L2 cache... a big deal?

[jasonlclei]

With the new PD that has 2 x 2 mb L2 cache, i am starting to wonder...

Is L2 cache significant in terms of performance gain?

And how does that compare to the x2's?

 

[BrownTown]

Well, L2 cache is a huge deal in that a processor with none will run horribly slow. But you get pretty rapidly deminishing returns past a certain point. The difference between the 8xx series 2MB of cache and the 9xx series 4MB of cache is very little because at that point the cache miss rate is already so small that doubling the cache only reduces the miss rate a fraction of a percent. Also, equally important is the speed of the cache, a small faster cache is often better then a slow fast cache. Basically the equation you use is something like this (L1 miss rate)*(L1 miss penalty) + (L2 miss rate)*(L2 miss penalty) + (main memmory mise rate)*(main memmory miss penalty). Where the miss penalities are the time it takes you to access information from the lower level cache. So for an L1 cache the miss penalty is the time it takes to access the L2 cache. And for the L2 cache its the time it takes to access the main memmory, and for main memmory its the time it takes to access the hard drive.

Actually thats not really the equation since the miss penalties for the high level caches also have to include misses in the lower level caches, but that euation illustrates the point good enough.

 

[stevty2889]

The extra L2 cache on the 9xx series hasn't made any noticable differace that I can see. I traded my 830 which wouldn't OC at all for a 920 that I can't get stable past 3.36ghz, but neither of them stack up against my X2 at stock let alone overclocked.

 

[BrownTown]

of course the 920 shouldn't stack up to an X2 becasue it costs less... and the fact you can't overclock it is as likely due to your motherboard as the CPU. But the point is still valid enough, intel CPUs have had lots more ccache for awhile now, and it hasn't helped beat AMD in performace. And also of note is that when you get to multicore processesors you have the added probelm of cach coherency. Where the cache of one of the cores doesn't the most up to date version of the information and therefore has to generate a cache miss even though the other core has the correct info. AMDs currently solvve this probelm with there corssbar interconnect which transfers data directly between the two cores. Intels 8xx and 9xx CPUs transmit this information over the FSB which cases all kinds of trouble for them since its much higher latentcy, and their FSB is already a bottleneck. Anyways, Yonah and all future Intel processors solve this problem by using a shared L2 cache. Although it will pop up again in quad and octo core processors since Intel will use the Multi Chip Moldule design like they did for Presler with these chips meaning that each chip will have shared caches, but the different chips in each module will not, so you get cache coherency problems again...

 

[jasonlclei]

wow thanks for all your info BrownTown.

If, for example, a CPU has 512 kb L2 cache vs 1 mb L2 cache with the same miss rates and etc. (assume everything's the same except the 512kb vs 1mb), how much would the gain be in terms of performance?

 

[BrownTown]

depends on the memmory latentcy (another area where AMD destroys Intel BTW), and also on the type of program you are running. Different types of programs can have much different types of memmory accesses. For example a program which often accessed memmory completely randomly would have a huge miss rate on any amount of cache. Whereas a program that did a bunch of work on a small data set wouldn't miss near as much since it would be able to have the whole working set in cache. But, worrying about the cache size and latentcy of processors doesn't make all that much sense becasue its hard to tell just how much of an effect it will have on programs. Its much better to just look at a lot of benchmarks and then decide how good a processor is based on that instead of getting caught up in all the numbers. If cache sizes and clockspeeds were what made the biggest difference in CPUs then Intel would be way better then AMD.

 

[jasonlclei]

i see.

thanks alot

 

[Shenkoa]

Nice info BrownTown but I have a question. Do you know how to speak english?