Death by Virtual Memory

Every pc user knows that pc’s become slower and slower over time until the point where they are almost unusable. This is where upgrading RAM will usually help – until eventually you have to buy a new pc. Apparantly that’s just the way pc’s wear out.

Actually, pc’s don’t wear out – readers of my blog probably know that, but users (knowingly and unknowingly) add software which consumes resources of which memory is the most important one.

RAM used to be very expensive and therefore a scarce resource. Programmers used to do all sorts of tricks to fit their increasingly complex programs in memory. To help them focus on the programming task and not worry too much about resource scarcity, operating system designers invented something called virtual memory or swap memory.

Swap memory allowed the operating system to remove running processes from the (expensive and therefor scarce) RAM and store the state of the process on disk (‘swap’ it out – hence the name), from where it could later be restored into RAM and start running on the cpu. The technique is still employed by all modern operating systems, and while the amount of RAM has grown considerably to a level where lack of it is usually not a problem, virtual memory techniques are still useful with long running processes that only need to run once in a while and where it is not a problem if the initial response time is a second or more – and when they’re not running, the RAM can be used for caching file system data and other important things.

But what happens if load increases, e.g. if the number of users grow or the system becomes otherwise loaded and the processes running on the system start competing for memory? The good news is that functionally nothing changes: Virtual memory is transparant to the process, so the code will execute the same as it did before. But the bad news is that execution time increases rapidly when real memory become exhausted and the OS has to start using VM. If this only happens during nighttime or at other times when users or external systems are’nt depending on the system, all is probably okay, but if not then you can be in real trouble. In fact, the problem can be so bad that the system becomes useless.

In fact, with much more RAM and larger programs in today’s computers, the relative performance penalty is much higher than it used to be. This is because when the OS starts swapping, the amount of data that needs to be transferred in and out of the hard disk(s) is probably a factor of 10 higher than it would have been say 10 years ago. During that time, however, hard disk access speeds has only doubled, so overall, the damage you risk of hitting the virtual memory “wall” is much higher now than it used to be.

An interesting factor which I have found useful to look for is the fact that anti virus systems installed on your servers often make the virutal memory problem worse. They do so because they install hooks into applications running on the system, monitoring all i/o. This monitoring performs well as long as the anti virus system can keep its database and code in memory, but when memory starvation starts occurring, it can turn into a real bad situation. How can we detect that situation (except by performance dropping)?

I’m not aware of any really useful tools that can sit in the background automatically detecting (or better: predicting) memory starvation problems on running servers or test systems. But there are ways to look for it: On Windows, I’ll be looking at the running processes, particularly focusing on the Page Faults Delta column, looking for processes consistently experiencing high numbers here:

This is an important performance testing subject. And one which is too often overlooked.

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