I say most apps because it's true. :) I would suggest that pretty much every app (other than video/audio streaming) people think are bandwidth-limited are actually latency-limited. Take the SpecFoo tests. Sure I would have rather seen SAP/TPC/etc that would be more relevant to Postgres but there aren't any apples-to-apples comparisons available yet. But there's something to consider here. What people in the past have believed is that memory bandwidth is the key to Spec numbers -- SpecFP isn't a test of floating point performance, it's a test of memory bandwidth. Or is it? Numbers for DC Opterons show lower latency/lower bandwith beating higher latency/higher bandwidth in what was supposedly bandwidth limited. What may actually be happening is extra bandwidth isn't actually used directly by the app itself -- instead the CPU uses it for prefetching to hide latency.

Scrounging around for more numbers, I've found benchmarks at Anandtech that relate better to Postgres. He has a "Order Entry" OLTP app running on MS-SQL. 1xDC beats 2x1 -- 2xDC beats 4x1.

order entry reads
2x248 - 235113
1x175 - 257192
4x848 - 360014
2x275 - 392643

order entry writes
2x248 - 235107
1x175 - 257184
4x848 - 360008
2x275 - 392634

order entry stored procedures
2x248 - 2939
1x175 - 3215
4x848 - 4500
2x275 - 4908

Greg Stark wrote:

William Yu <[EMAIL PROTECTED]> writes:

It turns out the latency in a 2xDC setup is just so much lower and most apps
like lower latency than higher bandwidth.

You haven't tested anything about "most apps". You tested what the SpecFoo apps prefer. If you're curious about which Postgres prefers you'll have to test with Postgres.

I'm not sure whether it will change the conclusion but I expect Postgres will
like bandwidth better than random benchmarks do.

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