52

Chapter 4. Physical and Virtual Memory

This system has about 256MB of RAM, the majority of which is in use, leaving only about 8MB free.

Over 100MB of the 512MB swap partition is in use. Although this system is certainly more limited

in terms of memory than the first system, to see if this memory limitation is causing performance

problems we must dig a bit deeper.

Although more cryptic than

free

,

vmstat

has the benefit of displaying more than memory utilization

statistics. Here is the output from

vmstat 1 10

:

procs

memory

swap

io

system

cpu

r

b

w

swpd

free

buff

cache

si

so

bi

bo

in

cs

us

sy

id

2

0

0 111304

9728

7036 107204

0

0

6

10

120

24

10

2

89

2

0

0 111304

9728

7036 107204

0

0

0

0

526

1653

96

4

0

1

0

0 111304

9616

7036 107204

0

0

0

0

552

2219

94

5

1

1

0

0 111304

9616

7036 107204

0

0

0

0

624

699

98

2

0

2

0

0 111304

9616

7052 107204

0

0

0

48

603

1466

95

5

0

3

0

0 111304

9620

7052 107204

0

0

0

0

768

932

90

4

6

3

0

0 111304

9440

7076 107360

92

0

244

0

820

1230

85

9

6

2

0

0 111304

9276

7076 107368

0

0

0

0

832

1060

87

6

7

3

0

0 111304

9624

7092 107372

0

0

16

0

813

1655

93

5

2

2

0

2 111304

9624

7108 107372

0

0

0

972 1189

1165

68

9

23

During this 10 second sample, the amount of free memory (the

free

field) varies somewhat, and there

is a bit of swap related I/O (the

si

and

so

fields), but overall this system is running well. It is doubtful,

however, how much additional workload it could handle, given the current memory utilization.

When researching memory related issues, it is often necessary to see how the Red Hat Linux virtual

memory subsystem is making use of system memory. By using

sar

, it is possible to look at this aspect

of system performance in much more detail.

By reviewing the

sar  r

report, we can look more closely at memory and swap utilization.:

Linux 2.4.18 18.8.0smp (raptor.example.com)

12/16/2002

12:00:01 AM kbmemfree kbmemused

%memused kbmemshrd kbbuffers

kbcached

12:10:00 AM

240468

1048252

81.34

0

133724

485772

12:20:00 AM

240508

1048212

81.34

0

134172

485600

...

08:40:00 PM

934132

354588

27.51

0

26080

185364

Average:

324346

964374

74.83

0

96072

467559

The

kbmemfree

and

kbmemused

fields show the typical free and used memory statistics, with the

percentage of memory used displayed in the

%memused

field. The

kbbuffers

and

kbcached

fields

show how many kilobytes of memory are allocated to buffers and the system wide data cache.

The

kbmemshrd

field is always zero for systems (such as Red Hat Linux) that use the 2.4 Linux

kernel.

The lines for this report have been truncated to fit on the page. Here is the remainder of each line, with

the timestamp added to the left to make reading easier:

12:00:01 AM

kbswpfree kbswpused

%swpused

12:10:00 AM

522104

0

0.00

12:20:00 AM

522104

0

0.00

...

08:40:00 PM

522104

0

0.00

Average:

522104

0

0.00

For swap utilization, the

kbswpfree

and

kbswpused

fields show the amount of free and used swap

space, in kilobytes, with the

%swpused

field showing the swap space used as a percentage.