be flushed. 
Normally there is a daemon (bdflush or update) that will sync the disks 
periodically. Essentially, the buffering and caching by the kernel is one 
of the things that makes Unix (and Linux) systems faster than Windows 
systems. 




On 2 May 2002 at 10:55, David Kramer wrote:

> On Thu, 2 May 2002, Drew Taylor wrote:
> 
> > I'm running a kernel 2.4.2-2 (stock from RH 7.1) and am wondering what is 
> > the difference between buffers and cached memory? I added 128MB and was 
> > surprised to see so little memory free, even knowing that Linux does 
> > aggressive caching.
> > 
> > [drew at nephi]$ cat /proc/meminfo
> >          total:    used:    free:  shared: buffers:  cached:
> > Mem:  195735552 194052096  1683456        0 13651968 122585088
> > Swap: 205590528   290816 205299712
> > MemTotal:       191148 kB
> > MemFree:          1644 kB
> > MemShared:           0 kB
> 
> Linux, as well as most versions of UNIX, take whatever memory they think 
> the system can spare and use it for disk cache.  It's not really "used" in 
> that it is not allocated to a userspace program.  Even though you only 
> have 1644K real memory free, you can allocate much more than that, because 
> as the system needs more memory it will take blocks of it out of service 
> as disk cache and allocate it to the user.
> 
> Because of this, it may be hard to come up with a definitive number of 
> "free memory".  AIX has a very wacky scheme that makes it virtualy 
> impossible, but at least here under Linux you are told how much is being 
> used for cache buffers.
> 
> ----------------------------------------------------------------------------
> DDDD   David Kramer         david at thekramers.net       http://thekramers.net
> DK KD  
> DKK D  You don't usually hear "hydraulics" and "simplicity" 
> DK KD  in the same sentence.
> DDDD                                             Cathy Rogers, Junkyard Wars
> 
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--
Jerry Feldman
Portfolio Partner Engineering
508-467-4315 http://www.testdrive.compaq.com/linux/
Compaq Computer Corp.
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