Not all memories are created equal. Some DIMM suppliers’
cards have margins that are better than others. And of course, the better the
margins, the better the performance of the system, and the fewer blue screens.
We all know that signal integrity is critical for optimal
system performance. High-speed signals on serial IO such as PCI Express, and on
memories like DDR3, should operate with very low bit error rates. What this
means is that under normal operating conditions, the margin (or shape and size
of the “eye diagram”) should be large enough so that there is a very low
probability of a bit flip. Single or multi-bit errors are either correctable or
uncorrectable, with either a performance impact, or a system crash or hang.
Notebook DIMMs are made by many companies, including
Corsair, Kingston, Patriot, Samsung, Crucial, and others. It is fairly
straightforward to measure the design margins of different DIMM suppliers, to
see which perform better and are more reliable. We hooked up ScanWorks for
Intel HSIO to a garden-variety notebook customer reference board and ran a 1-D
voltage margining test on two different vendors. The results for “Vendor X” and
“Vendor Y” are below:
The green indicates passing lanes, and red indicates failing
lanes. Vendor X has excellent margins; on the positive voltage side all lanes
are passing at the maximum allowed range, and on the negative voltage side we
are comfortable at the -40 voltage ticks (well past the defined eye mask at
which margins are unsatisfactory). Vendor Y, on the other hand, is right at the
very edge of the guard band. On the positive voltage side, a couple of lanes
are approaching 24 ticks, which is on the “hairy edge”. On the negative voltage
side there’s a little bit more margin, but maybe not enough to survive the
variances that will occur across different lots of silicon and circuit boards.
So whose memory would you buy, Vendor X or Vendor Y?
These are, of course, just the results of a single test. As referred
above, variances in the chips or boards may produce unacceptable
margins once a design has gone into volume production. An excellent e-Book on
the source of manufacturing variances is here: http://www.asset-intertech.com/Products/High-Speed-I-O-Validation/HSIO-Software/Manufacturing-Variance-e-Book.