Windows® Internals, Sixth Edition, Part 2

The main point to remember when thinking about robustion, and Superfetch optimizations in general, is that Superfetch constantly monitors usage patterns and updates its understanding of the system, so that it can avoid fetching useless data. Although changes in a user’s daily activities or application startup behavior might cause Superfetch to incorrectly “pollute” the cache with irrelevant data or to throw out data that Superfetch might think is useless, it will quickly adapt to any pattern changes. If the user’s actions are erratic and random, the worst that can happen is that the system behaves in a similar state as if Superfetch was not present at all. If Superfetch is ever in doubt or cannot track data reliably, it quiets itself and doesn’t make changes to a given process or page.

RAM Optimization Software

While Superfetch provides valuable and realistic optimization of memory usage for the various scenarios it aims to support, many third-party software manufacturers are involved in the distribution of so-called “RAM Optimization” software, which aims to significantly increase available memory on a user’s system. These memory optimizers typically present a user interface that shows a graph labeled “Available Memory,” and a line typically shows the amount of memory that the optimizer will try to free when it runs. After the optimization job runs, the utility’s available memory counter often goes up, sometimes dramatically, implying that the tool is actually freeing up memory for application use. RAM optimizers work by allocating and then freeing large amounts of virtual memory. The following illustration shows the effect a RAM optimizer has on a system.

The Before bar depicts the process and system working sets, the pages in standby lists, and free memory before optimization. The During bar shows that the RAM optimizer creates a high memory demand, which it does by incurring many page faults in a short time. In response, the memory manager increases the RAM optimizer’s working set. This working-set expansion occurs at the expense of free memory, followed by standby pages and—when available memory becomes low—at the expense of other process working sets. The After bar illustrates how, after the RAM optimizer frees its memory, the memory manager moves all the pages that were assigned to the RAM optimizer to the free page list (which ultimately get zeroed by the zero page thread and moved to the zeroed page list), thus contributing to the free memory value.

Although gaining more free memory might seem like a good thing, gaining free memory in this way is not. As RAM optimizers force the available memory counter up, they force other processes’ data and code out of memory. If you’re running Microsoft Word, for example, the text of open documents and the program code that was part of Word’s working set before the optimization (and was therefore present in physical memory) must be reread from disk as you continue to edit your document. Additionally, by depleting the standby lists, valuable cached data is lost, including much of Superfetch’s cache. The performance degradation can be especially severe on servers, where the trimming of the system working set causes cached file data in physical memory to be thrown out, causing hard faults the next time it is accessed.

ReadyBoost

Although RAM today is somewhat easily available and relatively cheap compared to a decade ago, it still doesn’t beat the cost of secondary storage such as hard disk drives. Unfortunately, hard disks today contain many moving parts, are fragile, and, more importantly, relatively slow compared to RAM, especially during seeking, so storing active Superfetch data on the drive would be as bad as paging out a page and hard faulting it inside memory. (Solid state disks offset some of these disadvantages, but they are pricier and still slow compared to RAM.) On the other hand, portable solid state media such as USB flash disk (UFD), CompactFlash cards, and Secure Digital cards provide a useful compromise. (In practice, CompactFlash cards and Secure Digital cards are almost always interfaced through a USB adapter, so they all appear to the system as USB flash disks.) They are cheaper than RAM and available in larger sizes, but they also have seek times much shorter than hard drives because of the lack of moving parts.

Random disk I/O is especially expensive because disk head seek time plus rotational latency for typical desktop hard drives total about 13 milliseconds—an eternity for today’s 3-GHz processors. Flash memory, however, can service random reads up to 10 times faster than a typical hard disk. Windows therefore includes a feature called ReadyBoost to take advantage of flash memory storage devices by creating an intermediate caching layer on them that logically sits between memory and disks.