A little off-topic, but does anybody know of any hacky ways to disable hyper-threading (on Haswell if it matters) if the firmware doesn't provide the option?
Intel Skylake/Kaby Lake processors: broken hyper-threading
191–200 of 278 posts
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#192Earlier quoted context omitted.
A few past lives ago, I used to work on the AIX kernel at IBM. I once spent a few weeks poring through trace data trying to investigate a very mysterious cache-aligned memory corruption induced by a memory stress test. Our trace data was quite comprehensive, and is always turned on due to its very low overhead. It was concerning enough (and took me long enough) that it eventually sucked in the rest of my team to aid…
Did you by chance see the paper a year back or so outlining that memory errors are more likely to occur near page boundaries? The author's premise was that a lot of 'cosmic rays' are just manufacturing flaws.
having often attributed these things to cosmic rays as well, anyone have any actual estimates on how often/likely cosmic rays are to cause errors in desktop-like setups?
had to have been a cosmic ray at least once right?
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#193Earlier quoted context omitted.
I wonder if one could update the microcode using a ring 0 virtualization platform and Windows as a guest.
Nope, can only be done in the host and not VMs for security reasons.
..and microcode updates aren't persistent, they have to be applied every boot anyway.
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#194Earlier quoted context omitted.
Did you by chance see the paper a year back or so outlining that memory errors are more likely to occur near page boundaries? The author's premise was that a lot of 'cosmic rays' are just manufacturing flaws.
having often attributed these things to cosmic rays as well, anyone have any actual estimates on how often/likely cosmic rays are to cause errors in desktop-like setups? had to have been a cosmic ray at least once right?
Cosmic ray errors (called soft errors in the industry) are measured in FIT rate, (failures in time) which is measured in failures per billion device hours. Basically, it's very unlikely to hit a desktop computer, somewhat likely to happen higher in the atmosphere/space, and very likely to happen in a system with hundreds or thousands of processors.
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#195Earlier quoted context omitted.
You really don't know what you're talking about. --- For Skylake, they probably optimized partial register writes to these four partial "high" registers (AH, BH, CH, DH), but the optimization was buggy in some hard-to- hit corner case. They did not do this. The high registers (AH/BH/DH/CH) are nearly written out of existence with the REX Prefix in 64bit mode. Within the manual(s) it is called out effectively not to u…
> High Register and 16bit registers are huge wart that it seems Intel is trying desperately hard to get us to stop using. Someone really ought to tell clang and gcc this; they both happily use 16-bit registers for 16-bit arithmetic. Anyway, Intel obviously already has special optimizations for many partial register accesses, dating back to Sandy Bridge. While it's quite possible that they left out the high registers…
Indeed, I've worked with CPUs that didn't have the register split of x86 and they are far less friendly to implementing certain algorithms, which would otherwise require many additional registers and lots of shift/add instructions to achieve the same effect. ("MOV AH, AL" being one simple example -- try doing that on a MIPS, for instance.)
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#196Earlier quoted context omitted.
If this "fix" ends up disabling HT, how can I get a refund not just for the CPU but for the $3k laptop I spec'd around it? Without needing to sue?
Contact your laptop OEM.
Have you ever had to do this?
The Dell XPS 9350 (Skylake) has numerous issues with GPU noise, USB-C compatibility, wifi reliability - Dell don't care unless your consumer laws are strong enough to make them care.
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#197Earlier quoted context omitted.
Did you by chance see the paper a year back or so outlining that memory errors are more likely to occur near page boundaries? The author's premise was that a lot of 'cosmic rays' are just manufacturing flaws.
Can you provide a citation? I'd like to read it.
Ah, found it: "Cosmic Rays Don't Strike Twice" http://www.cs.toronto.edu/~bianca/papers/ASPLOS2012.pdf
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#198So will this be affecting most Macbook Pros of the past few years? If so, there's a way to disable hyper-threading, but you need Xcode (Instruments). Open Instruments. Go to Preferences. Choose 'CPU'. Uncheck "Hardware Multi-Threading". Rebooting will reset it.
This is kind of like cutting off your leg because of a hangnail. I've been running a Skylake MBP for more than 6 months for compilation workloads and haven't seen a single processor hang. I'm much more annoyed by the completely unpredictable desktop assignment on monitors when hotplugging DisplayPort connections on multiple displays. This one bothers me every day.
I agree. I've been fine myself.
But if people feel the need to turn it off. ;)
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#199Earlier quoted context omitted.
That's absolutely true. When it comes to CPU/memory, skilled software engineers always think, "it must be my bug, it always is". So in that super rare case of actually running into a CPU defect, it's a mindfuck, it'll drive you crazy. You'll be looking for the flaw in your algorithm which makes it fail once a week under production load. But you just can't find it, it makes no sense ... (When it comes to drivers for n…
You have some lovely stories online about these things. Like the guy tracking down a stuck bit in RAM. Or on the network level, a VPN that failed only when traversing one possible route between company offices.
These problems have existed for years - I once tracked down a dead cell in core memory on a system I worked on in the Air Force. Because of the chance alignment, it caused incoming messages to never "end". Which was fun for the operators.
Re: Intel Skylake/Kaby Lake processors: broken hyper-threading
#200So, serious question: If the microcode "fix" for this ends up disabling HT, how does one get a refund not just for the CPU but for the $3k laptop I spec'd around it? Without needing to sue? This isn't a hypothetical; what did Intel do when the only fix for broken functionality was to disable TSX entirely?
I remember the pentium bug in the mid 90s, they actually shipped out replacement processors. Doubt that could be pulled off on laptops. Perhaps a microcode update can work around it.
Given my Surface Book got a 1/10 repair-ability score on ifixit... I dont think they'll just replace the chip :-P