[Beowulf] Is there really a need for Exascale?

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Thu Nov 29 06:52:51 PST 2012

Okay.. So SRAM instead of Cache..

Or at least cache that doesn't care about off chip coherency (e.g. No bus
snooping, and use delayed writeback)

A good paged virtual memory manager might work as well.

But here's a question... Would a Harvard architecture with separate code
and data paths to memory be a good idea. It's pretty standard in the DSP
world, which is sort of a SIMD (except it's not really a single
instruction... But you do the same thing to many sets of data over and
over.. And a lot of exascale type applications: finite element codes,
would have the same pattern)

On 11/29/12 6:47 AM, "Eugen Leitl" <eugen at leitl.org> wrote:

>On Thu, Nov 29, 2012 at 02:19:26PM +0000, Lux, Jim (337C) wrote:
>> On 11/28/12 11:46 PM, "Eugen Leitl" <eugen at leitl.org> wrote:
>> >On Thu, Nov 29, 2012 at 01:14:39AM -0500, Mark Hahn wrote:
>> >
>> >I've been waiting for cache to die and be substituted by
>> >on-die SRAM or MRAM. Yet to happen, but if it happens,
>> >it will be with embedded-like systems.
>> When running, SRAM consumes a lot more power and space than almost any
>> kind of DRAM.  2-4 transistors per cell vs 1, if nothing else.
>Yes, but we're talking cache. Cache is SRAM with extra logic.
>Even a cache hit is slower than it would take to access on-die
>SRAM. Cache coherency doesn't scale due to relativistically
>constrained signalling. There also cannot be any such thing
>as a global memory, unless you want it to be slow and spend
>a lot of silicon real estate to make multiple writes to the
>same location consistent.
>> A big problem is that the CMOS process for dense, low power, fast RAM is
>> different than what you want to use for a CPU. And even between DRAM and
>> SRAM there's a pretty big difference. (trenches, etc.)
>This is why we need stacked memories. Notice that MRAM might be compatible
>with CPU fabbing processes. ST-MRAM
>should have very good scaling in terms of performance and power
>dissipation and can potentially be fabricated on top of an
>ordinary CPU core http://www.cs.utexas.edu/~cart/publications/tr01-36.pdf 

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