Hacker News Reader

Comment by shiftingleft | Hacker News Reader

I don't think memory mirring features available today allow you to race two DRAM accesses and use the result that returns earlier?

View on HN

The memory controller sends the read to the DIMM that is not refreshing. It is invisible to software, except for the side-effect of having better performance.

Mirroring is more of a reliability feature though, no? From my understanding it’s like RAID where you keep multiple copies plus parity so uncorrectable errors aren’t catastrophic. Makes sense for mainframes which need to survive hardware failures.

Refresh avoidance is a tangential thing the memory controller happens to be able to do in a scheme like that, but you’d really have to be looking at it in a vacuum to bill it as a benefit.

Like I said, it’s all about cache. You’re not going to DRAM if you actually care about performance fluctuations at the scale of refresh stalls.

Clearly, hitting a cache would be the better outcome. The technique suggested here could only apply to unavoidably cold reads, some kind of table that's massive and randomly accessed. Assume it exists, for whatever reason. To answer your question, refresh avoidance is an advertised benefit of hardware mirroring. Current IBM techno-advertising that you can Google yourself says this:

"IBM z17 implements an enhanced redundant array of independent memory (RAIM) design with the following features: ... Staggered memory refresh: Uses RAIM to mask memory refresh latency."

I can google, thanks. My point is that nobody is buying mainframes with redundant memory to avoid refresh stalls. It’s a mostly irrelevant freebie on hardware you bought for fault tolerance.

Do you have evidence that this is a fact? Have you looked at the computing requirements documents for, for example, stock exchanges? I have it on good evidence that stock exchanges ran on mainframes. They are essentially the counterparty (in a computing sense not a financial sense) in each placed order. If someone is willing to run a fiberoptic cable from Chicago to New York or New Jersey to exploit reduced propagation delay, admittedly much larger than a refresh stall, wouldn't you think that they or someone else would also be interested in predicting computing stalls. An exchange would face at least a significant reputational risk if it could be exploited that way.

The low latency matching engines in colos run Linux these days, and we use microwave instead of fiber. Incoming orders are processed by hardware receive timestamp, so predicting jitter doesn’t give you an advantage. Clearing and settlement I’m not sure about, not latency critical though, mainframes wouldn’t surprise me there.

Reply on news.ycombinator.com