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Category: Computer Science

Optane/Xpoint and paradigm shift

So Optane is here. It’s a bit-addressable non-volatile storage with a higher density than DRAM. It’s not as good as initially promised, yet, but it’s a first iteration.

It is basically very slow RAM (or very fast flash), which is bit-adressable. So you are not, like with flash, erasing 64 KB sized pages, but you are doing things to individual bits and bytes.

It’s also faster than flash (but slower than DRAM), about 10x faster than old Commodore 64 memory.

And it’s persistent, so if you power off your machine, contents are not gone.

And it is very dense, denser even than the memory you currently use, because no transistors, so less space necessary per bit.

This is going to change a lot of things, but not right now. We need to rethink our approach to persistence.

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Qi in Apple, Otterbox makes wireless batteries, and David Foster left Google

So Google fucked the Nexus line, and did the lackluster and overpriced Pixel excursion. The guy who strategically fucked this up, David Foster, apparently goes back to Amazon.

Meanwhile it is an open secret that the iPhone 8 will have the Qi charging that the current Nexus and Pixels don’t have any more. Also, Otterbox and friends are already making contactless battery extensions. It’s not pretty and it’s not efficient, but after the “no ports for nobody” disaster of the current generation of Macbook Pros it’s at least consistent.

This leaves us with the question: Will Google make attractive phones again after Foster is gone now?

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Toybox: Writing a new command line from scratch

Rob Landley, of Busybox/Toybox fame, spoke four years ago about the Toybox project in the context of Android and whatever else was recent back then. The talk contains a brilliant deconstruction of the problems that GPL v3 has, and why it is in decline.

It also shows a lot of vision re containers, and what is needed in this context. If you are deploying Alpine today, with musl and toybox in it, here is why and how it came to be.

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jq

When dealing with Kubernetes, you will inevitably have to deal with config and data that is in JSON format.

jq is a cool tool to handle this, but while the man page is complete, it is also very dry. A nice tutorial can be found at The Programming Historian, which uses some real world use cases. My personal use case is Converting JSON to CSV, and the inverse of that. There also is a mildly interesting FAQ.

Learning jq takes about one quiet afternoon of time.

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Understanding sysdig

The open source sysdig is a piece of software that does not quite, but almost, what strace or oprofile do: It instrument the kernel, and traces system calls as well as a few other kernel activities.

It does not utilize the ptrace(2) kernel facility, though, but its own interface. This interface picks up data in the kernel and writes it into a ring buffer.

A userspace component extracts this data, interprets, filters and formats it, and then shows it. If the data source outpaces the userspace, the ring buffer overflows and events are lost, but the actual production workload is never slowed down.

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Signed pointers

So those real hackers keep telling me that back then in the times of the LISP machine they had tagged pointers and stuff.

Those pesky mobile Whizkids at Qualcomm could not let that stand, so they created signed pointers for ARM 8.3. Two families of new instructions have been made, one for signing pointers, the other for checking the signature. How does that work? The PDF at Qualcomm describes the details.

Basically, when pushing a return address onto the stack on subroutine call, that pointer is authenticated with a PAC* instruction, on return that pointer is checked with an AUT* instruction. The actual RET will fail with an address violation if the pointer has been messed with. PAC* and AUT* are out of NOP space, so they can be executed as NOPs on older CPUs.

PAC* signs the return address, AUT* checks it. On pre-8.3 CPUs, they decode as NOP instructions. RETing to an address that does not AUT is an illegal address exception.

A 64 bit pointer in an 40 bit cellphone processor is good for 24 bit signatures, but other partitions are possible depending on address space layout and size.

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