On 3/15/2010 5:33 PM, Gilad Shainer wrote:
To make it more accurate, most PCIe chipsets supports 256B reads, and the data bandwidth is 26Gb/s, which makes it 26+26, not 20+20.
I know Marketers lives in their own universe, but here are a few nuts for you to crack:
* If most PCIe chipsets would effectively do 256B Completions, why is the max unidirectional bandwidth for QDR/Nehalem is 3026 MB/s (24.2 GB/s) as reported in the latest MVAPICH announcement ? 3026 MB/s is 73.4% efficiency compared to raw bandwidth of 4 GB for Gen2 8x. With 256B Completions, the PCIe efficiency would be 92.7%, so someone would be losing 19.3% ? Would that be your silicon ?
* For 64B Completions: 64/84 is 0.7619, and 0.7619 * 32 = 24.38 Gb/s. How do you get 26 Gb/s again ?
* PCIe is a reliable protocol, there are Acks in the other direction. If you claim that one way is 26 GB/s and two-way is 26+26 Gb/s, does that mean you have invented a reliable protocol that does not need acks ?
* If bidirectional is 26+26, why is the max bidirectional bandwidth reported by MVAPICH is 5858 MB/s, ie 46.8 Gb/s or 23.4+23.4 Gb/s ? Granted, it's more than 20+20, but it depends a lot on the chipset-dependent pipeline depth.
BTW, Greg's offer is still pending... Patrick _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
