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The problem with prefetch

The problem with prefetch

Posted May 25, 2011 12:58 UTC (Wed) by Cyberax (✭ supporter ✭, #52523)
In reply to: The problem with prefetch by nye
Parent article: The problem with prefetch

No. SRAM uses 6 transistors, and in idle mode continuously seeps energy. That's OK if you have a small cache buffer but when you have gigabytes of SRAM it adds up very quickly.

DRAM uses capacitors and only needs to be refreshed from time to time.

Our embedded guys tell me that SRAM also is quite power-hungry during reads and writes, so high-frequency SRAMs consume significantly more power than DRAM. Sometimes very significantly more power.


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The problem with prefetch

Posted May 25, 2011 13:49 UTC (Wed) by nye (guest, #51576) [Link]

>No. SRAM uses 6 transistors, and in idle mode continuously seeps energy. That's OK if you have a small cache buffer but when you have gigabytes of SRAM it adds up very quickly.

> DRAM uses capacitors and only needs to be refreshed from time to time.

All of the resources I can find describing SRAM state that the power used continuously while idle is trivial compared to the power needed to constantly refresh DRAM, and hence it's only during heavy utilisation that its power consumption can get *up to* that of DRAM.

>Our embedded guys tell me that SRAM also is quite power-hungry during reads and writes, so high-frequency SRAMs consume significantly more power than DRAM. Sometimes very significantly more power

I'm sure what you're saying is true in your case, but how do you reconcile that with the fact that every other comparison between the two disagrees?

The only idea I can come up with is that you're comparing SRAM running *much faster* than DRAM so it's an apples-to-oranges comparison(?)

The problem with prefetch

Posted May 25, 2011 16:38 UTC (Wed) by bronson (subscriber, #4806) [Link]

> SRAM uses 6 transistors

Yep.

> and in idle mode continuously seeps energy

Not if made out of CMOS (and these days everything is CMOS). At steady state the only power loss in SRAM is gate and substrate leakage. Negligible.

> DRAM uses capacitors and only needs to be refreshed from time to time.

Yeah, 50 times a second. It adds up to quite a bit of power. Plus DRAMs have all the substrate leakage of the SRAM.

I'm pretty sure you're comparing supercomputer SRAMs from the early 80s, clocked to the limits of their lives, with low power modern DRAMs. If you compare equivalent parts you'll see that DRAMs are smaller but burn a lot more power.


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