Beginning SQL

Chapter 13
Gigahertz Networks or Compartmentalized Networks
Another area where you can sometimes get large speed increases is by analyzing how your network is
holding up to the load. Companies tend to grow haphazardly, adding employees all over the building
and plopping down new workstations with little thought to what is happening to network traffic. In the
Internet age, employees may be logging in to their workstations or the server using remote access, which
can cause significant network overhead. Internet music and video downloads also cause massive congestion.
It is not uncommon to end up with overloaded networks that can no longer keep up with the
traffic being encountered.
Oftentimes, new workstations can be purchased with gigahertz network cards. If the network is
approaching saturation, moving up to gigahertz networks can open up the bandwidth enough to allow
the applications to breathe again. Another thing that you can do is to compartmentalize the network
such that different sections of the company come in to the server on separate routers, switches, or network
interface cards (NICs). This is another area where there are tools to allow you to accurately measure
whether the network is a bottleneck. Networks require expert assistance if you need to modify
them, but if they are a bottleneck, then making those modifications can pay huge dividends.
Caches
A cache is a place where data is stored for fast access after it is retrieved from a location that takes significantly
longer to access. This definition is flexible enough to work for all the various caches that you find
on your computer, and there are many of them. The reason that you need to have at least a basic understanding
of caches is that your queries are filling the caches with data, and your queries are also flushing
the data out of the caches and replacing the old data with newer data. You might optimize a query and
discover that it becomes lightning fast. The reason it becomes fast, however, may have less to do with
your prowess as a SQL optimizer than because what you did allowed the data being worked on to
remain in one of the caches. If this is the case, then if the amount of data suddenly exceeds the size of the
cache, your query slows right back down again as the data needed gets flushed from the cache and
replaced with other necessary data.
Processor Cache
The processor has a cache for instructions and data. This is typically a small cache, often as little as 8K
for instructions and 32K for data, but in modern processors that amount may be in the hundreds of kilobytes
or even megabytes. The processor caches are the fastest caches in the system. Getting data from the
processor cache into the processor happens typically in a matter of one to a few clock cycles. At current
processor speeds of 3GHz, a clock cycle is 0.33 billionths of a second, or one-third of one-billionth of a
second. If your data fits in the processor cache, you are golden, and you get your results in the smallest
possible time.
Hard Disk Cache
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The physical disk storage medium tends to require milliseconds (thousandths of a second) to access.
Thus, getting data off the physical disk and into the disk cache (or into the system) takes as much as 10
milliseconds, or one-hundredth of a second — and that assumes that the disk is just sitting idle waiting
for you to ask for data, a very rare occurrence. Thus, accessing data on the physical disk is more than 10
million times slower than accessing data in the processor cache. Remember that this is the time it takes
to even start seeing the data.