How bad is an L1-8kb cash

[Greg12]

That seems to be all I get get now.


P4 2.4ghz L1-8kb, l2-512kb.

[tgpo]

http://www.amd.com/us-en/Glossary/0,,238_242~1042,00.html

There are different kinds of cache. Every CPU has a small amount of cache in the CPU. It's called Level 1 (L1) cache because it's the first place the CPU will look for information.

L1 Cache

The level 1 cache is the most important cache. It's located very close to the CPU and can therefore be accessed very quickly. L1 cache is usually not very big, 32kB for a Pentium II system and 64kB for an AMD K6-2. Although L1 cache is very small it's very important. I did a small test to illustrate this.

System:

* AMD K6-2 350 MHz, ASUS P5A (512kB L2 Cache), 128MB PC100 memory and a Matrox Millenium G200 8MB videocard.
* The tests were made with Final Reality 1.01 running at Windows 98 with DirectX 6 and G200 drivers version 4.26.
* I disabled the test 4 times; once with the L1 cache enabled and once without the L1 cache enabled. After that again but then with L2 cache disabled too.


Results:


Figure 1, Cache and Performance.

As can be seen from this picture L1 cache is very important. Without your system will be incredibly slow. The lack of L2 cache has also impact, but not even remotely close to that of the L1 cache.

The L1 cache runs at CPU speed, so speed-differences between different CPU's are not an issue. Therefore one is tempted to say more cache is better. Of course this is true, but there's more to it. Intel Pentium CPU's have L1 cache that is 4 way associative. AMD's cache is bigger but 'only' 2 way. This means that with random access of the L1 cache the Intel will be slightly faster.

Another option to make cache more efficient is write allocation. If information isn't found in the L1 cache it searching efficiently in the L2 cache is required. The better - more 'intelligent' - this is done the better the performance. AMD's K6-2 400 for instance has a different core than the slower models. Even if you clock the K6-2 400 at 350 it will still outperform the original 350. This is mainly because the new core has improved write allocation.



It doesn't seem to be all that good.

[sterno]

8k is fine if the CPU is designed to efficiently use a small L1 cache. It's hard to make high-speed processors with a large L1 cache because it's hard to make memory that runs that fast. Be happy that Intel's back to providing 512k L2 - I think all of the high-speed P3's only had 256k. As the difference between the CPU's internal clock rate and the system bus speed increases cache (both L1 and L2 in the case of traditional Intel systems, some other architectures have a more complex cache arrangement) becomes more important.

The P4 is actually a significant architectural change from earlier chips named "Pentium". The P2 and P3 were really just beefed-up Pentium Pros. You can't really compare cache size across different architectures, though within one family of CPU you can say that more is usually better up to a point.