Since I have had NUMEROUS requests for copies of the answer to my
original question about libs not dropping out of memory, I am going to
repost my question and the answers I recieved. Also, couriously, it
seems that i am NOT the only one who has this "feature" problem ect
ect.
For the flamers of the original question... Dang, must be ALOT of us
idiots out here on this list. (Sorry, couldn't resist the dig. hehehe)
ORIGINAL QUESTION -
==============================
Evening all. My question is, what is the kernel's time limit for
dropping unused libs from memory? Same thing for the modules. Not sure
exactly what is responsible for it but here is the problem.
When I boot I only use about 17MB of RAM with everything I need
loaded. OK, all well and good, so what's the problem you might ask.
Easy, when I load X11, memory usage increases to around 93MB. OK, not
a problem. The kernel will drop the libs out of memory when all
calling apps have not accessed them in memory for (I believe) 60
seconds, starting with the oldest lib first.
Wait a minute! It's 3 hours later and I have been out of X11 mode for
the last 2 1/2 hours! Why is my memory usage still being reported as
93MB? Well, I look and I see that over 48MB of it is cached.
OK, so first question is, how do I change the flush times on system
cache? I have played with bdflush (Running RH 5.2) and with sync.
Nothing. Second question, how do I change the system's memory purge
time so I can clean some of these stale libs out of memory? Third and
last question, if the kernel itself is not responsible for this type
of memory maintenance, what is?
JIM NANCE'S ANSWER -
======================
Hi David,
> Evening all. My question is, what is the kernel's time limit for
> dropping unused libs from memory? Same thing for the modules. Not
sure
> exactly what is responsible for it but here is the problem.
There is not a time limit. It will drop the least recently used pages
(4 K blocks) from memory when something else needs the memory.
> Wait a minute! It's 3 hours later and I have been out of X11 mode
for
> the last 2 1/2 hours! Why is my memory usage still being reported as
> 93MB? Well, I look and I see that over 48MB of it is cached.
Yes, this is the way its supposed to be.
> OK, so first question is, how do I change the flush times on system
> cache? I have played with bdflush (Running RH 5.2) and with sync.
> Nothing. Second question, how do I change the system's memory purge
> time so I can clean some of these stale libs out of memory? Third
and
> last question, if the kernel itself is not responsible for this type
> of memory maintenance, what is?
The cache is only flushed when something else needs the memory. Is
this
causing you problems? You can force the cache to get dumped by
loading
something else into memory, but I am not sure why you would want to.
Jim
GREGORY MAXWELL'S ANSWER -
============================
That's not libs eating your memory. It's read cache.
Linux works by the theorey that free memory is wasted memory, so it
uses
almost all your unused ram for disk cache. This is normal and good. It
takes almost NO time to deallocate this cache memory.
Here is the output of free on one of my servers.
The number 96828 is the number you want to look at. That means I have
about 96MB avail to apps.
total used free shared buffers
cached
Mem: 128040 123932 4108 57012 59668
33052
- -/+ buffers/cache: 31212 96828
Swap: 216768 4 216764
ONE OF THE NICER OF THE LESS TOLRANT - NAME WITHHELD BY ME
==============================================================
(off-list)
You obviously have little idea how memory works under a modern
operating
system.
David D.W. Downey wrote:
> Evening all. My question is, what is the kernel's time limit for
> dropping unused libs from memory?
The kernel doesn't interface with "libraries" at all.
> Same thing for the modules.
Depends on the distribution. I believe a cron job cleans them out or
something. I don't use modules myself, so I won't comment further.
> Not sure
> exactly what is responsible for it but here is the problem.
>
> When I boot I only use about 17MB of RAM with everything I need
> loaded. OK, all well and good, so what's the problem you might ask.
>
> Easy, when I load X11, memory usage increases to around 93MB. OK,
not
> a problem. The kernel will drop the libs out of memory when all
> calling apps have not accessed them in memory for (I believe) 60
> seconds, starting with the oldest lib first.
>
> Wait a minute! It's 3 hours later and I have been out of X11 mode
for
> the last 2 1/2 hours! Why is my memory usage still being reported as
> 93MB? Well, I look and I see that over 48MB of it is cached.
Yes, the libraries (and the binaries themselves, and their data) will
stay
mapped forever, until something else needs the ram (be it another app
or
disk cache - the kernel automatically sizes the cache based on how
much
you need and how much RAM you have). There's no reason to purge it
ahead of time - you don't get extra bonus points in heaven for using
as little RAM as possible. The kernel tries to use _all_ of your RAM
to the best extent it can. The only reason it leaves any free at all
is so it can respond to demand for new memory at very low-latency. If
you
have plenty of spare RAM, this means that a lot of rarely stuff is
probably still cached. Once you need more RAM, the kernel will start
reaping these unused parts, and your cache will shrink.
If you consider this a problem, I'd suggest using an operating system
that
doesn't dynamically grow its cache, such as Windows 3.1. This will
leave
your unneeded RAM completely unused which you seem to perversely
desire.
=======================================================================
There were other email responses, but these seemed to cover the
original question better than the rest.
Hope the info here helps out.
David D.W. Downey A.K.A PGPKeys
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David D.W. Downey A.K.A PGPKeys
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