Linux Server Memory Questions

JdS · #1 11-Apr-2006, 08:28

Some of you know that I recently leased a new web server for the sites. This new server is supposed to be set up with 1 GB worth of RAM. I have 2 questions about this (questions in bold).

When I type cat /proc/meminfo, this is what I see...

Code:

[jds@bella ~]$ cat /proc/meminfo
MemTotal:       970780 kB
MemFree:        778168 kB
Buffers:          1592 kB
Cached:          19772 kB
SwapCached:      20424 kB
Active:         168436 kB
Inactive:         5492 kB
HighTotal:       65472 kB
HighFree:         1080 kB
LowTotal:       905308 kB
LowFree:        777088 kB
SwapTotal:     1020116 kB
SwapFree:       938580 kB
Dirty:             132 kB
Writeback:           0 kB
Mapped:         160452 kB
Slab:            10088 kB
Committed_AS:   710308 kB
PageTables:       2364 kB
VmallocTotal:   106488 kB
VmallocUsed:      2128 kB
VmallocChunk:   104004 kB
HugePages_Total:     0
HugePages_Free:      0
Hugepagesize:     4096 kB

My first question: Is 1 GB equal to 970780 kB?

When I type top -- on the memory line, I see something like this:

Code:

Mem:    970780k total,   202352k used,   768428k free,     1756k buffers

From all my previous experiences using this command, I have never seen so much free memory. From what I have heard and read about Linux and how it uses memory, I think this is quite odd. The 768428k free bit is what concerns me...

Why is there so much free memory?

This is on a CentOS 4.3 Linux web server.

<mod edit>The Solution.</mod edit>

nniehoff · #2 11-Apr-2006, 09:01

As to your first question, that is really close to one GB. However Most of the machines I have with 1Gb all have > 1000000kB.

Secondly it seems really strange to have that much free memory. I don't use CentOS, I use Debian so there is a difference there and perhaps your webserver is running 0 extra processes. But that seems really unlikely. Here are a couple of my systems:

My laptop (only I use this):

Code:

MemTotal:      1035256 kB
MemFree:        435740 kB
Buffers:         55388 kB
Cached:         265164 kB
SwapCached:          0 kB
Active:         375136 kB
Inactive:       173852 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:      1035256 kB
LowFree:        435740 kB
SwapTotal:     1020088 kB
SwapFree:      1020088 kB
Dirty:              48 kB
Writeback:           0 kB
Mapped:         321620 kB
Slab:            37576 kB
CommitLimit:   1537716 kB
Committed_AS:   813316 kB
PageTables:       2324 kB
VmallocTotal:   245752 kB
VmallocUsed:      5868 kB
VmallocChunk:   239652 kB

A file server (not under heavy use at the moment since the office isn't open yet, however it is running some backups):

Code:

MemTotal:      2076368 kB
MemFree:          7240 kB
Buffers:         24156 kB
Cached:        1884536 kB
SwapCached:          0 kB
Active:          59112 kB
Inactive:      1886380 kB
HighTotal:     1179584 kB
HighFree:         1024 kB
LowTotal:       896784 kB
LowFree:          6216 kB
SwapTotal:     2096472 kB
SwapFree:      2095760 kB
Dirty:          110052 kB
Writeback:           0 kB
Mapped:          57340 kB
Slab:           111896 kB
Committed_AS:   172036 kB
PageTables:        896 kB
VmallocTotal:   114680 kB
VmallocUsed:      4104 kB
VmallocChunk:   110196 kB

My office webserver (not a heavily used server either):

Code:

MemTotal:       256244 kB
MemFree:          2328 kB
Buffers:         53160 kB
Cached:         105804 kB
SwapCached:          0 kB
Active:         114380 kB
Inactive:        93704 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:       256244 kB
LowFree:          2328 kB
SwapTotal:      499992 kB
SwapFree:       499992 kB
Dirty:              20 kB
Writeback:           0 kB
Mapped:          64516 kB
Slab:            39812 kB
Committed_AS:   168000 kB
PageTables:        960 kB
VmallocTotal:   711240 kB
VmallocUsed:     22904 kB
VmallocChunk:   687976 kB

My personal mail server (there are 3 users of this machine, incredibly low usage but a lot of mail volume and a lot of file server activity):

Code:

MemTotal:      1035544 kB
MemFree:         90148 kB
Buffers:        404916 kB
Cached:         183784 kB
SwapCached:       6008 kB
Active:         238056 kB
Inactive:       402496 kB
HighTotal:      131008 kB
HighFree:        23700 kB
LowTotal:       904536 kB
LowFree:         66448 kB
SwapTotal:      522104 kB
SwapFree:       485164 kB
Dirty:             240 kB
Writeback:           0 kB
Mapped:          69148 kB
Slab:           288204 kB
CommitLimit:   1039876 kB
Committed_AS:   382248 kB
PageTables:       2472 kB
VmallocTotal:   114680 kB
VmallocUsed:      3052 kB
VmallocChunk:   111344 kB
HugePages_Total:     0
HugePages_Free:      0
Hugepagesize:     4096 kB

From one of my sites hosted at dreamhost.com (I would imagine extremely heavy usage):

Code:

MemTotal:      4075076 kB
MemFree:        116720 kB
MemShared:           0 kB
Buffers:         80416 kB
Cached:        2159652 kB
SwapCached:      20696 kB
Active:         582244 kB
Inactive:      1678788 kB
HighTotal:     3211200 kB
HighFree:         5596 kB
LowTotal:       863876 kB
LowFree:        111124 kB
SwapTotal:     6313512 kB
SwapFree:      6238920 kB

From another site hosted at advancednetworkhosts.com

Code:

MemTotal:      2059896 kB
MemFree:         75560 kB
Buffers:        451352 kB
Cached:        1102344 kB
SwapCached:          0 kB
Active:        1062596 kB
Inactive:       794444 kB
HighTotal:     1163200 kB
HighFree:         2624 kB
LowTotal:       896696 kB
LowFree:         72936 kB
SwapTotal:     4192956 kB
SwapFree:      4192796 kB
Dirty:            6248 kB
Writeback:           0 kB
Mapped:         341284 kB
Slab:           103216 kB
Committed_AS:   758300 kB
PageTables:      12448 kB
VmallocTotal:   106488 kB
VmallocUsed:      5264 kB
VmallocChunk:   100120 kB
HugePages_Total:     0
HugePages_Free:      0
Hugepagesize:     4096 kB

All of these systems are Debian except the Mail server which is Fedora and the advanced networks one is RedHat Enterprise Edition. So I will let you decide for yourself if > 700 Mb of mem free makes since but to me it sounds a little far fetched. If this continues to sound fishy to you I would recommend you try rkhunter (www.rootkit.nl) just incase you got hacked and don't know it. This could be something a rootkit would do.

Nick

dsmith · #3 11-Apr-2006, 09:20

A Gigabyte of Memory would actually be equal to 1,048,576 Kbytes I believe (1024x1024). So no, the 970 shown is not quite a gigabyte.

I don't think it is terribly uncommon for the total memory to show less than what is there. On my system with 512Mbytes (524,288 Kb) it shows as 514552 available.

I think there is just a bit of overhead with BIOS, etc that takes some of the available memory. Also, it is quite possible that your video card is using shared memory which would account for the decrease.

I agree with nniehoff about the memory free. In my experience, Linux does not leave free memory lying around. I have a fairly lowly taxed system that currenlty has 9004 Kb of 644576 available. Linux memory management seems to leave memory occupied until it needs it. This is not centOS either. Perhaps centOS is using a different memory management algorithm? I will look into this and see if I can find out anything.

I wouldn't worry too much about this unless it is causing you problems. I am more curious about the centOS memory manager. Does anyone have any experience with centOS?

JdS · #4 11-Apr-2006, 09:29

Thank you for your feedback guys. I appreciate it.

My previous server was a CentOS 4.3 too. And it also had VERY little free memory "lying around"

On the VPS before that, with 4GB RAM (I think), the server also reported nearly all the memory was in "use".

I always knew that this is how Linux managed it's memory, grabbing (nearly) everything it can detect -- so seeing over 700MB of free memory on this new server was quite a shock to me.

My web host is convinced this is normal

dsmith · #5 11-Apr-2006, 09:48

In looking a bit closer at the memory of both what you and Nick posted and comparing it with mine I noticed that your file caching is significantly smaller than anything Nick posted as well as my systems.

On most of my systems file caching takes about 50% of available memory.

There are two explanations that I can think of:

This is a fairly new "boot" and there have not been very many open files. Therefore there is nothing in the file cache.
The kernel may be configured to minimize file caching. Perhaps, in your usage it is better to have alot of free memory that utilize file caching.

This has been pretty interesting to look into actually. Keep us posted on what you find out J.

Cheers!

JdS · #6 11-Apr-2006, 10:04

I will update this thread as soon as I figure it out myself. I will try asking for help on other web hosting forums as well, just to get more opinions and comments.

Right now, I know the server is handicapped. It's running as though it has only 256Kb of physical memory. I know this because swap memory is being used and it grows over the course of the day while the (over) 700MB free memory is left untouched.

dsmith · #7 11-Apr-2006, 10:53

Quote:

Originally Posted by JdS

Right now, I know the server is handicapped. It's running as though it has only 256Kb of physical memory. I know this because swap memory is being used and it grows over the course of the day while the (over) 700MB free memory is left untouched.

ahhh... I didn't realize that! Yes this would be a big concern to me as well.

I will let you know if I can find anything else...

cable_guy_67 · #8 19-Apr-2006, 14:05

I had some time and did some detective work. Now, I would like to say I understand very little of what I found in a concrete way. Conceptually I think I understand the problem enough to offer some insight though. After my searches, I think I may not understand it very well though.

Perhaps though, someone reading this will find the information useful and share some solutions rather than some filtered information.

It seemed to me, you were running against a glass ceiling of sorts when it comes to your memory. I thought for some reason your CentOS does not believe that there could possibly be more memory. (remember that bit above about me not knowing what the heck I'm talking about)

Now, that was the reason for my search terms. To that end, I found a couple of links that might help. Not exactly what I thought I might find, but they seem related to your problem.

CentOS Forum

So for some odd reason I ended up in the docs for 4.3 and got as far as the next link before realizing I was in danger of drowning from info overload for an OS I don't even have.
CentOS 4.3 docs

One of the respondants in the first link points to a weath of information that can only be described as a religous experience. I think I made it down a quarter of the page and blacked out. Here is that link.

Swappiness Debate

Anyhow, my assesment, recompile the kernal and keep your fingers crossed. It seems to be a common theme in my findings. I'd be curious to hear if you have made progress JdS.

HTH,
Mark

todays moral - assume you know nothing especially when you think you know the answer

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