amount of cache vs amount of memory.

[Moogle!]

When it comes to cacheing ram, I understand one needs:
64KB for 8MB
128KB for 16MB
256KB for 32MB
512K for 64MB
..and so on.

Is there any advantge to be had by having more than required amount for a given amount of ram? Like having 256K cache in a system having only 8 MB of ram?

 

[kgober]

In general, more cache is better but you get diminishing returns (i.e. twice as much cache is a bit less than twice as good). When caching was newly introduced to PCs it was entirely optional and you would often see motherboards with empty cache RAM sockets. If you wanted the extra performance (and were willing to pay for the expensive cache SRAMs) you were welcome to do that upgrade yourself, but the system worked fine without cache, just slower. Which is to say, there isn't any particular requirement that a certain amount of RAM must have a corresponding amount of cache. You can use more or less as your situation needs and circumstances permit.

In my experience, the amount of cache I'd have installed was directly tied to whether the system board I was using had cache sockets, and whether I could easily get my hands on the needed chips to fill those particular sockets.

 

[krebizfan]

What I remember was that the Packard Bell Pentiums with no L2 cache lost about 20% of the performance compared with the same clock speed Pentium with L2 cache. IIRC, most of the good performing Pentium 90s had 256k of L2 cache. Unless you plan on running a vintage database or vintage virtual machine software, increasing the cache size will have minimal effect on performance.

That can be modified by the specific CPU being used. The only one that comes to mind was the Pentium 4 where there never was enough cache. RISC workstations needed good amounts of cache but shipped with more than enough. Make sure the previous owner didn't strip the cache out.

 

[Svenska]

Keep in mind that the amount of cacheable memory might be limited.

In the Windows 95/98 era, there were quite many system with limited tag memory, so that only a limited amount of main memory (usually 64 MB) was cached. Since Windows liked to use memory at the end of the physical address space, that came with a rather heavy speed penalty. A good workaround for these situations is to put a RAM disk into the uncacheable memory to prevent Windows from using it (and have some fast temporary storage if needed). Or adding more cache to avoid the problem altogether, if the mainboard allows.

 

[Moogle!]

This question was asked with 386 and 486 machines in mind. Should have specified..

I know the cache figures are different for write through and write back.