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QPI vs Hyperthreading

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November 14, 2010 3:44:32 AM

Was just doing some research for shits and gig's, and even though i already knew intel clearly just kind of punches AMD in the face when it comes to performance, im stuck on exactly what the difference in speed for intels typical 4.8gt/s QPI vs AMDs typical 4000mhz hypertransport. Wouldnt the 4000mhz just be equal to 4gt/s?

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November 14, 2010 5:17:21 AM

There is no reason to compare statistics of each chip, as their architecture is very different.
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November 23, 2010 11:53:07 PM

lil_reeper_sniper said:
Was just doing some research for shits and gig's, and even though i already knew intel clearly just kind of punches AMD in the face when it comes to performance, im stuck on exactly what the difference in speed for intels typical 4.8gt/s QPI vs AMDs typical 4000mhz hypertransport. Wouldnt the 4000mhz just be equal to 4gt/s?


First of all, AMD's communication link is HyperTransport, not HyperThreading. Yes, 4000 MHz HyperTransport 3.0 link is similar to a 4.000 GT/sec Intel QPI connection because each transfers two bytes per clock cycle. So the 4.80 GT/sec QPI is 20% faster than a 4000 MHz HT link. However, that makes absolutely no difference in actual usage as any CPU with a 4000 MHz HT3 is a desktop CPU and the link is just used to talk to the northbridge. The link won't be saturated until the speed gets down below 1000 MHz or so if you don't have an IGP on board.

The real place where HyperTransport vs. QPI link speeds matter is in multi-CPU systems to talk to neighboring CPUs. All of AMD's current Opterons use 6400 MHz HT links, compared to some Intel Xeons using 4.80 GT/sec QPI links, some being at 5.87 GT/sec, and only the most expensive ones ($1000+) getting to run at the full 6.40 GT/s to equal the bandwidth of the HT links on a $99 Opteron.
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November 23, 2010 11:58:29 PM

HansVonOhain said:
There is no reason to compare statistics of each chip, as their architecture is very different.


QPI is actually very similar to HyperTransport. Lots of pundits even say Intel "stole" it from AMD, which is silly as HT is developed by a consortium and a lot of others besides AMD use it. Both buses are double-data-rate, point-to-point interfaces used to talk to northbridges and other CPUs in a NUMA setup, and in practice, both even transfer the same 16 bits per transmit. HT can be up to 32 bits wide while QPI is only effectively 16 bits wide (really 20 bits, but it uses an 8-bits-per-10-bits encoding scheme that "loses" four bits in overhead per transfer), but AMD has only gone up to 16 bits wide with HT.
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