2001.03_Memory Problems Fixed.pdf

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BEGINNERS

DR. LINUX

NEXT PATIENT

PLEASE

Complicated organisms, since that’s

what Linux systems are, have little

complaints all of their own. Dr. Linux

observes the patients in the Linux

newsgroups, issues prescriptions here

for current problems and suggests

alternative healing methods.

BY MARIANNE WACHHOLZ

Is there room for just one more?

The memory space is displayed to you in one

kilobyte units ( blocks ). You are given details on the

whole space on the respective hard disk partition,

diskette, CD, etc., with an indication of how much

of it is occupied ( used ) or free ( available ); plus the

memory space used as a percentage.

On checking these figures it will strike you that

the used (plus the still-available) memory comes to

only about 95 per cent of the total memory. This is

not a program or calculation error, but a deliberate

limitation of the memory space for normal mortal

users. The last five per cent is only available for the

superuser. This gives him or her the option of

making space in a full disk by means of a pack

program, which itself also needs some memory

space to work.

If you would prefer to have a specification in

mega- or gigabytes, give df the option -h with:

I have the feeling that the space on my computer is

getting a bit tight. How do I find out how much

hard disk space is occupied?

Dr. Linux: First of all, there are programs you

can call using a command line, for example the

command df (meaning d isk f ree):

user$ df

Filesystem 1k-blocks Used Available Use% Mounted on

/dev/hda6 3470648 2045904 1245220 62% /

/dev/hda2 932912 637288 248236 72% /RedHat

This promptly supplies you with the memory

occupied by all file systems which you currently

have mounted .

mount: This command integrates media (e.g. hard disk partitions and CDs) into

the Linux file system. This is normally reserved for root. Before you can remove a

mounted CD or diskette from the drive, an umount command is essential. And

hard disk partitions, too, can be put out of reach under Linux again in this way.

File system: The ways and means of organising data on a data carrier vary from

one operating system to another and for different storage media. For example

under Windows 9x there is usually an extension of the DOS file system FAT

named VFAT in use, while Linux likes its data partitions to be in the ext2 file

system. On data CDs on the other hand iso9660 is used.

Kilobyte: Memory is divided into memory cells, which contain either the value 0

or 1. Such a memory cell or the data stored in it is called a Bit. Several bits can be

combined into units such as a byte, word or long word: a byte for example

corresponds to eight bits. This is sufficient to store one (Latin) letter. The word

”space” accordingly needs five bytes of storage space. A page of text contains

approx. 1500 characters and therefore unformated approx. 1500 bytes for

storage. A kilobyte (kB, kByte) incidentally, corresponds not to 1000, but to 1024

bytes. One kilobyte times one kilobyte gives one Megabyte (MB, MByte), which is

exactly 1,048,576 bytes. A Gigabyte (GByte), thus one kilobyte times one kilobyte

times one kilobyte, is lots and lots of bytes, 1,073,741,824 to be exact.

user$ df -h

Filesystem Size Used Avail Use% Mounted on

/dev/hda6 3.3G 2.0G 1.2G 62% /

/dev/hda2 911M 622M 242M 72% /Red Hat

For users who prefer to work with graphical

interfaces, there are also a few applications

available. In the KDE menu you might find, in the

Utilities sub directory, the programs KDFree (Figure

1) and KDu (Figure 4).

If not, your distribution may supply these in a

kdu package for later installation. With SuSE Linux

this is in the series kpa . If you have no luck here you

can download the appropriate package from

http://rpmfind.net/linux/RPM/kdu.html .

kdfree offers in the first instance an overview of

all data carriers which are entered in your

/etc/fstab . These are also selectable individually via

riders, which provide a pie chart and information on

the selected drive (Figure 1). As with df only those

data carriers are taken into account which are

mounted under a mount point in the system.

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Dr. Linux

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In a direct comparison with this the program

GNOME Free Disk (Figure 2), which you will find in

the GNOME sub menu Tools looks a bit sparse in the

graphic representation. Here round instruments

show you a percentage value which represents how

full your disk is. This tool can also be invoked from

the command line using the command gdiskfree .

GNOME also offers you the option of inserting

an applet into the control panel with which you can

keep a constant eye on the memory space (Figure

3). To do this, in the GNOME menu, under

Panel/Add applet/Status display select the item Disk

space . On the command line, entering

diskusage_applet & achieves the same result.

What is displayed are, as with df, on one

command line the mount point ( MP: ) and after av:

as in " av ailable" the available memory space in

kilobyte blocks. If you click with the right mouse

button on the applet icon in the panel, a few more

setting can be configured via the menu entry

properties ; in particular, at this point the refresh rate

of the display may be of interest.

Dr. Linux: Your Linux system naturally comes

with a few programs which can be invaluable in

helping you to analyse your system. I would like to

introduce you to vmstat ( v irtual m emory stat istics).

Invoked on the command line, this produces

something like the following output:

[left]

Fig. 1: kdfree shows the memory

allocation of individual drives as an

overview or in detail

[right]

Fig. 2: The display of gdiskfree

therefore looks a bit sparse

procs memory swap io system cpu

rbw swpd free buff cache si so bi bo in cs us sy id

000 212 4936 3104 23556 0 0 3 0 128 266 2 0 98

Here is how to decrypt this jumble of figures and

abbreviations:

• r ( r un): The higher the number you find here, the

slower your system. This shows how many

processes would run if you did not have to wait

for machine time.

• b ( b lock): Processes displayed here are waiting for

specific events to be able to continue running the

program, but not for machine time.

• w : The number at this point shows you the

number of all processes currently backed up in

the swap domain

Memory space in directories

/etc/fstab: This configuration file is used on system start up by the mount

program to mount hard disk(partitions) and other data carriers in the file system

and keep information ready for later mount actions. The first four columns in the

file are the most interesting: The first specifies the device to be integrated, the

second the mount point, the third the type of file system used on the data carrier.

The mount options are listed in the fourth columns. The following example

automatically mounts the root file system on the first partition of the first SCSI

hard disk on booting, but on the other hand not the ATAPI-(IDE-)CDROM

(noauto). However, later on, normal users (thanks to the option user) have the

option, with the command mount /cdrom, of making any inserted data-CD

accessible under /cdrom as read-only.

How can I find out how much memory space

individual directories are occupying?

Dr. Linux: Change to a command line in the

directory whose memory occupancy interests you,

and enter the command:

user$ du -k

(short for: d isk u sage). You will receive an output of

memory occupancy in kilobytes for the sub directories

and the entire memory usage of a directory.

On the graphical user interface the

aforementioned KDu in the sub directory Utilities of

the KDE menu offers you the option of displaying

the memory usage of directories (Figure 4).

#Devicename mounted as Filesystem mount-options

/dev/sda1 / ext2 defaults 0 0

[...]

/dev/hdc /cdrom iso9660 ro,noauto,user 0 0

If you don’t ask...

Mount point: The directory in which the files of a data carrier are ”mounted” so

that they can be accessed under Linux and other Unix operating systems.

My computer seems so slow to me – what’s the

possible diagnosis?

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DR. LINUX

Fig. 3: The applet Disk

space and its

configuration menu

Fig. 4: KDu shows

the memory space in

directories

• swpd states the swap memory currently in use in

kilobytes

• free shows how many kilobytes of RAM are

unused right now

• buff shows the size of the areas of memory in

which the in-/output buffers are located; the unit

is again the kilobyte. If data are being produced,

there will not be a transfer to the hard disk for

each symbol or each block. All in-/outputs are

first placed in a buffer zone. If e.g. by reading, a

program tries to access a block inside the file, the

operating system checks whether the block

sought is already in the buffer. If so, it is loaded

and then made available to the program which

requested it

• Under cache can be found in kilobytes how

much integrated hard disk cache is currently

available

• si ( s wap i n) and so ( s wap o ut) show how many

kilobytes of data per second have been loaded

from the swap zone on the hard disk into the

main memory or swapped from the RAM onto

the disk

• bi ( b lock i n) and bo ( b lock o ut) refers to e.g.

hard disk activities, since the number of blocks

per second is displayed here which have been

sent to block devices or received from such.

Block devices are for example floppies and hard

disks.

• in shows the number of halts per second arising

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from hardware demands; the so-called

Interrupts

• cs ( c ontext s witch) shows how often per second

a switch is made from one program to the next

( multitasking ).

• us reflects what percentage of the processor

time used is consumed by application programs,

while

• sy shows the processor time used by the system

• id is the unused processor time, again as a

percentage

An id number in the double-digit range and

simultaneous massive activities in si and so can be

the first indications of too little main memory. A

high number for unused processor time means in

such a case that the system often has to wait to

access the hard disk. This can be confirmed, if swpd

is high, free,buff and cache on the other hand stay

relatively low. If id, si and so are constantly in the

region of practically zero, these are serious

indications of a processor which is too sluggish.

To track down problems, you should make

vmstat produce periodic messages while these are

going on. But you will probably have to be patient

at this point if everything is moving as slowly as a

tortoise in winter.

The following example shows a vmstat output

with six repetitions (second figure) in one-second

cycles (first figure):

is done with the command:

root# dd if=/dev/zero of= Filename bs=1024 co U

unt= Filesize_in_Kilobyte

/dev/zero: The content of the

zero device, as the name

indicates, consists of zeros

(which go on forever).

Everything written into this

special file is deleted.

This copies the file /dev/zero into the specified file

name s. As file size, enter a value between 40 and

131073 kilobytes. The specification of 131073

makes a file of around 128 megabytes, which is the

maximum value for swap memory. The smallest

manageable quantity of 40 kilobytes for making a

swap area is not really sensible.

Using the command:

root# mkswap Filename Filesize_in_kilobytes

the file is given a swap identification. Only then can

the file be used as swap storage. You can imagine

this procedure as like the installation of a file system

with the command mkfs .

Before you log the new file into the system,

make sure that only the superuser has read-write

privileges for it. Otherwise your system will questich

the wrong privileges or give error messages and

refuse access to the new swap area. In case of

doubt use chmod 0600 Filename to determine the

correct permissions.

Lastly, log the new memory with the command:

root# swapon Filename

onto the system. The swap memory will be available

to you immediately.

user$ vmstat -n 1 6

procs memory swap io system cpu

r b w swpd free buff cache si so bi bo in cs us sy id

1 0 0 208 1556 2616 20736 0 0 3 0 130 265 2 0 97

2 0 0 208 1552 2616 20552 0 0 230 0 2019 2358 36 16 48

0 0 1 208 1544 2616 18868 0 0 91 0 830 2607 41 19 40

0 0 0 208 1544 2616 18868 0 0 0 0 102 208 0 1 99

1 0 0 208 1500 2616 18912 0 0 11 0 358 734 0 5 95

0 1 0 208 2336 2616 19188 0 0 69 60 1150 727 10 5 85

Anyone not wanting to go rummaging in the Man

page, can use vmstat x to receive a brief

introduction to the use of this handy tool.

In Listing 1 you can see the acknowledgements

produced by Linux when you create a swap file.

Between the commands to create the file, the

command sync is inserted at this point. This makes

sure that everything is written to the hard disk,

before the next command executes further actions

with:

Space in the tiniest hut

When I ran up my Linux system I apparently created

too small a swap partition. Can I upgrade the swap

memory without repartitioning?

Dr. Linux: As a temporary solution, I would

recommend a swap file – which you may be familiar

with from Windows. Linux has two options for

swapping data from RAM onto the hard disk:

•When installing your system you inevitably came

across the swap partition. This is the first and the

faster option

• The swap file is used less often. This works more

slowly than a swap partition

Linux can manage up to 16 swap zones at once, so

if necessary swap files can simply switch over

The making and activating of a swap file is done

by the superuser on the command line. The former

root# swapoff Filename

you can log the additional swap memory off from

the system, if you no longer need it.

Listing 1: Creating and logging a swap file

root@maxi:/ # dd if=/dev/zero of=swapfile bs=1024 count=131073

131073+0 Records on

131073+0 Records off

root@maxi:/ # sync

root@maxi:/ # mkswap swapfile 131073

Swap area Version 1 with a volume of 134213632 bytes is made.

root@maxi:/ # sync

root@maxi:/ # chmod 0600 swapfile

root@maxi:/ # swapon -v swapfile

swapon for swapfile

6 · 2001 LINUX MAGAZINE 89

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