DRAM/SRAM relation, how important is it really?

DRAM/SRAM relation, how important is it really?

Post by Fredrik Raadesa » Tue, 16 Jul 1996 04:00:00



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

 
 
 

DRAM/SRAM relation, how important is it really?

Post by Wm. E. Davidsen » Tue, 16 Jul 1996 04:00:00



| 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.

 
 
 

DRAM/SRAM relation, how important is it really?

Post by Mark Ha » Wed, 17 Jul 1996 04:00:00


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.

well, cache granularity is called a 'line' rather than 'page',
16 (processors < p5) or 32 ( >= p5), but the idea's right.

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/

 
 
 

1. It`s IMPORTANT!!!

Muy buenas!!!,
Le escribo ya que tengo algunos problemas y dudas acerca de linux.
Espero que tu/Usted me pueda hechar una mano, ya que poco a poco me
estoy desesperando... He escrito muchsimos emilios a los diferentes
grupos de noticias, pero nadie me puede ayudar...:
Desde principios del verano del 97, llevo utilizando la versin de
S.u.S.E. Linux 5.2 (Krnl: 2.0.33, Xfree:3.3.2, con la version beta del
KDE.) Es la versin alemana que incluye 4 CD`s...
Mis dudas son:
1. ?Linux es compatible con mi procesador K6-2 a 350Mhz? Supongo que si,
ya que funciona,,, pero he odo algo por lo del cach de segundo
nivl...He odo que hace falta un patch para Linux que haga que Linux
utilice el cach de segundo nivel...?Es verdad?
2.En mi otro ordenador (un Pentium 133 con tarjeta grfica S3 Virge) he
conseguido hacer que funcione todo perfectamente. Todo, incluido el
entorno grfico...
Pero en mi nuevo ordenador (un K6-2/350 en placa ASUS P5A con tarjeta
grfica Trident 3DImge975 de 4 Mb) no consigo configurar el entorno
grfico. Cuando intento configurar la tarjeta grfica, con el programa
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me sale una ventana peque?a (ms o menos de 4cm de alto y tan ancha como
la pantalla) con mil lineas movindose de desordenadamente... Levemente
se v, que es el entorno grfico de "FVWM2"...pero en ese estado no se
puede hacer nada de nada...Me paso a la consola desde la cual he
ejecutado "startx" y presiono Ctrl&C...Desde la primera consola al
parecer no aparecen errores y parece estar todo bien...He probado mil
configuraciones diferentes de resolucin y colores pero nada,
tampoco...Mi targeta (Trident 3d Imge 975) est en la base de datos de
las targetas soportadas, pero aun as no la reconoce bien. En la caja de
versin de Linux pone que mi targeta est soportada, y ms alante pone
que estn soportadas experimentalmente y adems sin aceleracin... Esto
ltimo (que est en modo experimental) lo acabo de ver ahora y no me lo
imaginaba... <---puede ser que mi problema sea por los drivers??? A ver
si ahora me conecto, mando este mensaje y busco el driver
acutalizado...   Ahora que he visto que los drivers, para mi tarjeta
grfica, que incluye mi versin de Linux son experimentales me parece un
poco estpida haberte hecho esta pregunta... pero an as gracias por
leerla...
3. Por ltimo,: ?Sabes de algn programa para entrar en Internet por
RED? Para que desde el Pentium pueda entrar en internet con el K6. Tipo
"Internet-Share"....      Muchas gracias en adelanto, Daniel     --->

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