The cache on your CPU has become a very important part of today's computing. The
cache is a very high speed and very expensive piece of memory, which is used to
speed up the memory retrieval process. Due to its expensive CPU's come with a
relatively small amount of cache compared with the main system memory. Budget
CPU's have even less cache, this is the main way that the top processor
manufacturers take the cost out of their budget CPU's.
How does the CPU Cache work?
Without the cache memory every time the CPU requested data it would send a
request to the main memory which would then be sent back across the memory bus
to the CPU. This is a slow process in computing terms. The idea of the cache is
that this extremely fast memory would store and data that is frequently accessed
and also if possible the data that is around it. This is to achieve the quickest
possible response time to the CPU. Its based on playing the percentages. If a
certain piece of data has been requested 5 times before, its likely that this
specific piece of data will be required again and so is stored in the cache
memory.
Lets take a library as an example o how caching works. Imagine a large library
but with only one librarian (the standard one CPU setup). The first person comes
into the library and asks for Lord of the Rings. The librarian goes off follows
the path to the bookshelves (Memory Bus) retrieves the book and gives it to the
person. The book is returned to the library once its finished with. Now without
cache the book would be returned to the shelf. When the next person arrives and
asks for Lord of the Rings, the same process happens and takes the same amount
of time.
If this library had a cache system then once the book was returned it would have
been put on a shelf at the librarians desk. This way once the second person
comes in and asks for Lord of the Rings, the librarian only has to reach down to
the shelf and retrieve the book. This significantly reduces the time it takes to
retrieve the book. Back to computing this is the same idea, the data in the
cache is retrieved much quicker. The computer uses its logic to determine which
data is the most frequently accessed and keeps them books on the shelf so to
speak.
That is a one level cache system which is used in most hard drives and other
components. CPU's however use a 2 level cache system. The principles are the
same. The level 1 cache is the fastest and smallest memory, level 2 cache is
larger and slightly slower but still smaller and faster than the main memory.
Going back to the library, when Lord of the Rings is returned this time it will
be stored on the shelf. This time the library gets busy and lots of other books
are returned and the shelf soon fills up. Lord of the Rings hasn't been taken
out for a while and so gets taken off the shelf and put into a bookcase behind
the desk. The bookcase is still closer than the rest of the library and still
quick to get to. Now when the next person come in asking for Lord of the Rings,
the librarian will firstly look on the shelf and see that the book isn't there.
They will then proceed to the bookcase to see if the book is in there. This is
the same for CPU's. They check the L1 cache first and then check the L2 cache
for the data they require.
Is more Cache always better?
The answer is mostly yes but certainly not always. The main problem with having
too much cache memory is that the CPU will always check the cache memory before
the main system memory. Looking at our library again as an example. If 20
different people come into the library all after different books that haven't
been taken out in quite a while but the library has been busy before and so the
shelf and the bookcase are both full we have a problem. Each time a person asks
for a book the librarian will check the shelf and then check the bookcase before
realizing that the book has to be in the main library. The librarian each time
then trots off to get the book from the library. If this library had a non cache
system it would actually be quicker in this instance because the librarian would
go straight to the book in the main library instead of checking the shelf and
the bookcase.
As the fact that non cache systems only work in certain circumstances and so in
certain applications CPU's are definitely better with a decent amount of cache.
Applications such as MPEG encoders are not good cache users because they have a
constant stream of completely different data.
Does cache only store frequently accessed data?
If the cache memory has space it will store data that is close to that of the
frequently accessed data. Looking back again to our library. If the first person
of the day comes into the library and takes out Lord of the Rings, the
intelligent librarian may well place Lord of the Rings part II on the shelf. In
this case when the person brings back the book, there is a good chance that they
will ask for Lord of the Rings part II. As this will happen more times than not.
It was well worth the Librarian going to fetch the second part of the book in
case it was required.
Cache Hit and Cache Miss
Cache hit and cache miss are just simple terms for the accuracy of what goes
into the CPU's cache. If the CPU accesses its cache looking for data it will
either find it or it wont. If the CPU finds what's its after that's called a
cache hit. If it has to go to main memory to find it then that is called a cache
miss. The percentage of hits from the overall cache requests is called the hit
rate. You will be wanting to get this as high as possible for best performance.
About the Author: Stephen Orgill
Editor - www.pantherproducts.co.uk
Computer related
hardware articles and
component reviews
Article source - http://www.easyarticles.com
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