Chapter 4. Physical and Virtual Memory

53

To learn more about the swapping activity is taking place, use the

sar  W

report. Here is an example:

Linux 2.4.18 18.8.0 (pigdog.example.com)

12/17/2002

12:00:01 AM

pswpin/s pswpout/s

12:10:01 AM

0.15

2.56

12:20:00 AM

0.00

0.00

...

03:30:01 PM

0.42

2.56

Average:

0.11

0.37

Here we can see that, on average, there were three times fewer pages being brought in from swap

(

pswpin/s

) as there were going out to swap (

pswpout/s

).

To better understand how pages are being used, refer to the

sar  B

report:

Linux 2.4.18 18.8.0smp (raptor.example.com)

12/16/2002

12:00:01 AM

pgpgin/s pgpgout/s

activepg

inadtypg

inaclnpg

inatarpg

12:10:00 AM

0.03

8.61

195393

20654

30352

49279

12:20:00 AM

0.01

7.51

195385

20655

30336

49275

...

08:40:00 PM

0.00

7.79

71236

1371

6760

15873

Average:

201.54

201.54

169367

18999

35146

44702

Here we can view how many blocks per second are paged in from disk (

pgpgin/s

) and paged out to

disk (

pgpgout/s

). These statistics serve as a barometer of overall virtual memory activity.

However, more knowledge can be gained by looking at the other fields in this report. The Red Hat

Linux kernel marks all pages as either active or inactive. As the names imply, active pages are cur 

rently in use in some manner (as process or buffer pages, for example), while inactive pages are not.

This example report shows that the list of active pages (the

activepg

field) averages approximately

660MB

3

.

The remainder of the fields in this report concentrate on the inactive list   pages that, for one reason

or another, have not recently been used. The

inadtypg

field shows how many inactive pages are dirty

(modified) and may need to be written to disk. The

inaclnpg

field, on the other hand, shows how

many inactive pages are clean (unmodified) and do not need to be written to disk.

The

inatarpg

field represents the desired size of the inactive list. This value is calculated by the

Linux kernel and is sized such that the inactive list be large enough to act as a reasonable pool for

page replacement purposes.

For additional insight into page status (specifically, how often pages change status), use the

sar  R

report. Here is a sample report:

Linux 2.4.18 18.8.0smp (raptor.example.com)

12/16/2002

12:00:01 AM

frmpg/s

shmpg/s

bufpg/s

campg/s

12:10:00 AM

 0.10

0.00

0.12

 0.07

12:20:00 AM

0.02

0.00

0.19

 0.07

...

08:50:01 PM

 3.19

0.00

0.46

0.81

Average:

0.01

0.00

 0.00

 0.00

The statistics in this particular

sar

report are unique, in that they may be positive, negative, or zero.

When positive, the value indicates the rate at which pages of this type are increasing. When negative,

3. The page size under Red Hat Linux on the x86 architecture is 4096 bytes