Chapter 4. Physical and Virtual Memory

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CPU   The CPU is expending cycles doing the necessary processing to support memory manage 

ment and setting up the necessary I/O operations for paging and swapping.

The interrelated nature of these loads makes it easy to see how resource shortages can lead to severe

performance problems.

All it takes is a system with too little RAM, heavy page fault activity, and a system running near its

limit in terms of CPU or disk I/O. At this point, the system is thrashing, with performance rapidly

decreasing.

4.5.2. Best Case Performance Scenario

At best, system performance presents a minimal additional load to a well configured system:

RAM   Sufficient RAM for all working sets with enough left over to handle any page faults

2

Disk   Because of the limited page fault activity, disk I/O bandwidth would be minimally impacted

CPU   The majority of CPU cycles are dedicated to actually running applications, instead of

memory management

From this, the overall point to keep in mind is that the performance impact of virtual memory is

minimal when it is used as little as possible. This means that the primary determinant of good virtual

memory subsystem performance is having enough RAM.

Next in line (but much lower in relative importance) are sufficient disk I/O and CPU capacity. How 

ever, these resources only help the system performance degrade more gracefully from heavy faulting

and swapping; they do little to help the virtual memory subsystem performance (although they obvi 

ously can play a major role in overall system performance).

4.6. Red Hat Linux Specific Information

Due to the inherent complexity of being a demand paged virtual memory operating system, monitor 

ing memory related resources under Red Hat Linux can be confusing. Therefore, it is best to start with

the more straightforward tools, and work from there.

Using

free

, it is possible to get a concise (if somewhat simplistic) overview of memory and swap

utilization. Here is an example:

total

used

free

shared

buffers

cached

Mem:

1288720

361448

927272

0

27844

187632

 /+ buffers/cache:

145972

1142748

Swap:

522104

0

522104

We can see that this system has 1.2GB of RAM, of which only about 350MB is actually in use. As

expected for a system with this much free RAM, none of the 500MB swap partition is in use.

Contrast that example with this one:

total

used

free

shared

buffers

cached

Mem:

255088

246604

8484

0

6492

111320

 /+ buffers/cache:

128792

126296

Swap:

530136

111308

418828

2. A reasonably active system always experiences some page faults, if for no other reason than because a

newly launched application experiences page faults as it is first brought into memory.