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Upgrading and Repairing PCs Tip #3: How L1 Cache Works on Your PC

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In this excerpt from the 22nd edition of Scott Mueller's Upgrading and Repairing PCs, Scott explains how L1 cache works like a savvy waiter in a restaurant--most of the time.

Find more tips from Upgrading and Repairing PCs here.

From the book

To learn how the L1 cache works, consider the following analogy.

This story involves a person (in this case, you) eating food to act as the processor requesting and operating on data from memory. The kitchen where the food is prepared is the main system memory (typically double data rate [DDR], DDR2, or DDR3 dual inline memory module [DIMMs]). The cache controller is the waiter, and the L1 cache is the table where you are seated.

Okay, here’s the story. Say you start to eat at a particular restaurant every day at the same time. You come in, sit down, and order a hot dog. To keep this story proportionately accurate, let’s say you normally eat at the rate of one bite (byte? <grin>) every four seconds (233MHz = about 4ns cycling). It also takes 60 seconds for the kitchen to produce any given item that you order (60ns main memory).

So, when you arrive, you sit down, order a hot dog, and you have to wait for 60 seconds for the food to be produced before you can begin eating. After the waiter brings the food, you start eating at your normal rate. Pretty quickly you finish the hot dog, so you call the waiter over and order a hamburger. Again, you wait 60 seconds while the hamburger is being produced. When it arrives, you again begin eating at full speed. After you finish the hamburger, you order a plate of fries. Again you wait, and after the fries are delivered 60 seconds later, you eat them at full speed. Finally, you decide to finish the meal and order cheesecake for dessert. After another 60-second wait, you can eat cheesecake at full speed. Your overall eating experience consists of a lot of waiting, followed by short bursts of actual eating at full speed.

After coming into the restaurant for two consecutive nights at exactly 6 p.m. and ordering the same items in the same order each time, on the third night the waiter begins to think, “I know this guy is going to be here at 6 p.m., order a hot dog, a hamburger, fries, and then cheesecake. Why don’t I have these items prepared in advance and surprise him? Maybe I’ll get a big tip.” So you enter the restaurant and order a hot dog, and the waiter immediately puts it on your plate, with no waiting! You then proceed to finish the hot dog and right as you are about to request the hamburger, the waiter deposits one on your plate. The rest of the meal continues in the same fashion, and you eat the entire meal, taking a bite every four seconds, and you never have to wait for the kitchen to prepare the food. Your overall eating experience this time consists of all eating, with no waiting for the food to be prepared, due primarily to the intelligence and thoughtfulness of your waiter.

This analogy describes the function of the L1 cache in the processor. The L1 cache itself is a table that can contain one or more plates of food. Without a waiter, the space on the table is a simple food buffer. When it’s stocked, you can eat until the buffer is empty, but nobody seems to be intelligently refilling it. The waiter is the cache controller who takes action and adds the intelligence to decide which dishes are to be placed on the table in advance of your needing them. Like the real cache controller, he uses his skills to literally guess which food you will require next, and if he guesses correctly, you never have to wait.

Let’s now say on the fourth night you arrive exactly on time and start with the usual hot dog. The waiter, by now really feeling confident, has the hot dog already prepared when you arrive, so there is no waiting.

Just as you finish the hot dog, and right as he is placing a hamburger on your plate, you say “Gee, I’d really like a bratwurst now; I didn’t actually order this hamburger.” The waiter guessed wrong, and the consequence is that this time you have to wait the full 60 seconds as the kitchen prepares your brat. This is known as a cache miss, in which the cache controller did not correctly fill the cache with the data the processor actually needed next. The result is waiting, or in the case of a sample 233MHz Pentium system, the system essentially throttles back to 16MHz (RAM speed) whenever a cache miss occurs.

According to Intel, the L1 cache in most of its processors has approximately a 90% hit ratio. (Some processors, such as the Pentium 4, are slightly higher.) This means that the cache has the correct data 90% of the time, and consequently the processor runs at full speed (233MHz in this example) 90% of the time. However, 10% of the time the cache controller guesses incorrectly, and the data has to be retrieved out of the significantly slower main memory, meaning the processor has to wait. This essentially throttles the system back to RAM speed, which in this example was 60ns or 16MHz.

In this analogy, the processor was 14 times faster than the main memory. Memory speeds have increased from 16MHz (60ns) to 333MHz (3.0ns) or faster in the latest systems, but processor speeds have also risen to 3GHz and beyond. So even in the latest systems, memory is still 7.5 or more times slower than the processor. Cache is what makes up the difference.

The main feature of L1 cache is that it has always been integrated into the processor core, where it runs at the same speed as the core. This, combined with the hit ratio of 90% or greater, makes L1 cache important for system performance.

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