Editing Memory scrubbing (section)

== Overview ==
It is important to check each memory location periodically, frequently enough, before ''multiple'' bit errors within the same word are too likely to occur, because the ''one'' bit errors can be corrected, but the ''multiple'' bit errors are not correctable, in the case of usual (as of 2008) ECC memory modules.

In order to not disturb regular memory requests from the [[Central processing unit|CPU]] and thus prevent decreasing [[Computer performance|performance]], scrubbing is usually only done during idle periods. As the scrubbing consists of normal read and write operations, it may increase [[power consumption]] for the memory compared to non-scrubbing operation. Therefore, scrubbing is not performed continuously but periodically. For many servers, the scrub period can be configured in the [[BIOS]] setup program.

The normal memory reads issued by the CPU or [[Direct memory access|DMA]] devices are checked for ECC errors, but due to [[Locality of reference|data locality]] reasons they can be confined to a small range of addresses and keeping other memory locations untouched for a very long time. These locations can become vulnerable to more than one soft error, while scrubbing ensures the checking of the whole memory within a guaranteed time.

On some systems, not only the main memory (DRAM-based) is capable of scrubbing but also the [[CPU cache]]s (SRAM-based). On most systems the scrubbing rates for both can be set independently. Because cache is much smaller than the main memory, the scrubbing for caches does not need to happen as frequently.

Memory scrubbing increases reliability, therefore it can be classified as a [[Reliability, availability and serviceability (computer hardware)|RAS]] feature.

=== {{Anchor|PATROL|DEMAND}}Variants ===
There are usually two variants, known as ''patrol scrubbing'' and ''demand scrubbing''.  While they both essentially perform memory scrubbing and associated error correction (if it is doable), the main difference is how these two variants are initiated and executed.  Patrol scrubbing runs in an automated manner when the system is idle, while demand scrubbing performs the error correction when the data is actually requested from main memory.<ref>{{cite web
 | url = http://www.supermicro.com/manuals/motherboard/C600/MNL-1281.pdf
 | title = Supermicro X9SRA motherboard manual
 | date = March 5, 2014 | accessdate = February 22, 2015
 | publisher = [[Supermicro]] | pages = 4–10
}}</ref>