Very Computer

How big role does cache(SRAM) really play for performance?

I have a AMD486DX4 100mhz with 256kb SRAM and 8mb DRAM but suppose I upgrade
to 24mb DRAM, then should I upgrade SRAM too?

Assuming I did upgrade from (SRAM,DRAM)=(256kb,24mb) to say (512kb,24mb).

First of all, would that make sense? Would it be significant?
Anyone got any insight about how much I can expect it to do for
overall systemperformance? 5%, 1% or less?

I would like to hear if anyone has any knowledge about this, thanks.

 /Fredrik

| How big role does cache(SRAM) really play for performance?

You have the right idea, but the question is "cache misses."

| I have a AMD486DX4 100mhz with 256kb SRAM and 8mb DRAM but suppose I upgrade
| to 24mb DRAM, then should I upgrade SRAM too?
|
| Assuming I did upgrade from (SRAM,DRAM)=(256kb,24mb) to say (512kb,24mb).

The size of the cache is related to the size of your "working set" of
pages you access often. Some systems rarely reuse a page, due to apps,
and get little benefit from 512k or even a MB of cache. Others find that
more cache gives a big improvement, aven if the system has little memory
as RAM. It's not a fixed ratio between cache and RAM, although if you
add RAM because of load, often more cache will help, too.

| First of all, would that make sense? Would it be significant?
| Anyone got any insight about how much I can expect it to do for
| overall systemperformance? 5%, 1% or less?

Easy way to test, run some compute intensive process, and see how long
it takes. The go into the BIOS and disable L2 cache and run again. Hah,
took 4-5 times longer unless it was a very small program, because the L1
cache is tiny, intended to hold program loops.

| I would like to hear if anyone has any knowledge about this, thanks.

With a 486-66 we measured a 6% improvement from 256k to 1M cache. Given
that the Pentium beats memory a lot harder, and hurts more when it gets
a cache miss, so cache size jumps buy more in the Pentium.

If I could have afforded 512k cache on my PPros, I would have gotten it.
Unfortunately it's very expensive right now, so I live with what I have.

Hope some of this helps, it's not cut and dried with a nice table of
what cache buys what speed. Your applications and RAM speed are
important, too.
--
Stupidity, like virtue, is its own reward.

TMR does UNIX and other systems stuff, some real time, network and
system admin, security, C and other good stuff.

Quote:> | I have a AMD486DX4 100mhz with 256kb SRAM and 8mb DRAM but suppose I upgrade
> | to 24mb DRAM, then should I upgrade SRAM too?
> |
> | Assuming I did upgrade from (SRAM,DRAM)=(256kb,24mb) to say (512kb,24mb).
> The size of the cache is related to the size of your "working set" of
> pages you access often. Some systems rarely reuse a page, due to apps,
> and get little benefit from 512k or even a MB of cache. Others find that
> more cache gives a big improvement, aven if the system has little memory
> as RAM. It's not a fixed ratio between cache and RAM, although if you
> add RAM because of load, often more cache will help, too.

in any case, cache size is definitely related to memory size,
since a given cache will only cache for a certain size of memory.
here's how it works: the lower 18 or 19 bits of address are used
as an index directly into the cache (256 or 512k).  then the motherboard
provides another set of bits which contain the 'tag' which disambiguates
more of the address space.  this is usually 7 or 8 bits (7 for systems
that steal a tag bit to keep track of whether a cache line is dirty
or not - this is not needed for write-though caches.)  so with 256k
of cache, you probably also have 7 or 8 bits of tag, meaning that
18+7=25 or 26 bits of address space are cached.  that's 32 or 64M,
so 256k and 24M should be fine, even with writeback and 7 bits tag.

note that the things Bill Davidsen said about the speedups from various
cache sizes is ontrack.  in short: bigger cache will almost always
help, though sometimes a miniscule amount; it depends entirely on
your application.

regards, mark hahn.
--

                                        http://neurocog.lrdc.pitt.edu/~hahn/