[gentoo-user] Free memory issue

[gentoo-user] Free memory issue

[Raul Gonzales]

Hi all,

 I wonder what is going on with my system. I have a 2G of physical RAM 
but even without any major activity output of free,vmstat and top 
reports only ~64M free. Would any one know how to find more - ps doesn't 
show nothing wrong...




 free -m
             total       used       free     shared    buffers     cached
Mem:          1953       1888         65          0        143       1557
-/+ buffers/cache:        187       1765
Swap:         2055          0       2055


procs -----------memory---------- ---swap-- -----io---- -system-- 
----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy 
id wa
 0  0      0  66720 147196 1594400    0    0     3    10   27   75  0  0 
100  0


top - 20:34:42 up 1 day,  9:40,  1 user,  load average: 0.00, 0.00, 0.00
Tasks:  99 total,   1 running,  98 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.0%sy,  0.0%ni,100.0%id,  0.0%wa,  0.0%hi,  0.0%si,  
0.0%st
Mem:   2000552k total,  1934028k used,    66524k free,   147200k buffers
Swap:  2104504k total,        0k used,  2104504k free,  1594492k cached


thanks a lot,
Raul

[gentoo-user] Re: Free memory issue

[Nikos Chantziaras]

On 08/16/2009 09:41 PM, Raul Gonzales wrote:
> Hi all,
>
> I wonder what is going on with my system. I have a 2G of physical RAM
> but even without any major activity output of free,vmstat and top
> reports only ~64M free. Would any one know how to find more - ps doesn't
> show nothing wrong...
>
>
>
>
> free -m
> total used free shared buffers cached
> Mem: 1953 1888 65 0 143 1557
> -/+ buffers/cache: 187 1765
> Swap: 2055 0 2055

Er, I see 1765MB free here (187 used).  It's the "-/+ buffers/cache" 
line that reports free memory, NOT the first line.

Re: [gentoo-user] Free memory issue

[Philip Webb]

090816 Raul Gonzales wrote:
> I have a 2G of physical RAM but even without any major activity
> output of free,vmstat and top reports only  ~64M free. 
>
> free -m
>             total       used       free     shared    buffers     cached
> Mem:          1953       1888         65          0        143       1557
> -/+ buffers/cache:        187       1765
> Swap:         2055          0       2055

The crucial figure is '187', which is what is usable.
You have a lot of something cached: any idea what it might be ?
-- the kernel should delete that stuff, if it needs the space.

-- 
========================,,============================================
SUPPORT     ___________//___,   Philip Webb
ELECTRIC   /] [] [] [] [] []|   Cities Centre, University of Toronto
TRANSIT    `-O----------O---'   purslowatchassdotutorontodotca

Re: [gentoo-user] Free memory issue

[Andrey Falko]

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On Sun, Aug 16, 2009 at 11:56 AM, Philip Webb <purslow@ca.inter.net> wrote:

> 090816 Raul Gonzales wrote:
> > I have a 2G of physical RAM but even without any major activity
> > output of free,vmstat and top reports only  ~64M free.
> >
> > free -m
> >             total       used       free     shared    buffers     cached
> > Mem:          1953       1888         65          0        143       1557
> > -/+ buffers/cache:        187       1765
> > Swap:         2055          0       2055
>
> The crucial figure is '187', which is what is usable.
> You have a lot of something cached: any idea what it might be ?
> -- the kernel should delete that stuff, if it needs the space.
>
> All Disk I/O is cached into memory. A good example is if you are playing a
video game. Every time you load a new map, that map is cached in memory, so
if you load that map again, the kernel won't get it from disk, but from the
memory's cache. If your OS needs more ram, the cache is over-written with
actual memory. Here is a simple explaination of what I'm talking about:
http://linux.about.com/od/lsa_guide/a/gdelsa44.htm

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Re: [gentoo-user] Free memory issue

[Alan McKinnon]

On Sunday 16 August 2009 20:56:59 Philip Webb wrote:
> 090816 Raul Gonzales wrote:
> > I have a 2G of physical RAM but even without any major activity
> > output of free,vmstat and top reports only  ~64M free.
> >
> > free -m
> >             total       used       free     shared    buffers     cached
> > Mem:          1953       1888         65          0        143       1557
> > -/+ buffers/cache:        187       1765
> > Swap:         2055          0       2055
>
> The crucial figure is '187', which is what is usable.
> You have a lot of something cached: any idea what it might be ?
> -- the kernel should delete that stuff, if it needs the space.

top, free and every other tool that examines /proc lies through it's teeth.

Well, not really, but it's necessary to know what exactly it is measuring and 
also how the kernel is written.

But first: Raul, there is nothing wrong with your box. It is working as 
designed.

The "used" column literally means memory that in use for something, somewhere.
The "free" column literally means memory that is not in use anywhere. It's 
normally around 2 or 3 % of total physical RAM.

In Raul's case, the kernel has decided to allocate 1557M of RAM to cache. A 
particular block might have cached data in it, or it might not. It might have 
been used then marked as up for not in use anymore, or it might never have 
been written to since last reboot. It doesn't matter. All that does matter is 
that the kernel has decided that those blocks of memory *may* be used for 
caching.

The trick is that if more memory is needed by apps than what is listed as 
immediately available and free, the kernel will search out chunks that it can 
de-allocate, then it will do so.

Why does the kernel do this? It's a programming technique called lazy-
deallocation (similar to lazy-initialization) and it's used because it is 
efficient. It means the kernel only has to do work when it needs to do work. 
De-allocating memory as soon as it is not in use is expensive and usually a 
waste of time - it's very likely that that very chunk of memory will almost 
immediately be used once again. If so, why bother de-allocating it at all?

Bottom line: You, the user, do not really know what the kernel is doing with 
memory. It's also changing about 100 million times a second and your eyes 
don't work that fast. The kernel is designed to do these things in an 
autonomous fashion because it knows what to do, and you the user do not. The 
most you can do about this is provide hints as to how the kernel should 
operate and then let it get on with it.

top, free, et all, simply show snapshots of memory state at a point in time. 
You should ignore the absolute values they display and concentrate more on how 
the numbers are changing over time. A value that gradually increases while no 
real processes are running would indicate a problem for example.

This is all very correct and follows good engineering principles.

-- 
alan dot mckinnon at gmail dot com